The crustal scale seismic image of the Southern Uralide orogen from geology and the URSEIS CMP-reflection profiling show a doubly vergent fabric with a significant crustal root in a low-relief and little eroded and unextended mountain belt preserved since Early Mesozoic times. The structures are the result of two major, partly super-imposed collisional events: (1) the development of an Upper Devonian W-facing accretionary complex with an exhumed subduction-related hP-belt resulting from an continent-arc collision and (2) a Permo-Triassic collision with an eastward accretion of the Trans-uralian terranes and general intracontinental shortening. While the E-vergent accretionary prism involves the entire crust with the Moho as the basal detachment, the latter thick-skinned W-vergent backthrusts only include the upper part of the crust, emphasizing different rheologies of the plate fragments involved. The rather low thermal influence across the entire Urals emphasizes that the compositional features of the crustal units take the main role: mafic to intermediate rocks dominate in the accreted Siberian island arc and oceanic collage, while the former passive margin of the Archean East European craton is controlled by a quartz-feldspar rheology. The orogenic root zone as defined by wide-angle data is poorly imaged in the near vertical reflection data. Gravity and petrophysical modelling suggest that the crustal root is composed of mafic eclogitic rocks which inhibits Moho reflectivity and together with the supra-crustal oceanic fragments the buoyancy potential of the Uralian crust. The old, cold and low-density Archean lithospheric mantle below is not delaminated and screens the orogenic thickened crust from thermal overprint. In combination with permanent compressive boundary forces the Urals thus evolved near isostasy and never departed far from equilibrium.In contrast, the broad and dismembered Variscides suffered a major late- to post- collisional high-thermal overprint combined with processes of substantial exhumation, tectonic denudation, crustal melting and magmatic underplating. The continent-continent collisional systems involved Early Paleozoic (e.g.Cadomian-Panafrican) fragments. High buoyancy-potential and delamination of the juvenile lithospheric fragments and astenospheric upwelling cause the high crustal mobility associated with a substantial reorganisation and re-equilibration during basically Upper Carboniferous times.
Riphean - Early Paleozoic geological history of the Urals, is mainly the history of riftogenesis. During more than 1,1 Ga in the course of long but interrupted riftogenesis and extension it took place a transition from platform environment to rift ones, and then - to the generation of Urals paleoocean. During rift development the thicknesses of formations have been increasing with rapid decreasing of the share of sedimentary rocks and increasing of volcanic ones, while their alkalinity has been gradually lowering. The time of the beginning of oceanic spreading of Urals paleoocean is Late Arenigian. At the same time between Russian platform and Urals paleoocean it has being formed passive continental slope and rise, that is fixed by the beginning of cherty sedimentation, caused by the deepening of paleobasin. On the base of numerous new finds of conodonts collected mainly in the formations, which age was not properly determined, the view about stratigraphy and the history of the development of practically all Urals zones have been radically revised. For the first time many of the continental-slope formations, ophiolites and island-arc complexes were reliably dated and correlated. It has been studied the development and structural evolution of the Main Uralian Fault (MUF), which separates the western paleocontinental sector of the Urals from the eastern paleoisland-arc sector of the region. New geodynamic models of formation of the Urals eclogite-glaucophan belt and Platiniferous belt (Ivanov, Shmelev, 1996) were proposed. In middle Paleozoic MUF was a subduction zone, dipping to the east and consumed the crust of early Paleozoic Urals ocean. In the process there were generated coeval (in the South Urals - 380±10 Ma) eclogite-glaucophane complexes (western part of MUF zone) and andesite island-arc association which have been forming over them. Platiniferous Urals belt situated to east from MUF is a magmatic trace of subduction zone. The belts massifs, composed with dunites, clinopyroxenites, olivine gabbro, gabbro-norites and granitoids are island-arc complexes - the products of different depth crystallization of melts, which have been generating over subduction zone with the age about 420 Ma. Subduction consumed the crust of early Paleozoic ocean by the middle of Late Devonian. The direction of the collision of Urals terrains with Russian platform was oblique (north-western). So the Urals folded belt undergone all stages of complete cycle of geodynamic development: pre-rifting (Riphean-Vendian), continental rifting (Cambrian-Lower Ordovician), oceanic spreading (Middle-Upper Ordovician), island-arc (Upper Ordovician - Devonian), collision (Upper Devonian-Permian), limited post-orogenic extension (Triassic)and platform (from Jurassic up to present days).
Ivanov KS & Shmelev VR, J. Reports RAS, 347, 649-652, (1996).
Garnet amphibole pyroxenites were exhumed within the Mindyak ophiolite, Southern Urals, during the Palaeozoic Uralian orogeny. The plagioclase/spinel lherzolite ophiolite massif, situated along the Main Uralian Fault suture of the orogen, contains metamorphic garnet-bearing rocks in a tectonic breccia in contact with its mantle and transition zone sequences.
The garnetiferous blocks are compositionally varied but may be divided into two main types. Type 1 comprises predominantly grossular-rich garnet and diopside, and has high-MgO, slight LREE/HREE depletion (LaN/YbN of 0.3-1.3), positive Eu anomalies (Eu/Eu* = 1.33-1.51), and LILE enrichments. In contrast, Type 2 has more Fe-rich garnet and diopside, pargasite, and abundant accessory minerals including rutile, titanite, apatite, and zircon, lower-MgO, and LREE/HREE enrichment (LaN/YbN of 2-5.5) and LILE depletions. Both types have low 87Sr/86Sr400 0.7040-0.7044 and high (sum)Nd400 5.0-7.5 indicating that they had a depleted mantle source. We suggest that these rocks are metamorphosed, metasomatised, gabbros. An inferred depletion in Si, alkalis, and Al, and an enrichment in Ca and Fe relative to igneous compositions prior to metamorphism is attributed to rodingitisation of igneous protoliths. The metasomatised protoliths were taken to depth and metamorphosed during initiation of subduction of ocean lithosphere. Subsequent exhumation, during the Uralian orogeny, is evidenced by retrograde re-equilibration to amphibolite facies, and, later, second stage, rodingitisation.
Preliminary geochronological results include: a Pb-Pb single zircon age of 411 ± 4 Ma for crystal rims (in good agreement with a U-Pb population zircon age of 410 ± 5 Ma obtained by Saveliev et al., in press), and 467 Ma for crystal cores: and a Sm-Nd age of 414 ± 4 Ma for a whole rock-garnet pair. We suggest that the variously determined age of 410-415 Ma at the Silurian-Devonian boundary dates a metasomatic/metamorphic event in the history of the garnet amphibole pyroxenite blocks; and, tentatively, conclude that the older core age of 467 Ma is a magmatic zircon crystallisation age associated with ocean crust formation.
So, the petrogenetic history of the garnet amphibole pyroxenites traces a change from plate divergence and ocean formation, to plate convergence and, ultimately, to the Uralian orogeny.
http://www.dgt.uniud.it/petrology/
Saveliev A.A., Bibikova E.V., Savelieva G.N., Spadea P., Scarrow J.H., Pertsev A.N., Kirnozova T.I., Petrologia, (in press).
In the Polar Urals, eclogite-facies rocks crop out in the Marun-Keu complex (67°N, 66°E). This N-S striking complex covers an area of several 100 km2. It is situated along the western edge of the main suture zone of the Uralian orogen, and is bordered to the east by ophiolitic successions and island arc rocks. In order to understand the process of burial and exhumation of the HP rocks in the context of a continental collision, a detailed geochronological and petrological study of the eclogite complex was carried out. It was found that in the study area, major parts of the total eclogite volume resemble former hybrid, gabbroic to granitic magma bodies which originally intruded in a middle- to upper-crustal environment. Single zircon Pb evaporation data define crystallisation ages grouped around 540 and 490 Ma. Minor amounts of metasediments, with zircon ages exceeding 600 Ma are also present. During eclogite facies conditions, transformation of preexisting rocks to eclogites was incomplete and triggered by local presence of fluids. Fluid flow was partly channelised. Fluid pathways are represented by quartz-phengite-rutile veins. These veins generally show aureoles with rocks being transformed to eclogite-facies assemblages. Syn-metamorphic deformation is focussed in shear zones. In the main volume, however, magmatic, pre-eclogite facies macrotextural features are preserved. Internal Rb/Sr isochrons, based on eclogite facies minerals from a variety of lithologies, precisely define an age of 358 ± 3 Ma for all syn- and post- eclogite-facies processes studied. Within limits of error, the same age was obtained for amphibolite facies assemblages, with the exception of two samples showing slightly higher ages. The age of 358 ± 3 Ma also applies for minerals out of a fluid vein which caused local reamphibolitisation of eclogites. Rb/Sr biotite ages, interpreted as cooling ages, are indistinguishable from the age value given above. We conclude that transformation of the rocks to eclogites (600°C, 14 kbar; Udovkina, 1971) and subsequent uplift-related cooling was a rapid process, which took place in a narrow time interval of 6 Ma or less. In the Rai-Iz massif, a nearby ophiolite complex, phlogopite-amphibole-bearing fracture fills with serpentinisation aureoles were found. We interpret these veins as being formed by subduction-related dehydration fluids, passing the overlying mantle wedge. A provisional age for these veins is 381 ± 10 Ma. Implications of the data for the Uralian orogeny will be discussed.
Udovkina NG, Eclogites of the Polar Urals [in Russian], Nauka, Moscow, (1971).
The hinterland of the Middle Urals is dominated by island-arc volcanic rocks, separated by high grade, locally granulite facies, gneisses and major intrusive complexes. The high-grade rocks have previously been considered to be Precambrian complexes of (micro) continental affinities. One of the most well preserved granulite terranes in the Urals is the Early Devonian to Middle Carboniferous Salda Metamorphic Complex (SMC). The latter is partly covered by amphibolite and greenschist facies ophiolitic melange (Istok) of Silurian age that inturn is overlain by lower grade (sub-greenschist to greenschist grade) island-arc volcanics of mainly Silurian and Devonian age. Recent microprobe studies and pressure-temperature calculations combined with single zircon analyses isotopic age studies have allowed us to reconstruct the tectonic history of SMC. The oldest rocks found in the SMC are granulite-facies tonalite gneisses (Brodovo Suite) in the Brodovo-Maligino antiform. Zircons from these yield Early Devonian ages (387±12-405±6 Ma) and the minerals mainly display a prograde zonation. The second magmatic event was the emplacement of the Teliana gabbro-tonalite suite (354±3-359±3 Ma), these are mainly granulite-facies rocks but locally with an amphibolite to greenschist overprint. The well preserved granulites still show primary magmatic features and minor mineral zonations. The two magmatic suites are separated by a granulite- to upper amphibolite-facies shear zone that is well imaged by the ESRU96 seismic profile (Juhlin et. al 1998). Rocks of similar composition to the Teliana Suite inside the shear zone are dated to 341±8 Ma and display prograde metamorphism. The high-grade gneisses from the SMC are cut by subalkaline gabbros and monzonites dated to 337±5 and 333±5 Ma respectively. These intrusions only display a minor greenschist facies overprint. Earlier work by Grevtsova et. al (1967) and Bea et. al (1997) has shown the existence of Permian batholiths in the SMC as well as in the rest of the hinterland terranes. These intrusions show little Uralian tectonic overprint. We will propose the following tectonic history for the SMC: 1. An oceanic/back-arc environment in the Silurian (Istok Melange). 2. Early island-arc(?) setting in the Devonian (Brodovo Suite). 3. Late Devonian to Early Carboniferous evolved island-arc with intrusions in a granulite facies environment (Teliana) combined with eastward thrusting. 4. Early uplift in the Late Carboniferous (gabbro-monzonite intrusions). 5. Continued uplift in the Permian related to the batholith intrusions and 6. Widespread crustal extension and formation of Triassic West Siberian Basin and smaller graben structures across the Middle Urals.
Bea F, Fershtater G, Montero P, Smirnov V & Zin'kova E, Tectonophysics, 276, 103-116, (1997).
Grevtsova AP, Zukozurnikova GA & Dolgal AS, USSR Minestry of Geology (Report), (1970).
Juhlin C, Friberg M, Echtler H, Green AG, Ansorge J, Hismatulin T & Rybalka A, Tectonics, 17, 710-725, (1998).
The Bashkirian Anticlinorium of the southwestern Urals shows a much more complex structural architecture and tectonic evolution than previously known. Pre-Uralian Proterozoic extensional and compressional structures controlled significantly the Uralian tectonic convergence. A long-lasting Proterozoic rift process created extensional basement structures and a Riphean basin topography which influenced the formation of the western fold-and-thrust-belt with inversion structures during the Uralian deformation. A complete orogenic cycle during Cadomian times including terrane accretion at the eastern margin of the East European Platform resulted in a high-level Cadomian basement complex, which controlled the onset of Uralian deformation, and resulted in intense imbrication and tectonic stacking in the subjacent footwall of the Main Uralian Fault.
The Uralian orogenic evolution can be subdivided into three deformation stages with different oriented stress-regimes. Tectonic convergence started in the Late Devonian with the formation of an accretionary complex which prograded from SE to the NW. Continuous NW-SE directed convergence resulted finally in the formation of an early orogenic wedge thrusting the Cadomian basement complex onto the East European Platform. The main tectonic shortening was connected with these two stages and, although not well constrained, appears to be of Late Devonian to Carboniferous age. In the Permian a final stage of E-W compression can be observed throughout the SW Urals. In the West the fold-and-thrust-belt prograded to the W with reactivation of former extensional structures and minor shortening. In the East this phase was related to intense back thrusting.
The thick and old crust of the East European Platform was extremely cold when it collided with the Magnitogorsk magmatic arc during the Late Palaeozoic. This resulted in a rather narrow zone of intense crustal shortening, tectonic stacking and high strain at its eastern margin (Ural-Tau zone). While the first orogenic wedge is of thick-skinned type with the involvement of crystalline basement, even the later W-directed wedge is not typically thin-skinned as the depth of the basal detachment appears below 15 km and the involvement of Archean basement can be assumed. The structural architecture can be correlated with deep seismic images of the URSEIS '95 experiment. The western Uralian fold-and-thrust belt appears a typical example which developed on rifted, cold and rigid continental crust.
Among Palaeozoic orogenic belts, the Caledonide-Appalachian belt is notable for its subdivision into several 'orogenies'. While the Appalachians are treated in terms of six orogenies, two major episodes have been established within the Scandinavian segment of the same belt: an Early Ordovician (Finnmarkian) and a Mid-Silurian to Early Devonian (Scandian) event. However, recent concepts of mountain building processes make the relevance of subdividing an orogen into distinct episodes less obvious. The increasing evidence of syn-convergence tectonic erosion and diachroneity due to oblique collision make important criteria as unconformities and molasse ambiguous, and delimitation of an 'orogeny' in the traditional sense problematic. Amalgamation of terranes prior to accretion to the continental margin results in mountain belts composed of numerous segments with complex magmatic and metamorphic histories, rather than a few large segments, reworked during single orogenic episodes. Moreover, what is the critical mass of an orogen(y)? A single unconformity? A dated pegmatite dyke cutting a fold?
With regard to the Scandinavian Caledonides, increasing radiometric dating tends to erode the statistical basis for a subdivision into two distinct events. This is particularly true for the imbricated continental margin (Baltoscandian Margin, BM), which occurs as thrust sheets of strongly varying intensity of deformation, age and grade of metamorphism. Assessment of the extent of involvement of the BM during accretion of the mountain belt is fundamental to understanding thermal regimes of metamorphic processes and interpretations of isotopic signatures of marginal basin magmatism. Subduction and imbrication of the BM with eclogite facies metamorphism of rift facies dolerites at 505-475 Ma was accompanied by detachment of a sheeted-dyke complex from the continent-ocean transition. At 480 Ma, the detached dyke complex was penetrated by granitic dykes with inherited Precambrian ages, suggesting partial melting of the BM at depth at that time. Continuous imbrication and understacking of thrust sheets is indicated by rapid exhumation of eclogites (490-470 Ma) and ages of high-P metamorphism ranging between 470-415 Ma, and reflected in thrust zone fabrics cooling below 500°C at 450 Ma. Amphibolite to granulite facies metamorphism and partial melting of the BM occurred from 445 Ma and into the Scandian phase of ultimate continent-continent collision.
Thus, from a review of published and new radiometric data we conclude that the BM was continuously involved in tectonic processes at amphibolite facies and higher metamorphic grades throuhgout accretion of the Scandinavian Caledonides. This requires an active 'western' margin of Continent Baltica in Ordovician-Silurian plate reconstructions of the North Atlantic tract and modifies the alledged concept of an episodic orogenic process.
The Vendian to Cambrian evolution is interpreted as a stage of post-rift subsidence following the transition from continental rifting to ocean floor formation off western Baltica at c. 600 Ma ago. Although early Cambrian flooding events lead to temporarily higher sedimentation rates, post-rift subsidence generally decreased through time. Such a decrease is consistent with models of lithospheric stretching and thermal subsidence. The gradual decrease of thermal subsidence through Cambrian time shows that the Baltica lithosphere was essentially thermally re-equilibrated prior to earliest Caledonian, Finnmarkian tectonic activity in early Orodvician time. Finnmarkian activitiy lead to the deposition of Ordovician greywacke beds but not to any orogenic deformation at the western Baltica margin.
However, Silurian to Devonian, Scandian tectonic activity built up an orogenic wedge. This wedge can be divided into three mechanically distinct tiers: A: Lower Allochthon with exclusively Scandian structures and metamorphism, B: Middle Allochthon and Seve units of Upper Allochthon with traces of Finnmarkian events, and C: overlying exotic terranes (Köli of Upper Allochthon, Uppermost Allochthon). Detailed structural work documented shear criteria at the tier B-C (Seve-Köli) boundary, which consistently indicate a down-dip, top-to-the-west movement at a regional scale, immediately after peak PT conditions (c. late Wenlockian). Subsequently, the Seve-Köli boundary was deformed by regional folds, caused by the stacking of thrust systems in the underlying Lower Allochthon.
The overall shape of the orogenic wedge and its critical taper angle are dependent mainly on basal friction and wedge strength. Due to lithology and early cooling, the tier B of the Caledonian orogenic wedge is dominated by strong rocks, whereas the lower tier (A), containing horizons with low friction, is of relatively weaker strength. There, organic-rich black shales provide low-friction horizons, both at the basal detachment surface and within the wedge itself. As a result, the lower, external part of the wedge had a lower strength and a smaller critical taper angle than its internal part, so the orogenic load is upward concave.
Modelling the effect of such a load on the Baltica lithosphere shows a very small depression in front of the load (~2 km). The flexural depression produced by the main part of the orogenic load is filled up by the thickening thrust-and-fold belt, so that there is little space left for a foreland-basin. These results imply that the extremely small size of the peripheral or pro-foreland basin in front of the Scandinavian Caledonides is not due to subsequent erosion, but is a primary feature.
Global plate tectonics in the Paleozoic Era were broadly dominated by plate convergence, as various elements coalesced to form the short-lived Pangea supercontinent by Triassic time. Most of the plate motion related to Pangea assembly was driven by oceanic subduction, sometimes coupled with continental subduction/collision and genesis of very high-pressure rocks. In addition to this broad convergent setting, field evidence indicates that many regions were undergoing concomitant extension or crustal thinning, not necessarily related to 'exhumation' or 'gravitational collapse' scenarios. The importance of this type of Paleozoic crustal stretching and related finite strain patterns may go unaddressed in areas lacking extensive sedimentary or magmatic manifestations. We discuss the judicious application of structural, paleomagnetic and geochronologic analyses to the 'post-Caledonian' SW margin of Baltica where the paucity of late Paleozoic rocks has previously limited discussion of extensional tectonics during this time period.
First order field structural analysis has demonstrated that the current geometries of ductile extensional detachments in western Norway have been modified from their 'original', early to middle Devonian orientations; this was affected by imposition of post-early Devonian folding and/or several generations of younger, brittle faults. Paleomagnetic dating of brittle faults and dikes has indicated latest Permian ages for some of these brittle events; these ages are complemented by 40Ar/39Ar ages from the brittle fault rocks and dikes of latest Permian age (238 to 248 Ma for dikes and 250-260 Ma for some brittle faults). Further work with K-feldspar thermochronometry has demonstrated an Early Carboniferous unroofing event, possibly related to folding of one detachment. A natural concern in such studies is fluid flow in the faults (and dike margins) that may have disturbed the Ar- and/or magnetic signatures of the fault rocks. We have found that conducting profiles through fault zones, in areas with well-controlled tectonostratigraphy, is a satisfactory means by which to support the accuracy of the given age data. Examples from the Nordfjord-Sogn Detachment, the Oslo Rift, the Lærdal-Gjedde fault and the Møre-Trøndelag Fault Complex will be discussed.
The Seve-Kalak Superterrane (SKS) in the Scandinavian Caledonides contains the fragmented Late Precambrian continent-ocean transition between Baltica and the Iapetus Ocean. This passive margin was thrust eastwards over the Baltic Shield during Caledonian orogenesis (c. 520-390 Ma). The individual thrust sheets in the SKS went through different PTt-evolutions, resulting in dramatic metamorphic contrasts: Eclogite-bearing nappes are juxtaposed with nappes showing no evidence of Caledonian deformation or metamorphism in their interiors. A pronounced strain localisation to the marginal parts of the thrust sheets often left the interiors exceptionally well preserved. Excellent records of both pre-orogenic (rift) and early-orogenic (subduction and subsequent uplift) processes are thus preserved in the thrust sheets of the SKS.
Even though it has been transported several hundred kilometres, only the margins of the eastern part of the Sarektjåkkå Nappe (SN) are affected by penetrative Caledonian deformation. This part of the SN is dominated by pristine tholeiitic dykes and cross-bedded sandstones. The dykes are 608 Ma old and make up 70-80% of the nappe. Widely spaced (several km's), west-vergent thin shear zones of the Ruopsok fault system (RFS) are the only visible signs of Caledonian penetrative deformation in the interior of the nappe. Previously published Ar-Ar-datings indicate cooling below the closure temperature of hornblende at c. 470 Ma, but aberrant mica ages of up to 500 Ma have been recorded.
Ar dating of biotite and muscovite from a cross-laminated metapsammite in the SN gave well-defined ages of 428.5 and 432.4 Ma, respectively. Muscovite from a shear zone in the RFS gave 428.2 Ma, whereas hornblende from the same locality did not yield interpretable data. All data sets indicate that the rocks were completely degassed at some unknown earlier event, presumably the emplacement of the dyke swarm. No subsequent contamination with excess argon can be detected. These Ar ages date an event which is not recorded in any other way; there are no other indications of an event later than the emplacement of the dykes at 608 Ma.
The interior of the nappe - and thus the entire nappe complex, since the interiors should be the last to become affected by conductive heating - cooled below c. 350°C at around 430 Ma. Simple linear cooling from the c. 500°C at 470 Ma to 350°C at 430 Ma suggest a cooling rate of ~4°C/Ma. This is a low figure, but not unreasonable. It suggests a prolonged period of slow cooling (=exhumation?), following the initial, rapid uplift of the eclogite-bearing nappes and Early Ordovician amalgamation of the SKS. However, mica ages indicating ages older than 430 Ma are difficult to reconcile with this model; their geological significance should be critically reevaluated.
The Ordovician-Silurian accretionary wedge of the Southern Uplands and the structurally associated Ballantrae Ophiolite Complex formed part of a Caledonian convergent plate margin in SW Scotland. Provenance analysis of sediments from both tectonic units and the unconformably overlying Devonian Old Red sandstones were carried out to constrain source environments and elucidate processes of sediment recycling in a fossil convergent margin setting.
Detrital framework modes of sand size fractions in greywacke and sandstone samples were determined using the Gazzi-Dickinson method. The sedimentary cover of the Ballantrae Ophiolite Complex has a recycled orogenic provenance. The Ordovician units, with a distinctly magmatic source rock signature, show increasing compositional maturity with decreasing age. This may reflect either increasing distance to the source area with time, or continuous recycling and resedimentation. Samples from the Old Red sandstone unit show a clear cratonic provenance.
Our data from Southern Uplands show that the greywackes there were derived from a volcanic island arc. This interpretation is confirmed by earlier studies and by our results of XRF-analyses of detrital clinopyroxenes found in the greywackes. Here, a mixed calc- alkaline and tholeiitic signature is evident for mid-Ordovician to Silurian coarse clastics. In the oldest samples analysed (Tappins Group of Llanvirn age) clinopyroxenes are found, however, that are derived from alkaline basalts pointing to an intraplate volcanic source.
Geodynamic models for the Scottish Grampian orogeny envolve collision between a peri-Laurentian island arc and the passive margin of Laurentia. The collision produced large scale knappe stuctures, regional Barrovian metamorphism, syn- and post-metamorphic acid and basic magmas and deposition of flysch into neighbouring terranes. In Scotland, this is traditionally regarded as having lasted over a period of ~200 Ma. To testif the Grampian orogeny was actually a catastrophic Himalayan scale ~20 Maorogeny or not, both syn-metamorphic and post-metamorphic granites have been sampled. Prismatic, brown to light brown, transparent zircon grains with a c-axes length of up to 200 µm have been picked for dating by the single grain evaporation method of Kober (1986). Cathoden lominicense has shown that all grains are without cores and inclusions. These new data indicate that the Grampian orogeny in Scotland occurred between ~480 and~465 Ma. A study of the mineral composition of Ordovician and Silurian flysch from the neighbouring terranes reveals characteristics of a metamorphic/plutonic source very similar in type and age to the Grampian terrane. The depositional age of the earliest flysch is Middle Llanvirn. The emplacement of post-orogenic granites during the later Ordovician and though the Silurian into the Lower Devonian caused the Grampian terrane to be bouantly uplifted resulting in the continuum of erosion and flysh deposition.Comparison of these data from the west of Ireland (Friedrichs et al. 1997) suggest that the Grampian Orogeny was short catastrophic Himalayan styleorogeny between ~480 and ~465 Ma.
Friedrich et al, Terra Nova, 9, 31, (1997).
Kober, Mineral. Petrol, 93, 482-490, (1986).
Understanding mountain building evolution is based upon a good knowledge the pre-orogenic location and the collision period of the different accreted terranes. The traditional paleogeographic tools, paleomagnetism and paleontology, are not always able to resolve these problems. Espescially when remagnetization effects appears during orogenic events and when two areas contain same faunas which were separated by a narrow oceanic domain. All these limits lead to different paleogeographical models.
For example, in the northern Appalachian belt, several orogenic events occurred during the Paleozoic. The origin and the timing of the collision of the two exotic terranes (Avalon and Meguma) are discussed. There is no consensus about their collision time which can be occurred between middle Ordovician (450 Ma) and Late Devonian (370 Ma) after the different models. The gondwanian origin of the two exotic zones is well demonstrated but the precise location around this continent is not clear. Before the collision, they may came from the north of South America or from West Africa. Moreover, it is not clear if their accretion consisted to an arc-continent or to a continent-continent collision.
In order to resolve these problems, we use the Sm/Nd isotopic system in shales and we use published data of detrital zircons ages to constrain the location of the exotic zones during the Neoproterozoic. Assuming that the Nd isotopic initial composition in shales characterize a similar tectonic domain, we compare Sm-Nd isotopic data from the Humber zone, which corresponds to the sedimentary record of the Laurentian domain, with data from the exotic zones (Avalon and Meguma). All these data permit us to propose a model in which during the Neoproterozoic (670 Ma), the Avalon zone was located near the north of South America and the Meguma zone was situated near West Africa. We propose that the Avalon zone collided with north America in middle Ordovician (450 Ma) and the Meguma zone collided during early Carboniferous (between 370 and 330 Ma).
The Dalradian rocks of northwestern Britain and Ireland provide a classic example of collisional orogeny involving Barrovian metamorphism and clockwise PTt paths. This Grampian Orogeny was a thick-skinned event involving both basement and cover rocks. Controversy has surrounded the identification of basement elements as well as their affinity with Laurentia. The Dalradian was deposited on the rifted passive margin of Laurentia, during the Neoproterozoic to early Cambrian. Convergence leading to orogeny in the Early Ordovician cannot be the result of a simple Wilson cycle because the conjugate ocean margin is missing. Consequently models for the Grampian Orogeny have appealed to ophiolite obduction and collision with a magmatic arc.
Field observations and geochronology at a basement-Dalradian contact in northwestern Ireland lead us to suggest a new tectonic model involving indenter tectonics.
The metasedimentary Slishwood Division records pre-Grampian c. 600 Ma high-pressure granulite- and earlier eclogite-facies metamorphic events not seen within the Dalradian. Tectonic juxtaposition of these units occurred at c. 470 Ma during Grampian orogenesis, followed by peak Barrovian metamorphism at c. 465 Ma. Orogenic collapse is recorded by southeasterly-directed extensional deformation and pegmatite intrusion at c. 455 Ma.
At 600 Ma Laurentia is believed to have been in extension, a condition not conducive to high-pressure metamorphism. Although decompression from eclogite facies into the granulite field might be a lower crustal response to extension, it is our view that the early metamorphic history recorded in the Slishwood Division is unrelated to events on the Laurentian margin.
We propose that the Slishwood Division was the basement to a magmatic arc that collided with the Laurentian margin with the motion initially accommodated by subduction under its leading edge. Fluid infiltration into the Slishwood Division before collision is manifest by static growth of hydrous minerals, possibly in a supra-subduction zone setting. Hornblende geochronology dates this event at c. 480 Ma, 10 Ma before collision of the Slishwood Division with Laurentia.
Magnetic, gravity and seismic data suggest that the Slishwood Division does not extend laterally much beyond the present outcrop (c. 2000 km2). Transpressional structures either side of the Slishwood indenter have an opposite shear sense - dextral on northeast side, sinistral to the southwest - consistent with escape tectonics. The original geometry and extent of the indenter is unknown but sub-arc basement may be regionally extensive and could include the Tyrone Central Inlier and the enigmatic Midland Valley Terrane of Scotland.
Field and geochronological data suggest synchronous deformation, metamorphism and granite emplacement in transpressive orogens. Clockwise P-T paths are characteristic, and result from the effects of heat conduction to the surface during syntectonic erosion of the thickening orogen, dissipation of mechanical energy generated during deformation, and syntectonic mass transfer that advects hot material to shallow depths. In the Northern Appalachians, oblique translation during Devonian (Acadian) dextral transpression thickened the Silurian stratigraphic sequence of the Central Maine belt (CMB) and displaced isotherms toward the surface, creating a near-isothermal corridor. The typical high-T metamorphism of the CMB, however, reflects the additional effects of high heat production in the sequence, a consequence of high U and Th contents fixed in strongly reduced sediments of the precursor anoxic basin. Furthermore, basin subsidence likely prestressed the lithosphere, making this belt a weak link in responding to Devonian transpression.
Strain was accommodated heterogeneously, being localized into rheologically weaker strata. It was partitioned between steeply-dipping 'straight' zones of enhanced deformation that accommodated more displacement and record higher strain (higher strain zones, HSZs), and intervening zones, composed of rheologically stronger strata, that record lower strain (lower strain zones, LSZs). Perturbations in ductile flow caused folding and thrusting, but different rheological behavior between stratigraphic units resulted in enhanced fold tightening, overturning and limb shear strain in HSZs that was not recorded in LSZs. In HSZ rocks, penetrative continuous mica and quartz-ribbon foliation and bladed muscovite and biotite mineral elongation lineation define the tectonic fabric, recording apparent flattening-to-plane strain. In contrast, in LSZ rocks, foliation is weakly developed or absent and bladed muscovite that forms the prominent penetrative mineral elongation lineation defines the tectonic fabric, recording apparent constrictional strain. In both types of zone, deformation and mineral growth were coeval, because the same minerals define fabrics at the same grade.
At metamorphic grades above the contemporary solidus, stromatic migmatite and concordant or weakly discordant irregular 'sheet-like' bodies of granite occur in HSZs, suggesting percolative flow of melt along the flattening fabric and viscous flow of magma in planar conduits. Inhomogeneous migmatite and irregular 'rod-like' bodies of schlieric granite occur in intervening LSZs, suggesting transport of partially-molten material through these zones en masse by melt-assisted granular flow and in 'pipe-like' conduits. This relationship between fabric shape and the form of melt escape structures suggests flow was deformation-controlled and governed by strain partitioning. Thus, the coupled mechanical and thermal evolution of transpressive orogens enables transport of melt to progressively shallower crustal levels by differential stress-induced processes as the near-isothermal corridor is propagated upward, advecting heat to drive upper crustal high-T metamorphism. One consequence of this feedback relation is the syntectonic nature of the metamorphism, and granite ascent and emplacement in transpressive orogens.
The paper presents a detailed description of the interpretation of deep seismic data along profiles in the Palaeozoic Platform in Poland close to the Variscan Front. Total length of the seismic refraction and wide angle reflection profiles in this area is about 2000 km and several short (20-30 km) near vertical reflection profiles located in the Polish Basin. The purpose of the interpretation is to determine the thickness of the sedimentary cover, to investigate low velocities in the upper crust indicate a possible mid to late Palaeozoic basin to a depth of 20 km, and to study the nature of the lower crust and the transition zone between the crust and upper mantle in this area. Seismic models of the crust has implication for Caledonian and Variscan collision tectonics and for subsequent basin formation in the area.
In the Izera-Karkonosze Block, West Sudetes, the 580-540 Ma late- to post-Cadomian granodiorites and Neoproterozoic metasediments were intruded by the 515-480 Ma Izera granite that became gneissified in Palaeozoic times. The granite contains xenoliths of high-P metapelites (17 kbar, 680°C)and possesses a diatexitic envelope with HT-LP mineralogy, all obviously pre-Ordovician in age. Besides, the pre-515 Ma country rocks are also preserved in narrow schist belts occurring within the Izera granite. The mica schist belts differ from one another in their lithologic contents, structure and P-T-t-d histories and cannot represent any continuous sheet of metasediments. Metamorphic grade of these belts decreases discretely northward and along the S-N section across the Izera massif, the schistose remnants are represented by (1) rocks metamorphosed in the Kfs-Crd zone (T > 750°C, 4 kbar), (2) upper amphibolite facies Grt-Sil-Crd metapelites, (3) lower amphibolite facies mica schists (3-5 kbar, 500°C) and (4) greenschist facies rocks (3 kbar, max. 350-400°C ), all testifying to c. 400°C difference in PT conditions between the belts. The widest belt consists of rocks metamorphosed at temperature conditions different by 250°C, which, because of tectonic shuffling, are now set side by side in a 0.5-2 km wide belt. These schistose remnants are interpreted as derived from a c.15 km thick crustal section embracing upper amphibolite through greenschist facies rocks. The Neoproterozoic crust which was partially molten to produce the Izera granite was already strongly deformed and metamorphosed in a typically orogenic way, with HP and LP rocks derived from different portions of the orogen and tectonically intermingled prior to the 515 Ma onset of the rift-related granite magmatism of the next tectogenic cycle. Before the Izera granite intruded its schistose country rocks, they had already been stacked by the Cadomian N-vergent thrusting and folding, which brought into contact rocks from different crustal levels and produced the northerly tilt of the Cadomian metamorphic isograds. During the Palaeozoic the overlying country rocks were incised, in normal to oblique regime, into the Izera granite and further deformed. Accordingly, elements of the Cadomian crust are identifiable despite the Variscan overprint. These are also recognizable elsewhere in the Sudetes and their presence is commonly proved by zircon (grain cores) inheritance ages.
Contribution to 'Europrobe' TESZ and 'Orogene Prozesse...' projects.
The Sudetes on the NE margin of the Bohemian Massif expose a mosaic of various pre-Permian complexes, traditionally included in the Variscides. An idea of a significant influence of the Caledonian orogeny in that area has developed since 1920's, was subsequently rejected as a result of accumulating new data and developing new methods and concepts in geology. This idea was recently revived in models invoking Early Palaeozoic to, possibly, Early-Mid Devonian subduction and continental collision following the final closure of the Tornquist Sea and introducing a hypothetical Caledonian suture zone in the Sudetes. We reassess the evidence for the timing of orogenesis, and suggest that a regional pre-Upper Devonian unconformity traditionally regarded as the conclusive evidence in favour of the Caledonian orogeny, cannot be recognized in the Sudetes. The widespread Ordovician bimodal volcanism cannot be explained as subduction-related, but is, most probably, the result of continental break-up. No structural data from the Sudetes confirm the idea of a Caledonian orogeny. The palaeontological and palaeomagnetic data used as evidence of Caledonian orogenesis is inconclusive and its interpretation disregards large-scale Late Palaeozoic strike-slip displacements in SW Poland and the collage structure of its pre-Permian basement.
The only Early Palaeozoic ("Caledonian") orogenic record is confined to the gneisses of the Góry Sowie Massif, which occupies an exotic, allochthonous position within the Variscan tectonic framework of the Sudetes. By contrast, the evidence for a leading role of the Variscan orogeny in the Sudetes, includes Late Devonian blueschist metamorphism followed by Early Carboniferous regional high temperature event, widespread Late Devonian/Early Carboniferous flysch/molasse sedimentation and abundant granite intrusion in the Carboniferous to Early Permian. We conclude that the dominant tectonic imprint on the Polish Sudetes is Variscan.
Independently, we suggest that on the grounds of plate tectonic principles it is undesirable to extend the regionally localized term 'Caledonides' over other orogenic belts affected by any kind of early to mid-Palaeozoic tectonism. It seems more logical to term early events in the tectonic development of the Variscan belt rather as 'eo-Variscan' than to recourse to the broadly coeval, but little related, processes within the Caledonian orogenic domain.
Mafic meta-volcanic rocks of Upper Cambrian to Lower Devonian age in the Bohemian Massif may be correlated across much of the European Variscides from the Polish Sudetes, through the Czech Republic (West Sudetes, Mariánské-Lázne, Kladská) and Germany (KTB, ZEV, Münchberg). This magmatism was generated during fragmentation of the Gondwanan margin and subsequent basin formation.
Two separate categories of mafic-volcanics are recognised. Alkali basalts and enriched tholeiites (<epsilon>Nd +2.8 to +6.3) were formed by small degrees of partial melting of an upwelling asthenosphere (primitive plume mantle). Minor crustal contamination of these mafic volcanics involving continental crust and pelagic sediment occurred. Advection of heat from mantle-derived melts resulted in crustal melting; most associated silicic magmas contain a major continental crust component (<epsilon>Nd -3.6 to -4.6). MORB-like volcanic rocks (<epsilon>Nd +7.2 to +8.7) were developed in areas that underwent more extensive lithospheric thinning and decompressional melting. True oceanic crust was developed only where extensive, prolonged rifting and magmatism occurred.
Early Palaeozoic rift-related magmatism is also known from other parts of the European Variscides, notably Iberia and the Massif Central in France. This Cambro-Ordovician event is coeval with that preceding the separation of Avalonia from the Gondwanan margin. The exact timing of the migration of these micro-continents is enigmatic but it appears to have occurred throughout much of the early Palaeozoic. This fragmentation could have resulted from (1) back-arc spreading, (2) the continental margin over-riding an oceanic spreading centre or (3) plume activity. With the possible exception of the western Alps, compelling evidence for the existence of island arcs in Lower Palaeozoic rift zones of the European Variscides is lacking and the volcanism is unlikely to have been in a back-arc basin. The association of enriched alkali basalts and MORB may be best explained by plume activity in an extensional environment, as trace element ratios (e.g. Th/Nb, Ce/Nb) of the meta-basalts are comparable to that of modern plume-generated basalts. Compositional variability of the mafic volcanics attests to the utilisation of sources of different composition and depth in the sub-continental mantle. These heterogeneities in the source area may ultimately be explained by the presence of an upwelling mantle plume experiencing episodic periods of activity. However, the plume could have been situated close to a pre-existing oceanic spreading centre that was being over-ridden by the active Gondwanan continental margin.
We propose that the break-up of the Gondwanan margin generated an archipelago of microcontinents, of which Avalonia was the first to rift away. This break-up was produced by continental margin migration across an oceanic spreading centre with associated episodic plume activity. Such a model provides the simplest mechanism for repeatedly deriving relatively small microcontinents from the Gondwanan margin during early Palaeozoic time.
We have determined the U-Pb ages of single detrital zircons from Cambrian sandstones across the Teisseyre-Tornquist Line, in order to constrain possible source areas and the docking of terranes against the East European Platform (Baltica). In addition, Cambrian faunas and their biogeographic affinities were taken into consideration when assessing the clastic provenance.The Cambrian trilobite faunas from the margin of the East European Platform (EEP) are characteristic of the Baltic province. Preliminary data from Middle Cambrian sandstones (Okuniew IG-1 borehole; c. 15 km E. of Warsaw) show the presence of sources with Mid-Early Proterozoic and Late Archean zircon ages. Additionally, subconcordant/slightly discordant analyses unequivocally indicate provenance from a source with 650-550 Ma zircons, potentially the Late Vendian volcanic rocks of the EEP (Compston et al., 1995). In the same borehole the crystalline basement has been dated at 1800+/-3 Ma. In the Lysogory Unit (NE part of the Holy Cross Mtns.), which now adjoins the EEP, the Middle to Late Cambrian trilobite and brachiopod faunas suggest affinities with Armorica and Avalonia, respectively. The Upper Cambrian sandstones contain zircons with concordant/subconcordant 207/206Pb ages of 608, 1798 and 2668 Ma. Discordant data points suggest the presence of Late and Early Proterozoic and Late Archean zircon detritus.These new data show that provenance of detrital zircon across the Teisseyre-Tornquist Line (Trans-European Suture Zone) is more complex than expected since: (1) c. 600 Ma zircon ages are not restricted to Cadomian (peri-Gondwanan) sources; (2) Terranes with Avalonian faunas, like the Lysogory, may contain detrital zircons whose ages can be matched with sources in Baltica.
Acknowledgements: This project is carried out within the PACE network of EUROPROBE
Compston et al, J. Geol. Soc. Lon, 152, 599-611, (1995).
The palaeocontinent of Avalonia rifted from the northern margin of Gondwana in the Early Ordovician and started to collide with Baltica probably in the Ashgill, subsequently creating the so-called North German-Polish Caledonides at the Trans-European Suture Zone (Cocks et al., 1997; Giese et al., 1997; McCann, 1998). The northeast margin of Avalonia is a matter of debate, but based on detailed petrographic and geochemical evidence it seems likely that it lies close to the present Caledonian Deformation Front. The island of Rügen lies just south of this front in the southern Baltic Sea and sedimentary rocks underlying the island have been penetrated by extensive drilling. The Rügen rocks range in age from the late Tremadoc probably up to the early Caradoc and they underlie undeformed sediments of a Devonian and Carboniferous age. The microfossil group Chitinozoa, discovered by Alfred Eisenack in 1930, is found in great abundances in the Lower Palaeozoic of Rügen. The usefulness of Chitinozoa for Lower Palaeozoic biostratigraphy is well-known (Miller, 1996). Like other planktonic organisms, chitinozoans appear to exhibit a degree of palaeobiogeographic differentiation into latitudinal bands, principally controlled by climate. By analyzing chitinozoan assemblages from more than 38 samples deriving from six different wells in the vicinity of Rügen (i.e. Rügen 5/66, Binz 1/73, Lohme 2/70, G14 1/86, K5 1/88 and H2 1/90), we are able to complement earlier observations concerning the age of the youngest sediments below the unconformity, the general stratigraphy and the geographic affinities of the Lower Palaeozoic rocks underlying the island. An example is the presence of Siphonochitina formosa in the Llanvirn of Lohme 2/70 and Binz 1/73 that indicates high latitude and colder water conditions for this period of Rügen sediment deposition.
The concealed Lower Palaeozoic rocks of Rügen were interpreted as deposited in the Tornquist Ocean, but hitherto, all the fossils recovered (mainly graptolites, chitinozoans and acritarchs) are planktonic and doubts have been raised wheather these organisms are useful for attributing an origin to any particular continent (e.g. Cocks et al., 1997; Verniers & Cocks, 1998). In order to circumvent this problem, we have compared the newly obtained chitinozoan data with independent provenance studies of the Rügen lower Palaeozoic sediments (see papers by Giese and co-workers). In addition, our results will also be compared with studies of the benthic faunas (Zagora, 1997).
Cocks LRM, McKerrow WS & van Staal CR, Geol. Mag, 134(5), 627-636, (1997).
Giese U, Katzung G, Walter R & Weber J, Geol. Mag, 134(5), 637-652, (1997).
McCann, T, Geol. Mag, 135(1), 129-142, (1998).
Miller, MA, Palynology: principles and applications, AASP, 1, 307-336, (1996).
Verniers, J & Cocks LRM, Schr. Stat. Mus. Min. Geol. Dres, 9, 195, (1998).
Zagora, I, NJb. Geol. Paläont. Abh., 203(3), 351-368, (1997).
Three rock associations critical for reconstructing Variscan crustal evolution are: 1) widespread HT-LP (cord-sill-K-spar, 700-800oC, ca. 4 kbar) paragneisses cut by syn- to posttectonic, mostly S-type granites (330-290 Ma); 2) felsic HP-HT (Ky-mesoperthite, 1000oC, >15 kbar) granulites, yielding consistent zircon ages 5-10 Ma older than the granites and enclosing HP-HT garnet peridotite/eclogite/pyroxenite (30-35 kbar!, 1000-1200oC) and 3) an early (pre-380 Ma and possibly as old as 480 Ma) subduction-type eclogite facies metamorphism.
The Carboniferous evolution requires a heat source at mantle depths before 340 Ma (producing HP granulite) followed by high temperatures at mid/lower crustal levels, starting around 330 Ma (to produce the granites). Detachment of lithospheric mantle, with concomitant invasion of the asthenosphere, would provide heat but would also lead to significant basalt production and underplating unless the residual lithosphere was at least 50 km thick. No seismic evidence for a thick basalt layer exists and granite geochemistry reveals only a minor mantle signature. However, if detachment occurred following collision and major lithospheric thickening, juxtaposition of >50 km thick crust and asthenosphere is a realistic possibility. In addition, the thickened crust would itself heat up due to disturbance of the distribution of internal heat sources. This scenario allows for fast heating at granulite-forming depths and slower heating in the upper/middle crust. However, as granulite-bearing units were already at shallow crustal levels to be intruded by the granites, and if the common ca. 340 Ma zircon ages record the HP event in the granulites, minimum exhumation rates of 3 mm/a are required, over an extremely large area, to juxtapose granulites and HT-LP rocks. In fact, 340 Ma ages both for spherical, metamorphic zircons in HP minerals and prismatic, magmatic zircons in LP cordierite-bearing melt pods cross-cutting the granulites points to extremely fast extraction of these rocks from deep to shallow levels. Overturn of a hot, thickened, crustal root could explain this fast exhumation and would also, in itself, produce an extra advected heat source at mid crustal levels at the right time for major granite production. Is this catastrophic crustal collapse the reason for the great width of the Variscan high grade core?
A pyropic garnetite from the Podolsko Complex, Bohemian Massif, contains relic mineral assemblage of pyrope-rich garnet, Al-poor orthopyroxene and quartz, reflecting pressures of 26/28±3 kbar at 830±30°C (Kotková et al., 1997). It represents the first definitive record of a crustal material subjected to very-high pressures and exhumed on the surface in the Moldanubian Zone. An important event on the exhumation path is marked by extensively developed MP/HT reaction textures involving Spr, Cd, Spi, OpxII and Pl, overprinting the VHP mineral assemblage. The garnetite contains two distinct zircon populations: equant, spherical grains, similar to those from granulites, and longer prismatic grains, differing from the typical granitic or rhyolitic ones by rounded edges and terminations. Single-grain U-Pb dating yielded ages of 433+27/-17 Ma, interpreted as the protolith age.
Largely euhedral monazite displays distinct zoning with low-Th cores and high-Th rims. Total-Pb electron-probe dating of the Th-rich parts yielded a weighted average age of 341±15 Ma, probably reflecting a HP/MP metamorphic event. Similar ages were recorded by monazite enclosed in garnet as well as that within the late symplectite.
The Podolsko Complex comprising mainly orthogneisses and leucocratic migmatites with relics of granulites and mantle rocks has been correlated with the Gföhl Unit. By its location close to the N limit of the South Bohemian Moldanubicum towards the Tepla-Barrandian zone, it represents the northermost extension of this unit. Present juxtaposition of the Podolsko Complex and the Monotonous Unit results from a combination of the compressional and extensional tectonic movements. Petrological and geochronological evidence imply rapid exhumation of the garnetite from depths of ca 90 km, requiring an active tectonic unroofing prior to the final collisional stages of the Variscan orogeny. MP reaction textures could result from a short-lived thermal pulse resulting from processes such as a fast extensional uplift or nappe stacking (cf. Kotková et al., 1997).
Kotková J, Harley SL, Fiera M, Eur J Mineral, 9, 1017-1033, (1997).
In the present study, Rb-Sr age determinations were carried out on biotites (n = 7) and muscovites (n = 8) from lowgrade metapelites of the northern Bavarian Forest (Moldanubian unit around Lam, Germany) in order to elucidate the Variscan cooling history of the studied rocks. The Moldanubian unit is characterized by a multistage metamorphic history. The youngest and best preserved metamorphic imprint is of the LP-HT type. The age of this stage is well documented at around 320-330 Ma by different geochronological methods. Age data corresponding to the Devonian MP metamorphism (~ 380 Ma) of the adjacent units (i.e. ZEV, Münchberg Massif, Teplá-Barrandian) are completely lacking. The isotopic record of this event, if ever present, could have been totally erased by the younger LP-HT stage. Therefore, the lowgrade metapelites from the Lam area, where the LP-HT metamorphism reaches a minimum, are the most probable rocks in the Moldanubian unit to preserve Devonian age data.From our Rb-Sr age determinations we can say: (1) The biotite ages cluster around 318 Ma, in the range of cooling ages of the Variscan LP metamorphism. (2) Compared to the biotite ages, the muscovite ages exhibit a larger scatter and a tendency to older values (327-374 Ma). (3) The oldest muscovite ages lie around 373 Ma within the range of cooling ages of the Variscan MP metamorphism. These ages provide the first geochronological evidence of a Devonian metamorphic event in the Moldanubian unit. (4) Even closely spaced samples yield variable muscovite ages.We interprete these results in the following way: All samples were affected by an Early Devonian metamorphic event. During subsequent cooling the Rb-Sr system of muscovite was closed at around 373 Ma. Following this episode, the rocks were overprinted by the Carboniferous LP metamorphism reaching temperatures in the range between the closure temperatures of muscovite (500 ± 50°C) and biotite (320 ± 40°C). Final cooling to the closure temperature of biotite took place around 318 Ma. The considerable scatter of the muscovite ages which contrasts the uniformity of the biotite ages is thought to reflect variable degrees of isotopic resetting during shear deformation below the muscovite and above the biotite closure temperature. According to this interpretation, only the oldest muscovite ages are assumed to date the original, purely thermostatic cooling through the muscovite closure temperature. Comparing our results with geochronological data from the adjacent MP units, we suggest that the Moldanubian and the MP units experienced a parallel metamorphic evolution during the Variscan orogeny. The different age patterns preserved in the Moldanubian and the MP units are purely due to the fact that the latter were sitting at a shallower crustal level in the course of the Carboniferous LP metamorphic event.
Metaperidotites from a part of narrow continental rifted domain of Cambro-Ordovician age at the NE margin of the Bohemian Massif were investigated. Here, the mafic lower crust of Cambro-Ordovician age and upper mantle were exhumed during the Variscan convergence. At the present day, the boudins of spinel metaperidotites mark a major interplate thrust boundary between the Moldanubian/Lugian orogenic root domain and the Brunian Pan-African microcontinent.
Metamorphic evolution of spinel peridotites is marked by early hydration and serpentinization followed by prograde metamorphism resulting in development of a mineral assemblage consisting of orthopyroxene - magnesiohornblende - spinel - chlorite - olivine. Increase in temperature is documented by overgrowths of higher-grade minerals over lower-grade minerals, and by prograde chemical zoning of spinel, amphibole and orthopyroxene. The maximum temperature conditions estimated on the basis of calculated reactions and conventional thermometry correspond to 700-800°C at pressure conditions below ~10 kbar.
Proposed scenario of thermal evolution and tectonic significance of studied peridotites is based on their metamorphic evolution, metamorphism, structure and zircon geochronology of surrounding rocks and thermal and rheological modelling. Upwelling of upper mantle and thinning of lower crust occured during Cambro-Ordovician rifting, and consequently both lithologies were exhumed to rather shalow level. After ~ 150 Ma of cooling at the beginning of Variscan collision, both lithologies were "cold" and very "strong". Such previously rifted domain cannot be thickened because it is stronger than the adjacent continental lithosphere. Therefore, the prograde metamorphism of partly serpentinized mantle rocks can be seen as a result of thermal effect of underplated magma, indicated at the present erosion surface by close granodioritic sill. The heat from magma was sufficient to considerably weaken thin layer of the uppermost mantle and to allow initiation of a ductile thrust in peridotites which exhumed cold and brittle part of the lower mafic crust.
The Erzgebirge is situated at the northern margin of the Bohemian Massif. It is composed mainly of crystalline units (Erzgebirge Crystalline Complex:ECC) and of low-grade Palaeozoic sequences at its margins. This region has been traditionally interpreted as a NE-SW-trending anticlinal zone with an axis plunging to the SW. The eclogite-bearing units within the ECC and the low-grade Palaeozoic sequences have been interpreted as part of the para-autochthonous Saxothuringian zone.
This study focuses on the timing of the high pressure metamorphism and the protolith age in the ECC using the Sm-Nd, the conventional and Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) zircon technique. In addition, LA-ICP-MS was used for the trace and REE study on zircon crystals as well as on glass pellets.
Zircons were extracted from an eclogite at Wolkenstein and Voigtdorf (HP unit 2, Schmädicke et al.1994), from Eppendorf and Forchheim (both HP unit 1, Schmädicke et al.(1994). Oscillatory zoning as detected by cathodoluminescence (CL) using a scanning electron microscope, demonstrate the magmatic origin of zircon from Wolkenstein. All analyses of the magmatic domains yield a 206Pb/ 238U mean SHRIMP age of 490 ± 8 Ma (95%c.l.). LA-ICP-MS analyses of the same zircons yield a 206Pb/ 238U mean age of 495 ± 8 Ma. These ages are interpreted to reflect the emplacement age of the protolith of this eclogite of Wolkenstein. Conventional U-Pb single zircon analyses for the eclogite Forchheim yield concordant data points with an age of 339 ± 1.6 Ma; the LA-ICP-MS method detected for the zircons (eclogite Forchheim and Eppendorf) concordant 206Pb/ 238U mean ages of 338 ± 5 Ma as well as 344 ± 6 Ma. All Carboniferous ages are interpreted to reflect the time of the HP-metamorphism. There is no information on the protolith age.
Sm-Nd whole rock, garnet and cpx analyses display for the eclogite Wolkenstein an age of 343 ± 1.8 Ma, for the eclogite Voigtsdorf 335 ± 4 Ma and for the eclogite Eppendorf 341 ± 2 Ma. These ages reflect the time of the Carboniferous metamorphic overprint. This is similar to Carboniferous HP-metamorphism as detected in garnet-peridotites of the Czech part of the Erzgebirge (SHRIMP U-Pb zircon), the Moldanubian Zone or at the southern rim of the Bohemian Massif in Austria.
At the present stage of investigation the existence of Cambro-Ordovician protoliths - eclogitic and granulitic rocks - at the northern margin of the Bohemian Massif seems quite common.
The Saxo-Thuringian belt on the N flank of the European Variscides resulted from SE-ward subduction of a Cambro-Ordovician rift basin under the Teplá-Barrandian (Bohemian) margin. The resulting HP rocks are now exposed in different tectonic settings, and were exhumed in different modes, developed in different thermal regimes. Eclogites contained in the tectonic klippen of Münchberg, Wildenfels, and Frankenberg originated from early Devonian (c. 400 Ma) subduction to ~70 km depth. Cooling ages in the klippen, combined with clast spectra and mineral ages in the foreland flysch record exhumation in Famennan time (Schäfer et al., 1997). The HP rocks rose in a narrow corridor along the suture zone, were retrogressed under amphibolite facies conditions, and rapidly recycled into the foreland flysch. Rocks exposed in the cores of the Saxonian Granulites and the Erzgebirge record high-grade metamorphism at c. 340 Ma, although peak pressure might be older (Kröner et al., 1998). Eclogites were subducted to 150 km depth (Massonne, 1998); HP granulites were heated to 1.050°C (Hagen et al., 1997). These rocks were emplaced immediately after peak metamorphism, under the floor of the Saxothuringian foreland basin. This is most clearly documented in the core complex of the Saxonian Granulites, in which HP granulites are juxtaposed against LP/HT rocks of the hanging wall. The interface is an extensional zone of HT simple shear, which cuts out ~30 km of crustal thickness (Kroner, 1995). It appears that granulite emplacement under the foreland crust was accomodated by extension, which permitted continued sedimentation on the hanging wall. The 340 Ma rocks do not appear in the clastic record of the flysch.Emplacement of hot, low viscosity rocks was probably driven by buoyancy and the hydraulic gradient between the crustal root to the SE and the lower crust of the foreland. Unlike the earlier HP rocks exposed in the klippen, the younger HP rocks were thermally softened, and - instead of piercing their cover - intruded into the foreland. In more internal parts of the belt (Erzgebirge), the injected high grade rocks were subsequently affected by thrusting and polyphase refolding (Nega, 1998).
Schäfer J, Neuroth H, Ahrendt H, Dörr W & Franke W, Geol Rundsch, 86, 599-611, (1997).
Kröner A, Jaeckel P, Reischmann T & Kroner U, Geol Rundsch, 86, 751-766, (1998).
Massonne H-J, Ext Abs 7th Int Kimberlite Conf , Cape Town, 552-554, (1998).
Hagen B, Rötzler J & Hoernes S, Terra Nostra, 97/5, 58-60, (1997).
Kroner U, Freiberger Forschungshefte, C 457, 1-114, (1995).
Nega M, Dissertation TU München, 1-162, (1998).
Strength profiles through the continental lithosphere show the lower crust as a particular zone of low strength and low viscosity, respectively. It has been suggested that the lower crust can behave like a fluid on a geological time scale and can flow according to pressure gradients resulting from lateral differences in lithostatic head. Such pressure gradients exist, for example, between an orogen and its foreland. In addition, the increased radiogenic heat production within the thickened crust as well as heat from external sources (e.g., delamination) will further decrease the strength of the crust in the orogenic realm with respect to the surrounding. Consequently, there usually is a spatial correlation between lower crustal flow and thickened crust as the cooler and stronger foreland crust prevents any outward movement of lower crustal rocks. Thus, in order to allow substantial foreland-directed flow of lower crustal rocks an abnormal high temperature field in the foreland reducing the strength of the crust is required. Possible geodynamic scenarios which could generate such a hot foreland crust are active mantle plumes or a tandem of parallel subduction zones in which one back-arc mantle plume heats the foreland of the adjacent collision zone.
In order to gain a quantitative insight into the processes of lower crustal flow in orogenic settings a two-dimensional thermo-mechanical finite element model was developed and applied specifically to the rapid exhumation of the Saxonian Granulites in East Germany. Modeling results show that if a favourable temperature field exists in the foreland the lower crust is indeed capable to flow over substantial horizontal distances (> 100 km). The flow pattern of the lower crust can be described by a plug flow showing pronounced high-strain zones at the boundaries to the upper crust and mantle lithosphere. This process of foreland-directed lower crustal flow could be an effective mechanism to exhume HT/HP rocks like the Saxonian Granulites in areas which never experienced much crustal thickening.
The scope of this contribution is to present the occurrence of coesite-bearing eclogites in the Eastern French Massif Central (Mt. du Lyonnais unit) and to discuss the exhumation of these very high-pressure rocks. Detailed mineralogical investigations and geochronological data allow construction of both P-T and T-time paths; the combination of both datasets suggests a depth-time path and related exhumation rates, where the change in geotherm has been taken into account during the calculation. High-pressure metamorphism (minimum 28 kbar or ca. 90 km) is constrained between 380-400 Ma. The rocks cooled below 500°C at pressures of 8-9 kbar at 360 Ma and below 350 °C at 340 Ma. Thus at 360 Ma, the rocks had been exhumed to ca. 30 km depth. These kinetic results conform with the geological constraints extracted from the tectonic and sedimentary record of the Eastern French Massif Central.
These multidisciplinary approaches provide new information on Palaeozoic orogeny and allow us to discuss the relative roles of subduction and collision in exhumation of very high pressure rocks. We suggest that a significant amount of exhumation of these rocks occurred during subduction in a back-arc environment, prior to continental collision; significantly, continental collision itself was responsible only for the final stages of exhumation of these rocks under a transpressive regime.
The Hoher-Bogen shear zone is situated in the SW-Tepla-Barrandian at the border to the Moldanubian unit (sensu strictu). Here the NNW-SSE striking West-Bohemian shear zone and the WSW-ENE striking Central Bohemian shear zone merge in a large bend. Neither ductile nor brittle faults associated with these shear zones continue to the south or west of the Hoher Bogen. The Hoher-Bogen shear zone is characterized by an up to 4 km wide zone of high temperature amphibolitic mylonites, a thin zone of ultrabasic mylonites, and rapid increase of metamorphic grade to the SW from greenschist to granulite grade. The bordering Moldanubian unit here with 450°C shows a minimum temperature of metamorphism, but calculated pressures are about 10 kb at both sides of the Hoher-Bogen shear zone. The early Variscan age of deformation in the shear zone is deduced from K/Ar hornblende cooling ages with 380 Ma on average and Rb/Sr white mica cooling ages of 375 Ma (see Ihlenfeld et al., this volume). Intrusion ages for the metabasites are Cambrian. The shear zone is mostly made up by amphibolitic mylonites with ubiquituous large plagioclases, that increasingly recrystallize into elliptic plagioclase domains. A transition from gabbroic to amphibolitic rocks, that we interprete as metagabbros, can be observed following a NE-SW profile. We measured the strain distribution in the shear zone and investigated the regional distribution of amphibole and plagioclase microstructures. High temperatures (750°-850°C) for the ductile deformation were deduced using amphibole/plagioclase equilibria and the Al-in sphene thermometer on syntectonic sphene. Strain generally increases to the Tepla-Barrandian/Moldanubian border, but with an inhomogeneous strain pattern. The increase in deformation towards the Tepla-Barrandian/Moldanubian border is contemporaneous with the regional increase in metamorphism, and increasing recrystallization of amphibole and plagioclase.The major ductile deformation occured during uplift from lower crustal depths of about 35 km during the Devonian, forming a steeply to the NNW to E dipping foliation and steep NE stretching lineation. The uplift is also shown by the regional distribution of shear sense criteria, that generally indicate uplift of the SW part. A reconstruction of a SW-NE profile through the Hoher Bogen shear zone using average strain values indicates vertical stretching of about 385%. These lower crustal ductile movements are interpreted to be the result of a Devonian SW-NE collision between the Tepla-Barrandian and Moldanubian blocks, where the relatively strong amphibolites formed an indenter at the SW-edge of the Tepla Barrandian. This indenter was pushed deep into the Moldanubian, that is folded around it, and was squeezed upwards to mid to high crustal depths together with the Moldanubian unit during the Devonian. The shape of the indenter results from the inherited geometry of the pre-Devonian plate margin.
Brittle microtectonics, the application of techniques of mesofracture analysis (Hancock 1985), is a well established approach to the solution of tectonic problems. This study will concentrate on the commonest of brittle mesofractures, joints, and attempt to characterise them in relation to their role in the tectonic evolution of the Irish Variscides. It is widely believed that joints are more reliable indicators of stress/strain trajectories in weakly deformed rocks than in fold-thrust belts and regions with higher and more complex strain histories. It is normally assumed that systematic jointing in the latter case post-dates the main compressional orogenic event. However, while systematic jointing in SW Ireland is a late stage episode in the Late Palaeozoic structural evolution of the area, it is linked in a very real way to the Variscan orogenic stress/strain regime.
A single joint set dominates the mesoscopic fracture pattern of Beara, with a modal range of 165°-180°. Photo-lineament data from the area indicate the presence of two trends, a dominant set with a modal range of 040°-070° and a minor set with a modal range of 000°-020°. The overall fracture pattern for the study area is surprisingly systematic for a fold/thrust belt setting, thereby prompting the need for a re-examination of the significance of these structures in such settings. It is argued that the dominant photolineament set represents Variscan tectonic fabric control on joint development during late stage uplift. The dominant mesofracture joint set is thought to be a consequence of tensile failure influenced by a waning Variscan stress regime, again during uplift. The minor macrofracture set post-dates the influence of the Variscan palaeostress regime and is tentatively postulated to be related to Tertiary opening of the North Atlantic
Hancock, P, Journal of Structural Geology, 7, 443-457, (1985).
The final convergence stage of the Variscan orogeny in Central Europe is characterized - among others - by HT/LP metamorphism and widespread granite intrusions. The low strength of the lithosphere provides an important constraint for the maximum crustal thickness in the Variscides and may have caused a widening of the deformation zone both perpendicular and parallel to the orogen rather than further crustal stacking. The resulting syn-convergent extension in combination with erosion can provide an effective mechanism for the rapid exhumation which is typical for several Variscan metamorphic complexes. In order to get a quantitative insight into the complex strain distribution in zones of continent-continent collision this study utilizes numerical modeling techniques and compares the results of the numerical simulations to field observations from the Variscides. The numerical simulations are based on a two-dimensional plane-strain finite element approach. The modeling concept and boundary conditions follow work of Beaumont & Quinlan (1994) who studied evolutionary models of doubly vergent compressional orogens. In addition to various parameter studies the numerical model is applied to two examples from the Variscan orogen. In each case the modeling results are compared to field data, in particular petrologic and geochronologic data as well as seismic sections. The first case study deals with the synconvergent evolution of the Black Forest (SW Germany). The second example concentrates on a NW-SE cross-section through the northern Variscan orogen and examines the effect of two collision zones and laterally varying lithospheric strength on the resulting deformation pattern. A special aspect of this model setup is the overlap of the retro- and pro-side shear zones in the area between the two collision zones.
Beaumont C & Quinlan G, Geophys. J. Int, 116, 754-783, (1994).
Southeastern sectors of the Bohemian Massif are locally transected by two generations of lamprophyric dykes which post-date internal Variscan deformation of the variably metamorphosed constituent nappe units. Dykes of the first generation trend ESE, are locally foliated and have been variably affected by low grade metamorphism. This generation is interpreted to have been emplaced during final WNW-ESE shortening of the Variscan nappe complex. A 322.5 ± 0.3 Ma (2 sigma internal error) 40Ar/39Ar biotite plateau age a representative, most external,unmetamorphosed dyke of the first generation is interpreted to reflect post-magma crystallization cooling. Most of the second generation dykes are unfoliated and unmetamorphosed. These follow a major NNE-trend, and were emplaced in all major tectonic units in central sectors of the orogen postdating regional Variscan deformation and metamorphism. Three biotite concentrates from the younger dykes record 40Ar/39Ar plateau ages of 316 - 306 Ma (2 sigma internal error). Chemical compositions of second generation dykes vary from gabbroic, potassic mafic to trachyandesitic compositions of a high-K series. They are interpreted to have originated from a crust-contaminated mantle source which likely resulted from post-collisional remelting of subducted lithosphere. Emplacement was related to late stage orogenic extension which allowed ascent of magmas generated by post-collisional remelting of subducted lithosphere or within the asthenosphere following lithospheric break-off. The new data constrain a c. 15-10 Ma interval between Variscan plate collision and subsequent extension. A similar, short duration between compression and subsequent lamprophyre emplacement has been observed in other orogens, e.g., Dabieshan. We suggest, therefore, that post-collisional lamprophyre emplecement result from similar geodynamic scenarios.
Sm-Nd isotope and geochemical data were obtained for clastic (meta)sediments from the Saxothuringian flysch basin and from possible source regions to determine the provenance of siliciclastic detritus deposited and to reconstruct aspects of the sedimentation history. Since Sm-Nd isotope systematics are in general preserved throughout diagenesis and metamorphism, Nd-model ages reflect the average crustal residence age of a sediment, e.g. the predominance of older or younger crustal materials. In combination with main and trace element geochemistry changes in source material are monitored.
Clast spectra and isotopic ages of detrital mica and zircon (Schäfer et al., 1997; Neuroth, 1997) are interpreted to document the dominant influx of debris from the southern border of the Saxothuringian basin, e.g. the Cadomian basement, from middle Cambrian until lower Carboniferous times. <epsilon>Nd for the same strata decreases from -7/-8 for the Upper Proterozoic/oldest Cambrian in the Barrandean basin to -9/-10 for the Ordovician/lower Silurian indicating the entry of higher amounts of older and highly reworked material. While the inset of sedimentation from Ordivician sources (Neuroth 1997) in both allochthonous and autochthonous sediments of lower Carboniferous age coincides with an increase of <epsilon>Nd to about -8, a further increase to about -4 in strata from the uppermost lower Carboniferous might coincides with the finding of early Variscan detrital muscovites possibly from the Bohemian Massif (Neuroth, 1997).
Episodical peak values for <epsilon>Nd of 0 to -3 in the Devonian are related to the local influx of volcaniclastic debris. Young Nd model ages (1.2-1.4 Ga) and high <epsilon>Nd (0 to -3) from the Upper Precambrian of the Tepla-Barrandean and the ZEV are to be seen in the context of other sedimentary strata in Europe of similar age and Nd systematics (Michard et al., 1985; Nägler et al., 1995; Ugidos et al., 1997) being still controversely discussed.
Michard A, Gurriet P, Soudant M & Albarède F, Geochim. Cosmochim. Acta, 49, 601-610, (1985).
Nägler TF, Schäfer H-J, & Gebauer, D, Chem. Geol, 121, 345-357, (1995).
Neuroth H, Göttinger Arb. zur Geologie und Paläontologie, 72, 143 pp, (1997).
Schäfer J, Neuroth H, Ahrendt H, Dörr W & Franke W, Geol. Rdsch, 86, 599-611, (1997).
Ugidos JM, Valladares MI, Recio C, Rogers G, Fallick AE & Stephens WE, Chem. Geol, 136, 55-70, (1997).
The Mid German Crystalline Rise (MGCR) is a crustal segment of crystalline rocks within the Saxothuringian zone of the Variscan orogenic belt. It consists predominantly of granites and a variety of metamorphic rocks. The chemical composition of the magmatic rocks is typical for a subduction-related origin. This implies, that the MGCR was a magmatic arc terrane within the Variscan orogenic belt. In this contribution we present constraints on the timing of magmatic activity and we try to quantify the arc addition and the crustal growth in the MGCR during the Variscan orogeny.
The arc activity in the MGCR was not continuous throughout the time of the Variscan orogenic cycle. Three distinct episodes of arc-related igneous activity can be identified. The first magmatic phase lasted from about 440-410 Ma. Rocks of this Silurian period are exposed in the northern part of the MGCR and in the Northern Phyllite zone. The second pulse of igneous activity lasted for about 10 Ma from 370-360 Ma. This late Devonian magmatic phase is only documented in the western part of the MGCR. The most important and wide spread magmatic episode took place in the lower Carboniferous from about 340 Ma to 320 Ma. The number of precise ages now provides a sufficient data base to identify these distinct magmatic pulses. This age distribution underlines that even in a small region such as the MGCR the Variscan orogeny was not a single event but is characterized by three distinct episodes.
The contribution of juvenile, subduction-related material is known from Cenozoic arcs to be 20-40 km3km-1Ma-1 (km3 material per km arc length and million years, Reymer & Schubert, 1984). We used the Nd and Sr isotope data of the igneous rocks to estimate the amount of reworked crust and the addition of mantle-derived material. These values and the geochronological data of the arc-related magmatic episodes were taken to calculate the arc addition rate for the MGCR. The resulting rates are 12-32 km3km-1Ma-1 for the Silurian, 32 km3km-1Ma-1 for the Devonian, and 56-80 km3km-1Ma-1 for the Carboniferous episode. The Silurian and Devonian rates are in the same range or slightly lower than the rates of modern arcs. The addition rate of the lower Carboniferous arc, however, is significantly higher than that of modern arcs. We suggest that this was triggered by a high convergence rate of the Variscan terranes during the Lower Carboniferous.
Reymer A & Schubert G, Tectonics, 3, 63-77, (1984).
Palaeontological criteria, for a longtime, favoured a Gondwanan origin for the metasedimentary units and acidic extrusive rocks of the Upper Austroalpine nappes in the Eastern Alps (Noric Terrane: Frisch and Neubauer, 1989). In the Central and Western Alps, Variscan amphibolite facies metamorphism and anatexis strongly transformed pre-existing units, but lithostratigraphic comparisons and increasing isotopic data show that a zone containing Ordovician migmatites and intrusive granitoids can be followed through the different Alpine realms. The striking resemblance of the granitoids is confirmed through geochemical data, and most carry the fingerprints of a former arc volcanic situation. A mainly twofold evolution must be envisaged for these rocks: i) Late Precambrian oceanic crust and Cambrian island arcs indicate different tectonic regimes at and off the Gondwana margin (Schaltegger et al 1997). Early Cambrian ultramafic, mafic and granitoid rocks document the general tendency of rifting from that time until the Ordovician, leading to the break-up into the so-called peri-Gondwanan microcontinents. An Ordovician, Gondwana-directed orogenic evolution is preserved in meta-eclogites, occurring together with ultramafic and metabasic rocks in the Alpine basement areas (Biino 1995, Zurbriggen et al 1997). The minimum age of such rocks is constrained by Ordovician migmatites and the intrusion of Late Ordovician granitoids. All these relics of pre-Variscan crust in the Alpine domain can be explained by a former peri-Gondwanan organization (Stampfli and Mosar, 1998).
ii) Besides traces of Devonian evolution, all domains testify the large-scale Variscan super-collision amalgamating different microcontinental blocks into Pangea. The linear distribution of Visean durbachites could be a relic of the Variscan suture, comparable to other Variscan areas (Matte 1998), and could help to find a Visean palinspastic reconstruction for the Alpine domains. Late- and post-Variscan events are characterised by regional anatexis and different pulses of magmatic rocks associated with renewed rifting.
Biino G, Eur. J. Mineral, 7, 57-70, (1995).
Frisch W & Neubauer F, Geol. Soc. Amer. Spec. Paper, 230, 91-100, (1989).
Matte P, GFF, 120, 209-222, (1998).
Schaltegger U, Nägler TF, Corfu F, Maggetti M, Galetti G & Stosch HG, Schweiz Mineral Petrogr. Mitt, 77, 337-350, (1997).
Stampfli G & Mosar J, www-sst. uni. ch, (1998).
Zurbriggen R, Franz L & Handy M, Schweiz Mineral Petrogr. Mitt, 77, 361-381, (1997).
Large-scale correlations of the metamorphic terranes across the Ibero-Armorican arc have been hampered by several difficulties. New structural and petrological data in some key areas in southern Brittany reveal striking similarities with the tectonic evolution described in the well-exposed cross-section observed in northern Galicia.
The key element is provided by the basal unit of the Champtoceaux nappe (Cellier), closely similar to the basal units of the Galician nappes (Malpica-Tuy - Santiago). Both units present the same lithology: various types of granites-rhyolites, of the same chemistry and age (about 480 Ma), basaltic to doleritic lava flows or dykes, and rare hornfelses. The metamorphic evolution is closely comparable, with an early eclogite-facies event followed by decompression at decreasing temperatures. Moreover, geochronological data show that the age of the high-pressure event is uppermost Devonian in both cases (about 360 Ma: U-Pb, Sm-Nd) and that cooling (Rb-Sr and Ar-Ar) occurred shortly after the peak P-T conditions.
The basal units are overthrusted by intermediate units consisting of mafic (amphibolites) and ultramafic rocks which could represent a former oceanic crust and are devoid of eclogite-facies relics. In the upper unit (i.e. in the Champtoceaux unit, which could be compared to the Agualada unit), a few poorly-preserved eclogites are observed in partially-melted orthogneisses. Thrusting of the upper unit is accompanied by intense ductile deformation at subsolidus conditions in the hangingwall. In the footwall, thrusting results in an inverted metamorphic gradient, where Grt-Cld assemblages are overprinted by Grt-St-Bt assemblages, with occasional Ky. Microstructural data show that the HT minerals partly overgrow the fabric associated to the thrusting. This inverted gradient is also known in Galicia. The highest unit in the nappe pile in Brittany consists of Precambrian micaschists (Mauges), and are intruded by gabbros (le Pallet). This unit could be correlated with the Ordenes schists and Monte Castelo gabbro in Galicia.
The proposed correlations permit an estimation of the amount of displacement along the South-Armorican shear zone (at least 200-300 km), and indicate a protracted history of crustal thickening up to the Devonian-Carboniferous boundary.
In SW Iberia the suture of the Variscides is exposed in the SW vergent branch of this bilateral orogen. Early obduction to N and NE, in Lower Devonian, is synchronous with subduction in the same sense and followed by SW directed continent collision between the Iberian and South Portuguese Terrane, SPT, since the Lower Carboniferous. Hot obduction emplaced the Beja-Acebuches ophiolite complex and cold obduction followed by collision emplaced ophiolite Klippe that travelled at least 50 km to NE of the suture and footwall duplexes of high pressure metamorphic rocks up to eclogite facies. Subduction and collision are oblique and induce transtension followed by transpression both in the SPT and in Ossa-Morena Zone, OMZ, the domain of the Iberian Terrane in the footwall to the suture; there is a combination of predominant orthogonal compression and subsidiary sinistral strike-slip in the NW-SE oriented structures. The most proeminent, located 60 to 120 Km NE of the curved suture, is the Tomar-Badajoz-Cordoba shear zone, reactivating an earlier Cadomian suture; it is a flower structure that separates the NE vergent structures that transport OMZ over the Centro-Iberian Zone, CIZ, and OMZ over SPT, respectively in the NE and SW branching of the Flower Structure. The SW vergent strucures of the OMZ and SPT represent a pro-wedge above the NE dipping subduction zone and the NE vergent structures of the OMZ and CIZ represent a retro-wedge in models of asymmetrical orogens modified to account for the transpressive tectonic regime. The amount of NE subduction is unknown and probably much greater than the amount of SW underthrusting of CIZ below OMZ; the last has minimum in the order of 50 Km. The deep structure of the suture and the possible thin-skinned floating character of OMZ remains to be investigated by seismic profiling, the main goal of the SW-Iberia project (EUROPROBE, ESF), with major implications in terms of Resources and Risks.
Variscan and pre-variscan orogens with a NW-SE trending are exposed in the Southwest branch of the Iberian Armorican Arc (Matte & Ribeiro 1975; Ribeiro et al. 1995). Three main tectonostratigraphic domains are exposed, from North to the South: the Central Iberian Zone (CIZ), the Ossa Morena Zone (OMZ) and the South Portuguese Zone (SPZ). Tectonic trenches parallel to the orogen trending have been originated during sinistral wrench transtensional pulses, controlling the frontiers between the tectonostratigraphic domains. These troughs correspond to half-graben structures bounded to the South by growth shear zones, with low angle dips to the North. In lower structural levels, this regional stretching tectonic style was linked to high-pressure rocks exhumation. Synorogenic flysch, diachronically filled these basins since the Precambrian times in the central parts of the orogen (CIZ), through the Lower Palaeozoic (in the OMZ), until the Upper Carboniferous (Upper Westphalian) in the forelands (SPZ).
Upper Palaeozoic basins decrease the age towards the SW according to a migration of the orogeny in the same sense. From the Middle Devonian until the Upper Carboniferous, the Iberian virgation has been induced by the counter-clockwise rotation of a kinematic indenter component (Brun & Burg 1982). The previous sinistral NE dipping accidents associated to a regional shear sense with top to the North, gradually turned the movement to the West and Southwest. Thus, the prior transtensional regime gives place to a transpressional one, favouring the orogenic shortening.
The early simple shear regime acting along the boundary between the OMZ and the SPZ, favoured the syntectonic intrusion of a mafic and ultramafic magmatic complex, the so called Beja-Acebuches Ophiolite (Quesada et al. 1994). Unless a MORB geochemical affinity has been described, this banded magmatic complex is intruded in the Lower Palaeozoic metasediments of OMZ. From the Upper Devonian onwards, this suture zone evolved to an intracontinental subduction geometry, associated to synorogenic Carboniferous flysch in the SPZ and to centrifugal vergences towards the SW, according to the thin-skinned model (Silva et al. 1990).
A sinistral transtensional-transpressional regime worked along an almost linear strike-slip orogen, probably since the Cadomian times. The diachronic succession of Palaeozoic basins controlled by a similar geometry, kinematics and dynamics, suggests the existence of a continuous process, according to an heritage of the opening and closure of the Iapetus Ocean, situated further West from the Iberia. The progressive deformation of a linear transcurrent orogen (Badham 1982), evolving to an arcuate structure coeval with foreland Carboniferous basins, is made with no turning points. Therefore, we do not recognize the existence of a major variscan ocean in SW Iberia.
Matte Ph & Ribeiro A, C. R. Ac. Sc. Paris, Sér. D, 280, 2825-2828, (1975).
Badham JP, Journ. Geol. Soc. London, 132, 493-504, (1982).
Brun J & Burg J, Earth Planet. Sci. Letters, 61, 319-332, (1982).
Silva JB, Oliveira J & Ribeiro A, Pre-Mesozoic Geology of Iberia, 348-362, (1990).
Quesada C, Fonseca P, Munhá J, Ribeiro A & Oliveira J, Bol. Geol. y Minero de España, 105-1, 3-49, (1994).
Ribeiro A, Dias R & Silva JB, Geodinamica Acta, 8-4, 173-184, (1995).
Within the Iberian Massif, the Central Iberian Zone (CIZ) and the Ossa-Morena Zone (OMZ) constitute two distinct tectonostratigraphic and metamorphic domains, along an WNW-ESE trending orogen. Different criteria can be used for the boundary characterisation between these two domains, depending upon the age of the geodynamic events.
High-grade rocks occur along the Coimbra-Cordoba Blastomylonite Zone, a major sinistral wrench shear zone (Burg 1981). The lack of consensual geochronological data (Schäfer 1990; Ordoñes-Casado 1998) supports divergent tectonic interpretations for the real significance of eclogites (Abalos et al. 1993; Azor et al. 1993) as suture zone testimonies, which have been found within high-grade gneisses. Nevertheless, this major shear zone is situated inside the OMZ in the northern parts, and it should not correspond to a Variscan boundary with the CIZ.
According to palaeogeographic criteria, the CIZ overlies the OMZ through a continuous unconformity of Ordovician (Arenigian) Armorican quartzites covering Upper Proterozoic units of the OMZ. This North dipping stratigraphic contact could be more acceptable for the boundary between OMZ and CIZ, but its pattern has been modified by the Variscan deformation.
During Upper Palaeozoic times, the transpressional mechanisms linked to the Iberian Armorican Arc virgation, generated a dissymmetric flower structure in the northern parts of OMZ. Due to a North directed vergence in the northern branch of this flower structure, some OMZ units, tectonically overlie the CIZ. Coalescent flower structures at various scales have been formed, bounded by scissors-type faults (Schreurs & Colleta 1998) with opposing vergence. Left-lateral strike-slip and thrust fault systems, controlled the horizontal and vertical growth of the flower structures, and such major planar structures show variation of geometric and kinematic features along the orogenic trending. The previous Ordovician unconformity works during the Variscan times, as a thrust contact with shear sense to the North.
A distinct Lower Palaeozoic stratigraphic sequence and a late Proterozoic basement with OMZ affinities, record a polycyclic geodynamic history. Upper Palaeozoic geodynamic events developed under low to high-grade metamorphic conditions transpose the former Cadomian evolution. The exposure of metamorphic rocks of different grades along different shear zones is related to a variable amount of movement along major fault boundaries in different times. Thus, the OMZ / CIZ boundary worked as a continental transpressional zone with major flower structures transposing a continuous history, from the Proterozoic through the Upper Palaeozoic times.
Abalos B, Ibarguchi G & Eguiluz L, Tectonophysics, 217, 347-353, (1993).
Azor A, Lodeiro FG & Simancas JF, Tectonophysics, 217, 343-346, (1993).
Burg J, Iglesias M, Laurent P, Matte Ph & Ribeiro A, Tectonophysics, 76, 161-177, (1981).
Ordoñes-Casado B, Ph D, ETH, Zurich, 12940, (1998).
Schafer HJ, Ph D, ETH, Zurich, 9246, (1990).
Schreus G, Colletta B, Continental Transpressional & Transtensional Tectonics, Geol. Soc. Special Publ., 135, 56-79, (1998).
The southern border of the Ossa-Morena Zone displays a relatively complex fracture network that affects all geologic and lithostratigraphic units belonging to three main domains: the Autochthonous Iberian Terrane, the oceanic exotic terrane known as the Beja-Acebuches Ophiolite Complex and a calc-alkaline intrusive complex - the Beja Igneous Complex. The fracture network comprises mainly four different systems: 1) NNE-SSW to NE-SW left-lateral strike-slip faults, represented by several major accidents (e.g. the Messejana Fault), whose development is usually ascribable to the late-variscan stress-field; 2) N-S to NW-SE right-lateral faults, generally of minor extension and commonly interpreted as the conjugate system of the prevailing structures referred to in the previous item; 3) ENE-WSW left-lateral, brittle shear zones (e.g. the Ficalho Fault) that form complex structural arrays with the structures mentioned below; and 4) major E-W to WNW-ESE shear corridors of predominant sinistral kinematics, generated during the variscan collisional events (under a left-lateral transpressional deformation regime) between the Iberian and the South Portuguese Terranes (Quesada et al., 1994; Fonseca, 1995) and object of late, polyphasic reactivation.The Ferreira-Ficalho Thrust, which presently forms the northern border of the South Portuguese Terranes, is one of the most important tectonic accidents that belong to the last mentioned fracture system. In fact, there are many subparallel, E-W to WNW-ESE shear corridors that can be followed for several tens of kilometres and whose geological importance is noteworthy, since they often mark the main contacts between all the domains, destroying the initial geometric relationships, and being also responsible for the strong tectonic dismembering of the ophiolite sequence. Structural and geological mapping together with the available magnetic and gravimetric surveys confirm the lateral continuity of the shear zones and their importance in the regional structural arrangement of the OMZ southern border.According to the proposed geodynamic models (Quesada et al., 1994, Fonseca, 1995), the structural/metamorphic evolution of the OMZ southern border can be generally envisaged as a continuous succession of phenomena developed during three main variscan deformation phases; the genesis of the major shears took place during the late stages of continental collision.
(*) Contribution from MIZOMOR Project (PBICT/P/CTA/2112/95), JNICT-Portugal
Fonseca PE, PhD Thesis University of Lisbon, (1995).
Quesada C, Fonseca PE, Munhá J, Oliveira JT & Ribeiro A, Bol. Geológico y Minero, 105-1, 4-39, (1994).
In SW Iberia (Ossa Morena Zone) the Variscan tectonics of Alvito-Viana do Alentejo's critical sector is dominated by two main deformation events afecting several tectonostratigraphic units: late Proterozoic Série Negra's metassediments; (supposed) early Cambrian marbles; Silurian (?) Xisto de Moura's metassediments and imbricated felsites and metabasites (including eclogites) of unknown age. The early (D1) deformation event is characterised by a pervasive planar and linear fabric imprinted at mesoscopic and microscopic scales. Sin-D1 cleavage (S1) shows a variable geometry on top of which a roughly N-S trending stretching lineation is recognisable. At mesoscopic scale sigmoidal and delta shaped rigid mafic 'boudins', embedded in a more ductile marble matrix, indicate a top to the North (NNW, sometimes NNE) sense of shear. In Série Negra's metassediments (e. g. garnet mica schists) thin sections show helicitic inclusions in rotated garnet porphyroclasts confirming, among other kinematic shear criteria, a northwards sense of shear for the early, D1, Variscan structures. At a regional scale the second deformation event (D2) overprints D1 structures by folding S1 according to a N-S trending antiform macrostructure. Its geometry is characterised by an eastern normal long limb (N-S, East deeping) and by a western, locally inverted, short limb (N-S, subvertical), thus geometrically defining a West verging structure. At chosen outcrops it is also possible to recognise development of sin-D2 axial plane cleavage (S2) cutting S1.Mesoscopic structures related with D1 and D2 geometric interference are well observed in the Marbles' unit near Viana do Alentejo. The D2 refolding of sin-D1 structures originates a geometric pattern, mainly characterised by a complex set of folds with vertical axis. Some of these structures are interpreted as a consequence of possible diachronic actuation of the two main Variscan deformation events (D1 and D2). D1 and D2 are also responsible for the final geometry of the possible (originally sin-D1) westwards thrusting of the Série Negra's late Proterozoic metassediments over the Cambrian (?) marbles. Although true nature of this contact is still understudy, N-S sinform structures comprising Série Negra lithologies on top of the marbles allow us to speculate about the existence of klippen associated to this possible thrust.
Acknowledgements F. Rosas benefits of a PhD scholarship (PRAXIS XXI/BD/9220/96) granted by FCT (Fundação para a Ciência e Tecnologia).
Despite the great effort that has been drawn to the tectonic, geochemical and geochronological study of the Variscan Orogen in Europe, there is still great controversy as to the geodynamic setting (including root and suture zones) and age of its high-grade polymetamorphic Terranes. Recent tectonic and isotopic data on the Bragança CAT suggest a polycyclical evolution for this Terrane: Grenvillian (c. 1.05 Ga) HP/HT granulites overlying Panafrican (> c. 510 Ma) paragneisses and eclogites, late Panafrican to early Variscan (c. 545-470 Ma) mafic/ultramafic intrusions, and Variscan (c. 395 Ma) nappe emplacement. Available geochemical data, together with relative positions and ages, suggest that a deep seated subduction and suture zone is still preserved in the CAT, with Panafrican eclogites and metasediments subducted below a Grenvillian lower continental margin or mature island arc. Intrusives show a continental affinity, and are interpreted as late Panafrican underplating and/or early Variscan deep seated continental rifting.
We now propose that the root zone of the Bragança CAT (and other similar CAT in NW Iberia) can be found in the deep portion of the Avalon Terrane, suggested by: 1. palaeogeographic reconstructions of the Permian Pangaea, in which the Iberian Peninsula lies adjacent to Avalon (Van der Voo et al. 1984); 2. the Cadomian suture preserved in the Bragança CAT, and the similar geodynamic evolution recorded by both terranes (subduction and underplating under Grenvillian continental crust - Keppie et al. 1991; Marques et al. 1996). Similarity between CAT and Avalon could also reach upper crustal levels; for instance, the Lagoa orthogneisses of the Morais CAT in NE Portugal could be related with the upper Proterozoic volcanism of Avalon, and the mafic dykes that intrude them related with continental rifting in upper Proterozoic/lower Cambrian of Avalon.
From the above, it seems that we presently see at surface in the CAT, what is deep seated in the Avalon Terrane.
Keppie JD, Nance RD, Murphy JB & Dostal J, The West African Orogens and Circum-Atlantic Correlatives, RD Dallmeyer and JP Lécorché (eds), 315-333, (1991).
Marques FO, Ribeiro A & Munhá JM, Tectonics, 15, 747-762, (1996).
Van der Voo R, Peinado J & Scotese CR, Plate reconstructions from Paleozoic Paleomagnetism, R. Van der Voo et al. (eds), Am. Geophys. Union. Geodyn. Ser, 12, 11-26, (1984).
Recent structural studies of Variscan plutons in the Central Pyrenees have suggested that the Axial Zone of the Pyrenees acted as a large scale transpressional ductile dextral shear zone during the main (D2) Variscan deformation phase. Several of these plutons which intruded early during D2 exhibit an asymmetrical geometry similar to <sigma>-type porphyroclasts. As such they were used as kinematic indicators. In order to test this hypothesis a detailed regional structural analysis of the Palaeozoic metasediments which comprise the "matrix" of the postulated shear zone is being undertaken. First preliminary results from field studies are presented.
The study area is located in the French and Andorran Pyrenees, extending 70 km along the E-W strike of the Axial zone and 25 km across strike, covering the area west of the Bassiès Pluton to the east of the Hospitalet Dome. Near the contact with the Bassiès Pluton and along the southern margin of the Hospitalet Dome the metasediments have experienced sub-greenschist to greenschist grade regional metamorphism. Foliation generally dips steeply to the north and northwest and parallels the exposed contact of the plutons. Several kilometers southeast of the Bassiès pluton regional metamorphism reached amphibolite grade. In the higher grade rocks orientation of schistosity is less homogeneous but generally dips moderately northerly. Mineral stretching lineation is prominent and plunges shallowly to the WNW and ESE. Two younger generations of nearly vertical plunging crenulation fold axes can be distinguished.
Macroscopic kinematic indicators are scarce. In the low grade slates these are predominantly boudinaged crosscutting quartz veins and C'-type shear bands. Higher grade schists have experienced higher strain, evident from the mylonitic microfabric and -shaped quartz boudins. Prominent features of the schists are centimeter long andalusite porphyroclasts. The andalusite crystals appear to have grown in the early stages of D2-deformation and are not aligned with their length axis parallel to the mineral stretching lineation. In addition to rotation within the plane of schistosity there appears to be a rotational component perpendicular to it. The kinematic behaviour of these clasts is the subject of current studies. First results from kinematic analysis indicate a top to the southeast sense of shear. This is in concordance with transpressive ductile shear with a dextral strike-slip component as postulated by workers studying the Variscan plutons. More detailed results of the ongoing study are necessary though to substantiate the existence of the proposed Variscan mega-shear zone in the Axial Zone of the Pyrenees.
Multi-chronometric ages and Sr-Nd isotopic data of eclogites and granitic rocks place strong constraints to the tectonic evolution of the Dabieshan UHP terrane in central China. In the Southern Dabie UHP Terrane (SDT), comparable age patterns (zircon U-Pb, garnet Sm-Nd, mica Rb-Sr, mica Ar-Ar) of UHP eclogites and country gneisses indicate their "in-situ" tectonic relationship (see also Chavagnac et al., EUG-10). The age of UHP metamorphism is estimated at ~ 220 Ma based on the new ion probe zircon U-Pb and Sm-Nd isochron ages of eclogites from Bixiling and Maowu (see also Rowley et al., 1997).
In the Northern Dabie Complex (NDC), zircon U-Pb ages obtained for homogeneous and gneissic granitoids range from 138-125 Ma (Xue et al., 1997; Hacker et al., 1998; this paper). This period corresponds to the most significant post-collisional thermal event in Dabieshan, but conspicuously not recorded in UHP rocks of the SDT.
Rapid cooling, hence rapid exhumation, is recorded by a variety of chronometers (hb Ar-Ar, bio Ar-Ar, bio Rb-Sr, K-spar Ar-Ar, and apatite FT) in UHP eclogites and gneisses of the SDT as well as in post-tectonic granitic and mafic intrusions of the NDC.
Sr-Nd isotope tracer study clearly established that the Cretaceous granitoids of the NDC, characterised by ISr of 0.709-0.710 and <epsilon>Nd(T) of -15 to -20, were derived by partial melting of the lower-intermediate crust of probably Mesoproterozoic ages (TDM = 1.7-2.2 Ga). Thus, the NDC cannot be a newly created magmatic complex as might be indicated by the dominantly young zircon ages (Hacker et al., 1998). The gneisses from the NDC have similar isotopic characteristics (<epsilon>Nd(T) = -16 to -22) as the Cretaceous granitic plutons. New zircon U-Pb analyses of a granitic gneiss from Lannaio yielded a Proterozoic age of ca. 1.8 Ga. By contrast, most granitic gneisses from the SDT have higher <epsilon>Nd(T) values of -2 to -10, except those of the Shuanghe locality. This implies a drastic difference in the origin of the two terranes in addition to their constrasting thermal and subduction/exhumation histories.
Hacker BR, Ratschbacher L, Webb L, Ireland T, Walker D, Dong S, E. P. S. L, 161, 215-230, (1998).
Rowley DB, Xue F, Tucker RD, Peng ZX, Baker J, Davis A, E. P. S. L, 151, 191-203, (1997).
Xue F, Rowley DB, Tucker RD, Peng ZX, J. Geology, 105, 744-753, (1997).
Chavagnac V, Jahn BM, Villa IM, Whitehouse MJ, J. Conf. Abs., 4 (1999)
The modelling presented here is based on the extensive structural geological and geochronological studies conducted in the West Lachlan Orogen over the past decade (see Gray, 1997; Foster et al., 1998 for reviews). The goal of this work is to present a detailed quantitative analysis of the deformation, fluid flow and thermal transport associated with the evolution of this orogen from the late Cambrian (approximately 510 Ma) to late-Devonian (approximately 376 Ma), a period of ca. 130 million years. The ultimate purpose of the work is to gain a quantitative understanding of the localization of the Stawell-Ballarat-Bendigo gold mineralization.
Modern seismic studies indicate a crustal structure consisting of a lower crust extending from ca. 18 km to ca. 36 km of unknown composition but here assumed to be comprised of layered mafic/felsic granulites. A detachment at ca. 17 km is proposed as the base of an approximately 10 km thick imbricate stack of mafic volcanics. Overlying this imbricate stack is an approximately 8 km thick package of folded Ordovician sediments. During the peak of metamorphism this package of folded Ordovician sediments could have reached a thickness of 25 km prior to subsequent erosion. The total crustal thickness now is ca. 36 km.
Sedimentation, deformation, metamorphism and plutonism evolves from West to East across the presently exposed orogen extending from the edge of the Stawell Zone in the West to the edge of the Melbourne Zone in the East; a distance of ca. 325 km. Deformation begins at ca. 450 Ma in the West and extends through to ca. 380 Ma in the East. Plutonism occurs in the period 410-390 Ma predominantly in the Stawell Zone and west part of the Bendigo-Ballarat Zone and extends to 380-360 Ma in the Bendigo-Ballarat Zone and in the Melbourne Zone.
The starting point for modelling involves placing constraints on the thermal/plutonic history. We assume, as a base level scenario, that the Ordovician sediments have a composition similar to those of the Bega-Kosciusko region (B. Chappell, private communication), namely, 3.54% K2O, 16.9 ppm Th, and 3.5 ppm U. In the Cambrian/Ordovician this composition is equivalent to a heat production rate of ca. 3 microWatts/cubic metre. We assume the Ordovician/Silurian sediments to have had this internal heat production rate uniformly with depth. In order to be compatible with modern heat flow measurements, this then constrains the internal heat production rate of the mafic volcanics and the lower crust to be, on average, ca. 1 microWatt/cubic metre (assuming a modern heat flux at the Moho of 20 milliWatts/square metre).
These assumptions lead to temperatures at the Moho in the Cambrian /Ordovician of ca. 1000°C for a Moho heat flux of 30 milliWatts/square metre and ca. 820°C for a Moho heat flux of 20 milliWatts/square metre. Thus, it seems that the granites typical of the West Lachlan Orogen can be generated simply by thickening of the crust by folding and imbricate stacking without calling upon ad hoc processes (accompanied by high heat flow) in the mantle. We also explore the time lag between deformation and plutonism (ca. 50 million years) and show that this is compatible with melting arising from the progressive Eastward evolution of tectonic thickening.
The orogen is modelled as an Eastward thinning wedge of Mohr-Coulomb, dilating material underlain by a lower-crust and mantle which has a non-linear viscous response. The material erodes as it thickens due to deformation and the resultant material propagates the wedge eastwards. Fluid flow in the wedge is coupled to the deformation through deformation induced porosity (and hence, permeability). The resultant fluid flow takes place via the porosity wave mechanisms proposed in Hobbs et al. (this volume).
Several boundary conditions are explored including a basal shear stress condition that simulates the movement of a subduction zone. In each scenario, dilatant shear zones develop which control the localization of fluid flow. We explore the controls on the timing and spacing of these zones and the conditions that result in focussing of fluid flow into regions such as Stawell and Bendigo-Ballarat.
The linking of thermal history and fluid flow enables regions of devolatilization to be delineated in the lower crust together with the plumbing systems that enable these fluids to be transferred to the upper crust.
This modelling enables an holistic, quantitative model for the sedimentation/deformation/plutonism/fluid flow/mineralization process to be developed.
Foster, D.A., Gray, D.R., Kwak, T.A.P. and Bucher, M., Ore Geology Reviews, 13, 229-250, (1998).
Gray, D.R., In: Orogeny Through Time. Burg, J.-P. and Ford, M. (eds). Geological Society Special Publication, 121, 149-177, (1997).
Data concerning provenance and depositional area of Lower Ordovician sedimentary rocks in the Southern Puna was obtained by sediment petrographical and geochemical analyses. These rocks are associated with mafic and ultra-mafic magmatic rocks and intermediate lava flows. This association have been interpreted as an ophiolite sequence which formed during the amalgamation of the Puna Terrane and Gondwana iniciated by the collision of the exotic Arequipa-Antofalla Terrane in Upper Ordovician times. In the Southern Puna crop out Tremadocian quartz-rich turbidites (Tolar Chico Formation), overlain by volcaniclastic rocks of Tremadoc-Arenig age (Tolillar Formation) as indicated by Araneograptus murrayi. These rocks are tectonically associated with mafic to ultramafic bodies. The overlying volcaniclastic rocks are characterised by a higher amount of volcanic components (Diablo Formation) and are intercalated with synsedimentary lava flows. All units were deformed in the Oclóyic Orogeny (Upper Ordovician), with isoclinal folding verging to the west. According to component relations, the quartz arenites are classed as having a "continental block" provenance while the overlying volcaniclastic rocks show an increase in volca-ni-genic debris and have a "dissected arc" to "transitional arc" signature. Data on illite crystallinity show that the studied rocks experienced very low grade metamorphism (anchimetamorphic zone) which can be correlated to pumpellyite-prehnite mineral facies conditions. The average CIA value of the sedimentary rocks is 66. Trace element and REE ratios show that the quartz arenites have a "rifted margin" signature, which is interpreted as a result of reworking. From base to top, the geochemical features of the volcaniclastic rocks indicate a change of the provenance from active continental margin to continental arc sources. The REE patterns of the samples show affinities to the PAAS standard and their characteristics are correlatable to felsic and intermediate source rock compositions. Ratios of REE and HFSE show average values of the upper continental crust. The <epsilon>Nd and the TDM values point to variable provenances for the different units. The Formation Tolar Chico has TDMs of 1.9 Ga and <epsilon>Nd(t) of -9 and imply a mesoproterozoic basement source. The Formation Tolillar has TDMs of 1.5-1.6 Ga and <epsilon>Nd(t) of -5 which are similar to the basement values of the Late Proterozoic Sierras Pampeanas Terrane. The Formation Diablo contains the youngest TDMs of 1.2-1.3 Ga and <epsilon>Nd(t) of -1.9. We interprete the whole sedimentary succession as recycled from older continental crust with a slightly primitive input from a Lower Ordovician volcanic arc (Sierra Famatina and Faja Eruptiva de la Puna Occidental) which evolved on continental crust.
The Early Paleozoic Ross Orogen is well exposed in the Transantarctic Mountains over a distance of about 3000 km. Matching counterparts are preserved along strike in Australia (Lachlan Fold Belt) and South America (Sierras Pampeanas). In the Transantarctic Mountains, there is ample evidence for an active margin setting of the orogen, such as subduction-related plutonism, inboard/outboard polarity, high pressure metamorphism, ophiolites, oceanic segments and accreted terranes. The main common feature of the orogen is its magmatic arc, comprising large calc-alkaline granitoid bodies. The subduction-related intrusive activity had its peak at around 500 Ma.
In the Ross Sea sector of the Ross Orogen, a major suture has been identified during joint German-Italian field investigations in North Victoria Land. It separates the inboard granitic arc, hosted in medium to high-grade (low-pressure) metamorphic rocks, from two outboard sedimentary terranes of Cambro-Ordovician age. The inner terrane consists of volcanic rocks of primitive island arc signature, overlain by a clastic to conglomeratic sequence containing exotic limestone blocks and this in turn covered by a thick, deltaic to fluviatile, quartzitic sandstone sequence. The outer terrane comprises a very thick, regularly folded turbidite sequence.
There are controversial opinions about the nature of these two terranes: (1) They have been explained as exotic terranes, derived from a far oceanic realm or an opposite passive margin, but (2) they could also represent a volcano-sedimentary accretionary wedge and a turbiditic forearc basin, both formed at the margin itself.
The suture between the arc and the outboard terranes is characterised by the occurrence of: a small belt of medium to high pressure metamorphic rocks, lenses of ultramafic rocks in the form of cumulates, layered gabbros and eclogites, an overprint of greenschist metamorphism, retrograde on the arc side and prograde on the terrane side, a system of thrusts carrying the three major tectonic units outwards, the inboard units on top of the outboard ones.
The distribution of arc granites very close to the suture suggests that the subduction zone has jumped outwards at least once during the accretion of the terranes.
There is no evidence for a later continent/continent collision and therefore the accretionary orogen is well preserved. In principal, the Early Paleozoic Ross Orogen shows the same active margin setting between Gondwana and Proto-Pacific as the later Andean Orogen between the South American craton and the recent Pacific. Wilson cycles seem to be absent in this part of the globe for the whole Phanerozoic period of time.
The Southern Urals provide the unique possibility to correlate the PTt evolution of a high-pressure complex with the geodynamic history of a Paleozoic arc-continent collision. The high-pressure Maksyutov Complex occurs within, and flanks the hinterland side of an accretionary complex related to the arc-continent collision. The HP rocks are juxtaposed against the very low-grade Magnitogorsk arc in the east along the east-dipping arc-continent suture (Main Uralian Fault). The Maksyutov Complex consists of two distinct units; the lower, Unit 1, represents subducted continental crust of the East European Craton that experienced peak metamorphic conditions of 20 kbar and 550°C, whereas the upper unit, Unit 2 contains relicts of oceanic crust that experienced lower PT-conditions (P<10 kbar, T<400°C). The early convergent evolution of the Magnitogorsk arc and the East European Craton is recorded by the Emsian-age Baimak-Buribai arc-tholeiites that locally contain boninitic lavas, dikes and sills that are typical of a forearc region. From the Emsian through to the Givetian predominantly calc-alkaline volcanism records mature island arc evolution. The onset of dacitic volcanism in the Givetian may indicate the arrival of the passive margin of East European Craton in the subduction zone. Strong evidence for metamorphism of East European continental crust at eclogite facies conditions in the Givetian is provided by isotopic ages from eclogites/blueschists from Unit 1 of the Maksutov Complex. With the entry of the continental crust into the subduction zone, volcanism waned and stopped and the subsequent history of the arc-continent collision is recorded in the forearc basin sediments. These Givetian to earliest Tournaisian-aged volcanoclastic sediments indicate significant uplift and erosion of the arc edifice that resulted in breaching of the outer arc high and deposition of arc-derived sediments across the subducting East European Craton to form an accretionary complex. During this phase in the collision history, buoyancy-driven exhumation of Unit 1 of the Maksutov Complex led to its juxtaposition against Unit 2 as indicated by Frasnian to Fammennian Ar-Ar and Rb-Sr ages of retrograde shear zones. At the same time, Paleozoic sediments of the East European Craton were being offscraped from the downgoing continental crust to form the Suvanyak Complex and underplated at the base of the growing accretionary complex. By the Tournaisian the arc-continent collision appears to have stopped and shallow water carbonates were deposited on top of the mildly deformed arc. Exhumation of the Maksutovo complex appears to have continued, however, until the Visean when it was emplaced in its current position within the accretionary complex.
The south Urals foreland thrust and fold belt is a west-verging, basement involved thrust stack that records a long tectonic history spanning much of the Precambrian and into the Paleozoic. Extensive involvement of the Precmabrian basement resulted in reactivation of pre-Uralide structures and the inclusion of crystalline thrust sheets in the Uralide thrust belt. Recognition of these different tectonic events and the structures they produced is of importance for the interpretation of reflection seismic profiles through the area. For example, the Uralide reactivation of the Zilmerdak thrust and the Zuratkul fault make if difficult to interpret reflection seismic fabrics in thrust sheets bound by these faults in the context of the Uralide deformation. In the URSEIS vibroseis profile the Zuratkul fault is imaged as a zone of moderately east-dipping reflectivity that can be trace into the middle, and possibly lower crust. Its hangingwall consists of bright, concave-downward to steeply inclined reflectivity that corresponds to the Uraltau antiform. The Zilmerdak thrust is imaged as a zone of moderately west-dipping reflectivity that can be traced to mid-crustal levels, and which appears to merge downward into a zone of gently east-dipping diffuse reflectivity that may corresponds to the basal thrust. Understanding that these thrust sheets and their bounding faults underwent a long and complex deformation history prior to the Uralide event, and that Uralide reactivation resulted predominantly in uplift without any thermal overprint provides constraints for interpreting the seismic reflection fabric. For example, the features imaged within these thrust sheets, such as the bright, concave downward reflectors in hangingwall to the Zuratkul fault, are related to pre-Uralide features in Precambrian basement, and not to Uralide accretion of high pressure rocks. Such an interpretation can lead to a very different architectural and kinematic picture for the thrust belt.
The Main Uralian Fault has been regarded as a 2500 km long, roughly linear feature that represents the suture of the arc-continent collision that occurred between the East European Craton and a series of island arcs. Some of its characteristics, however, suggest that it is not a single feature, but rather that it has a different kinematic, mechanical and tectonic history along strike. A comparison between vertical incidence reflection seismic data from the Middle (ESRU93,95 & 96) and the Southern Urals (URSEIS & R114) indicates, for example, that there are remarkable differences in upper crustal reflectivity in the area of the Main Uralian Fault. In the Southern Urals, it appears as an east dipping, weakly reflective event that corresponds to the change in the reflectivity pattern between the East European Craton to the west and the Magnitogorsk volcanic arc to the east. On the contrary, in the Middle Urals it appears as a reflective package that, upon migration, transforms into two different sets of east dipping reflections. The reflective pattern of the Main Uralian Fault in the Middle Urals leads us to suggest that it does not represent the same arc-continent suture as in the Southern Urals. The geometry of the large scale shear zones in the Middle Urals implies that oblique convergence occurred late in the tectonic history of this area, and possibly reworked the original arc-continent suture, significantly overprinting, or even destroying its original signature. In addittion, late extension related to the opening of the West Siberian Basin may have significantly affected the arc-continent collision in the Middle Urals.
The Ural orogen is considered an example of an incomplete "arrested" collision, where thick, cold and rigid crust of the East European Platform collided obliquely with a set of microcontinental and island arcs terranes. Unlike other Palaeozoic orogenic belts the Ural escaped post-ororgenic collapse and extension, and resembles a well preserved orogen since the late Palaeozoic. (Echtler et al., 1996)
Two major phases of deformation for the Uralian orogeny in the SW-Urals have been identified and have to be considered in balanced reconstruction and kinematic restoration: (a) an accretionary phase during the Middle to Late Devonian with obduction and development of an accretionary wedge. During this stage the predominant tectonic transport was directed to the north-west; (b) a later, east-west directed compressional phase with development of a westerly propagating fold and thrust belt and the pre-Uralian foredeep on the East European Platform. To account for this twofold tectonic transport a 3-dimensional balancing model, using 3D-Move software (Midland Valley), is applied in this study.
The Bashkirian Mega-Anticlinorium (BMA) was formerly interpreted as a wide antiformal structure. It consists of up to 18 km of dominantly terrigeneous and carbonaceous sediments of the Riphaen and Vendian. These are in part unconformably overlain by Ordovician sediments in the east and Devonian to Permian sediments on the western flank. The BMA is structurally subdivided in several, large scale antiforms and synforms, separated by major east dipping thrusts.
Along three traverses across the BMA (one subparallel the URSEIS95 section, the other approx. 50 and 100 km further north) a detailed structural inventory has been accomplished, accompanied by investigations of illite christallinity, conodont alteration and thermochronological data. These data reveal no significant internal deformation and only a diagenetic-anchizonal overprint in the western part of the fold and thrust belt (Giese et al., in press). Consequently, considerable volume changes are not to be expected in this area. A first 3-dimensional model will therefore focus on this western part of the fold-and-thrust-belt. The URSEIS95 seismic section, digitised topographic and geological map data sets and well data will be incorporated in the model.
Echtler HP, Stiller M, Steinhoff F, Krawczyk C, Berzin R, Suleimanov A, Spridonov V, Knapp JH, Yunusov N, Menshikov Y & Avarez-Marron J, Science, 274, 224-226, (1996).
Giese U, Glasmacher U, Kozlov V, Matenaar I, Puchkov V, Stroink L, Bauer W, Ladage S & Walter R, Geologische Rundschau, in press
A multidisciplinary approach was applied to quantify the thermal history of Precambrian and Paleozoic strata along a NW-SE transect 60 km north of the URSEIS ´95-Transect in the western fold-and-thrust belt of the southern Ural, Russia. Also the metamorphic complex of Beloretzk and the Tirlyan Synform were included. The transect extends from Devonian to Permian sedimentary units of the Pre-Uralian foredeep, crossing the Precambrian siliciclastic and carbonate strata of the Bashkirian Mega-Anticlinorium and the metamorphic complex of Beloretzk (MCB) into the Paleozoic units of the Zilair Synclinorium. The section was subdivided into several structural units which show different tectonic inventories and structural histories and are separated by thrusts or faults. Amphibole (718±5 Ma), biotite (»480 Ma) and muscovite (about 550±5 Ma) concentrates of metamorphic rocks were analyzed with the 40Ar/39Ar step heating technique to quantify the upper temperature segment of the cooling path of the MCB. Microcline of granitic and syenitic pebbles of Vendian conglomerates (530 - 550 Ma) and orthoclase pebbles of Upper Riphean conglomerates (910 - 950 Ma) in the western fold-and-thrust belt clearly indicate the absence of a thermal overprint above 200°C during the Uralian orogeny. To quantify the low temperature tail of the cooling history the fission-track technique was applied on apatite grains. These grains were obtained by a narrow sampling (4 km spacing) of the Paleozoic and Precambrian sandstones and magmatic rocks along the 120 km transect. Fission track ages and track length distribution indicate a complex cooling and exhumation history in Upper Paleozoic and Mesozoic time. All thermochronological data will be discussed in relation to the structural evolution of the western fold-and-thrust belt of the southern Urals. Also apatite fission-track data from Devonian and Vendian sedimentary units which give hints to the exhumation history of the source area (eastern Neoproterozoic orogen) will be presented.
The data, which were obtained during the investigation of pre-Mesozoic formations in outcrops (Inner Carpathians), boreholes (Transcarpathian depression, Carpathian foredeep and adjacent areas of the platform) and rock fragments in flysch and molasses (Outer Carpathians) are composed the factual basis for the reconstruction of pre-Alpine tectonic events on the territory of the Ukrainian Carpathians and adjacent regions. Combined analysis of these data have allowed to determine the character features of pre-Mesozoic formations, their type and tectonic setting (eugeosynclinal, miogeosynclinal, orogenic, platform), to make correlation between regions, to reconstruct the final structure of the Earth crust of this territory and stages of its formation, and to establish the main borders of these stages, on which folding, metamorphism and granitization of different intensity, uplifting of the territory, denudation, essential and, sometimes, complete rebuilding of the structure, replacement of one formations by another, in particular - of geosynclinal formations by orogenic and later, in some places (Southern Poland), by platform formations, were taken place. These borders were connected with the boundary between the Riphean and Vend - Baikalian (Assinian) folding, the end of the Silurian - beginning of the Devonian - Late Caledonian folding (Ardennian -Zigerland phases) and boundary between Early and Late Carboniferous - Hercynian folding (Sudetic phase). Beside these main borders, on the considered territory another tectonic phases are also clearly observed, such as: Sardian (Early Ordovician), Taconic (Late Ordovician), Uralian (between Ordovician and Permian) and Pfalzian (between Permian and Triassic). As a result of Hercynian tectonic events all ancient complexes, independently from their previous history, were juxtaposed in the united mountain and folding construction. In foredeep of this construction (at present time buried under the Carpathians) and in all other subsidence areas of relief, the molasses of the Upper Carboniferous and Permian were accumulated. Thus, the pre-Alpine history of the Carpathians was finished by creation on this territory of the crust of a continental type, which, at the beginning of the Alpine stage, was undergone the extension and breaking, that caused, in some places, complete destruction of the granite-metamorphic layer. So, the Alpine geosyncline is not inherited from previous stages, but a new formed. All mentioned above pre-Alpine events, have a good correlation on the pracarpathian territory, as well as on the south-west margin of the East-European platform. The comparison with a nearest fragment of the Gondvana (Adriatic region) does not show such correlation. It prove, that the continental crust of the Carpathians is composed of the complexes, which were formed in perifennosarmatian part of Mediterranean belt, but not in Gondvanian, as it is sometimes considered.
Baltica was born by the break-up of Rodinia in the Neoproterozoic. In Silurian - Devonian it collided with Laurentia to form the Scandinavian Caledonides, and later in the Carboniferous - Permian with Siberia and intervening terranes to form the Uralides.
Lithologies in the Scandinavian Caledonides related to the break-up and rifting are superbly exposed in the Särv and Seve Nappe Complexes. In the Särv Nappes Neoproterozoic feldspathic sandstones, Vendian tillites and Cambrian quartzites are cut by numerous Late Precambrian dolerite dikes at high angle to bedding. The over-lying Seve Nappes represent the outermost margin of the rifted continent with both continental and oceanic components; it is overlain by the Köli Nappes of island-arc affinity. In the Seve Nappes, occurring along almost the entire orogen, varied environments are represented; in the north remnants of sandstones swim in mafic dikes, further south quartzites appear over vast areas, while in the middle part of the orogen amphibolites, of both intrusive and extrusive origin alternate with mica schists of various composition, quartzites and marbles. This nappe complex also contains reworked remnants of the crystalline basement, ultramafic bodies, and eclogites. The Seve rocks reach the highest grades of metamorphism in the Scandinavian Caledonides.
In the Uralides, the Central Uralian Zone (CUZ) and the Schistosity zone (SZ) along the Main Uralian Fault correspond to a similar structural position, under the Tagil Magmatic Arc. CUZ, Neoproterozoic phyllites, sandstones, limestones, Vendian tillites, quartzites and shales, form the basement to Ordovician up to Early Carboniferous shelf sediments and Late Carboniferous and Permian molasse mainly occurring in the West Uralian Zone; SZ is built up by phyllites, mica schists, marbles, mafic dikes and extrusives. Mafic dike swarms, the bulk of supposed Vendian - Cambrian age, occur in CUZ. The metamorphic grade in SZ is markedly higher than in the units towards the foreland and in the Tagil Arc. Blue schists occur in CUZ and rocks exposed to elevated pressure in SZ.
The rocks of SZ, have been referred to as the Main Uralian Fault zone, up to recently commonly considered to be the suture zone of the orogen. It is shown to be of Ordovician age on modern maps. However, single-zircon analysis on grains from a granite within the SZ has yielded a weighted average 207Pb/206Pb age of 580±3 Ma. On older more detailed maps, the same lithologies occur both in CUZ and in what is shown on overview maps as part of the Tagil Zone.
The lithological, structural and metamorphic data together with the new date 1) provide evidence that SZ constitutes the margin of Baltica and 2) similar results from the northerly Timan Range and Pechora Basin imply activity along the whole eastern Baltica margin.
N-S oriented linear structures are long known at the lowermost portion of the Jotun Nappe especially at its northwestern margin. Field work carried out in the area of Sogndal (inner Sognefjord region) showed that these N-S trending linear structures were formed in an anastomosing network of shear zones, which has been shown to be largely parallel to the base of the Jotun Nappe. The deformation is concentrated at lithological discontinuities within the Jotun Nappe. From the orientation of the individual shear zones within different parts of the shear zone network, it becomes obvious that the deformation is in some way linked to the basal thrust of the nappe, because the orientation of the individual shear zones successively becomes parallel with the basal thrust approaching the basal thrust. However, the lineation also rotates towards the Caledonian (WNW-ESE) trend approaching the base of the Jotun Nappe, which complicates the distinction between different phases of deformation that may have occurred.
The deformation within the shear zone network is completely isolated within the Jotun Nappe and no link can be made to the tectonic process that may have produced these mylonites. The emplacement of the Jotun Nappe has occurred during the Scandian period, which is expected not to be responsible for the formation of the high strain zone within the Jotun Nappe. Paleogeographic reconstruction of the pre-Scandian configuration restores the Jotun Nappe to a position NW of the Western Gneiss Complex, which is well established in literature. An earlier tectonic event may be recognized if the provenance of the Jotun Nappe is different to this pre-Scandian position. This has been studied by comparing the geochronological and metamorphic record of the Jotun Nappe basement rocks with the Western Gneiss Complex and the Sveconorwegian province of the Baltic Shield. The provenance is inferred to be further south than previously expected. For such a tectonic setting the internal deformation of the Jotun Nappe may be related to this N-directed translation prior to SE-directed Scandian thrusting of the Jotun Nappe.
The c.100 km wide (WNW-ESE) Varanger Peninsula, in NE Finnmark, is cut by the WNW-ESE trending dextral strike-slip Trollfjorden-Komagelva Fault (TKF). It has been suggested that the diagenetic late Precambrian South Varanger Region (SVR), south of the TKF, lies in the foreland of both the Caledonian and Baikalian Orogenies. To the north of the TKF, late Precambrian sediments of the North Varanger Region (NVR) are overthrust in the NW by the Tanahorn Nappe (TN), part of the Caledonian Middle Allochthon. Overthrusting occurred prior to TKF dextral strike-slip movement. The NVR/TN underwent lower (NVR)-middle greenschist (TN) metamorphism, with the development of a slaty (NVR) to penetrative (TN) cleavage. In the west the cleavage has been inferred to be Caledonian, whilst in the east a Baikalian (late Precambrian) age has been assumed on the basis of a suspect U-Pb zircon intrusion age of a cleavage-cutting undeformed dolerite dyke. The NVR, like the SVR, has also been proposed to straddle the Baikalian-Caledonian boundary. The age of cleavage in the NVR is thus crucial to understanding the region.
Preliminary 40Ar/39Ar data from the TN and western NVR have been obtained from the 2-6 and 6-11µm pelitic fractions. The TN basal mylonite gave ages of c. 464 Ma, with an high-T step at c.470-480 Ma; phyllites in the nappe gave a 2-6µm plateau age at 415 Ma with a 460-480 Ma high-T step and 6-11µm ages similar to those in the mylonites. Slates in the NVR gave 440-450 Ma ages, but the 6-11µm fraction from the Kongsfjord Fm. (basal NVR) yielded an age of 470 Ma. More data will be available shortly.
These data confirm the Caledonian age of the cleavage in the western part of the NVR. The TN mylonite ages are comparable to Rb-Sr Kalak mylonite ages from Porsangerfjord (479 Ma) and suggest that Finnmarkian deformation was longer-lived and over-rode Baltica further than previously thought. Whether the NVR ages are very late Finnmarkian, early-Scandian or simply a separate event is unclear as yet. The data also seem to confirm the post Caledonian age of the cleavage cutting undeformed dykes in the Varanger area and the pre-deformation age of the penetratively deformed Kongsfjord dyke swarm (correlated with >550 Ma old Båtsfjord dyke swarm).
Restorations, using balanced-sections and branch-lines, suggest that the TKF had a c.350 km dextral movement, implying a major offset in the proposed `Caledonian-Baikalian´ boundaries. The Rybachi Peninsula restores to west of the SVR.The development of a Caledonian cleavage in the western NVR, restored to c.350 km WNW of the SVR makes the suggestion that the cleavage in the eastern NVR is of Baikalian age difficult to envisage.
The Sarek mountains (67°N) is the largest area of pronounced alpine topography in Sweden. The landforms are strongly dependent on the structure and distribution of the hard rocks, which dominantly belong to the allochthon of the Scandinavian Caledonides. Recent investigations in the central and southern parts of the Sarek National Park have drastically changed the geological map, elucidating some problematic and poorly understood aspects of the tectonic evolution of this part of the orogen. The investigated area is dominated by thrust sheets belonging to the tectonically shortened margin of continent Baltica (TSMB) and the passive margin between Baltica and the Iapetus Ocean (the Seve-Kalak Superterrane; SKS).
Our new map connects the Akkajaure area in the north with the Ruoutevare-Kvikkjokk area in the south and it is unequivocally demonstrated that the Ruoutevare Anorthosite Complex is not only correlatable with, but IS in fact the same unit as the Akkajaure Nappe Complex, both belonging to the TSMB.
In contrast to the simple layercake geometry depicted in older published maps, a major structural culmination - the Tielma Culmination (TC) - is connected to a large and complex folded imbricate structure in the Ålkatj-Tielma-Rita-Sarvesvagge-Luottolako area. Structurally, the culmination is complicated by the presence of large semi-lateral ramps and very large transposed open-tight folds.
A second important component is the Skarja Nappe (SN), dominated by a monotonous garnet mica schist, here assigned to the SKS. The SN forms an enormous lenticular pinch-and-swell structure together with the garnet amphibolite-dominated Sarektjåkkå Nappe immediately north of the TC.
The imbrication involves all levels in the tectonostratigraphy, and several repetitions including units from both the TSMB and the SKS are exposed in the Sarvesvagge Valley. It is thus commonly observed that high-grade metamorphic rocks underlie thrust sheets with low-to medium grade crystalline rocks of the TSMB.
The intercalation of thrust sheets with strongly contrasting metamorphic and structural characteristics indicate that the imbrication event was late-orogenic, probably connected to the final emplacement of the allochthon in the frontal part of the orogen. Still, it dictated the regional strain distribution and determined the present structural geology of the entire region.
The geenshist facies rocks contain mylonites, Proterozoic basement and late Proterozoic to Cambrian sediments. The rocks belong to a middle structural level of the orogenic wedge and suffered deformation not only during their own emplacement but during the development of the underlying level too (Greiling, 1989). This Middle Allochthon is exposed under the overlying Upper and the underlying Lower Allochthon. The nappe slices extend in a range of some hundred metres to some tens of kilometers. The external part shows more continuous imbrication, while the internal side is built up of smaller slices. Here, additional shortening oblique to the direction of tectonic transport can be observed. This effect decreases towards the external side.
The sedimentary succession of arkoses, quarzites and shales are hardly deformed in many places. Deformation is dominated by brittle mechanics. Detailed mapping led to a lithostratigraphic record for the sediments (Bartusch, 1998) that could be used for the construction of the tectonic horses. Now, the lithostratigraphy can be roughly compared to the Lower Allochthon, both in the more internal and external parts of the orogenic wedge (Bångonåive window, Greiling et al., 1993 and directly underlying Lower Allochthon, parts of which were investigated simultaniously).
Deformation varies in intensity from penetrative - near thrusts and in pelitic rocks - to weak - in the more internal parts of the horses. Foliation is commonly associated with the growth of white mica and rare biotite and garnet. This early fabric is overprinted by a mylonitic foliation and folding features related to shear zones separating the structural units within the Middle Allochthon, or to folding related to later stacking in the Lower Allochthon (Greiling, 1989). These structures relate very well to the pattern of the Lower Allochthon, that could be outlined more easily due to the earlier knowledge of the sedimentary succession (Gee & Zachrisson, 1979).
Basement rocks usually reveal a more mylonitic character and dominate the slices towards the more internal parts of the Middle Allochthon, where an alternating succession of basement-cover rocks reveals the duplex character (Bartusch & Febbroni, 1998). There, the basement shows significant differences in deformation grade and stacking scale, compared with the structures and deformation in the Proterozoic Sediments. The rather ductile deformation gives rise to suggestions, that implacement has taken place under conditions closer to the adjacent Upper Allochthon. Penetrative deformation, local growth of biotite and garnet and - in places - excessive growth of white mica together with a strong foliation make the rocks appear comparable if not confusable.
Bartusch JM, Terra Nostra, 3, P9, (1998).
Bartusch JM & Febbroni S, Freiberger Forschungshefte, C471, 18-20, (1998).
Gee DG & Zachrisson E, SGU, C769, 48pp, (1979).
Greiling RO, The Caledonide Geology of Scandinavia, Graham & Trotman, London, 69-77, (1989).
Greiling RO, Gayer RA & Stephens MB, Geol Mag, Cambridge, 130, 471-482, (1993).
The peraluminous, late-Caledonian Leinster Granite Complex, SE Ireland (U-Pb 405 ± 2 Ma) is one of the largest outcropping granite bodies in Europe. The granite complex outcrops over an area of 1500 km2 and intrudes the Lower Palaeozoic Avalonian margin sequence of the Leinster Massif, SE Ireland. Geophysical surveys indicate continuation of the granite complex and Leinster Massif to the west along the northern margin of Upper Palaeozoic Munster Basin of SW Ireland.
Recent investigations of the siting, ascent and emplacement of the granite complex indicate the fundamental tectonic control by a long-lived, lithospheric detachment within the Lower Palaeozoic Leinster Massif. Structural and kinematic data indicate that this structure was actively transtensional during intrusion, with ascent facilitated by a multiple sheeting mechanism within the shear zone and emplacement accommodated by an anastomosing high strain system propagated from the original detachment. The initial nature of this detachment can be seen to be a reactivated collisional suture within the Leinster Massif produced during Lower Palaeozoic amalgamation of the Massif.
When this kinematic data is combined with recent isotopic dates for the lowest exposed parts of the Upper Palaeozoic Munster Basin, SW Ireland (U-Pb 385 ± 0.7 Ma) and geophysical modelling of the basin architecture, it is apparent that the pre-basin tectonic framework of the Lower Palaeozoic Leinster Massif, provided the fundamental control on basin initiation, sedimentation and Variscan inversion.
Late Paleozoic granitic magmatism is more widespread than previously believed in the southern New England Appalachians, and marks the end of the final collisional/extensional cycle associated with the formation of the supercontinent Pangea. In southeastern New England, late Paleozoic granitic magmas occur as distinct sheet-like plutonic bodies and as extensive pegmatitic/granitic dikes, all emplaced into Late Proterozoic basement gneisses of Pan African affinity. Recent field, microtextural and geochronological study of several bodies of granitic gneisses mapped as part of the Late Proterozoic basement gneisses reveals that several of these are late Paleozoic magmas. The granitic gneisses are weakly to strongly foliated, with foliation passing continuously and variously into concordant layers, lenses, and plexuses (cm- to m-scale) of unfoliated granite and pegmatoid granite. Transitions vary from sharp to diffuse. Small-scale "shear zones" (asymmetric deflections of foliations) are commonly filled with unfoliated granitic rock. Granite in these "shears" locally narrow to the width of a single grain of feldspar or quartz, but are themselves never deformed. In thin section, evidence of solid-state recrystallization or equilibrated microtextures is lacking. Even foliated granite gneisses contain equant quartz and tabular feldspars forming a granular magmatic texture characterized by complexly intergrown (interpenetrating) grain boundaries. The presence of mesoperthitic feldspars further precludes subsolidus recrystallization. These features indicate the foliation, shear deflection, and granite dikes are the product of melt segregation and redistribution within a deforming mush.
Gneissic and unfoliated granite yield concordant monazite U-Pb ages of 275-285 Ma, but complex age patterns for zircon. Discordant conventional zircon U-Pb ages cannot be uniquely interpreted, but in combination with 207Pb/206Pb ages obtained by direct evaporation, appear to reflect mixtures of Late Proterozoic cores and late Paleozoic magmatic overgrowths, followed by variable modern Pb loss. The large proportion of zircon overgrowth relative to core and the concordant monazite U-Pb ages suggest that both zircon overgrowths and monazite are the products of growth from melt of late Paleozoic age.
These results indicate that Late Proterozoic basement gneisses residing at deep crustal levels were mobilized during the late Paleozoic Alleghanian orogeny, contributing partial melts and restitic material to magmas rising from deeper levels. Foliations in late Paleozoic magmas were acquired during crystallization. Unfoliated granite lenses developed by segregation into dilational zones within a deforming crystal mush, due to emplacement of the magmas within a deforming crustal section. This magmatism slightly postdates major penetrative ductile deformation of the Late Proterozoic basement (295-305 Ma), and arguably is the result of overthickening of the crust during late Paleozoic convergence between Laurentia and Gondwana. A mid to lower crust rendered weak by these events may have been instrumental in the ensuing Permian extensional collapse of the thickened crustal section.
Despite study for over a century the Sudetes remain controversial, with on-going argument as to whether Caledonian, or Variscan influences were dominant in shaping its geology. The Kaczawa Complex is a vital piece in this jigsaw, as it contains mélange that is interpreted to have formed within the Rheic Ocean (that here separated Bohemia from Laurorussia) at a subduction trench. Our examination of outcrops and core has revealed a variety of structural/sedimentation features in the mélange including: incipient crenulation cleavage locally preserved with clay minerals 'injected' along the cleavage planes, sandstone bodies that preserve web structures and rootless slump folds in structural continuity with the cleavage. These features imply that the mélange matrix was extensively deformed very soon after it was deposited.
Parts of the Kaczawa mélange are dated as Upper Devonian-Lower Carboniferous on the basis of conodonts found within detrital limestone boudins/olistoliths. The mélange is tectonically interleaved with various rocks, mostly of poorly constrained age, including: Silurian T-MORB pillow basalts; arc-related volcanoclastic rocks, and both Devonian and Ordovician turbidite-dominated sandstones. It covers the remainder of the Kaczawa Complex which consists of stratigraphically coherent Upper Cambrian (?) to Ordovician rift-related volcanic rocks, sandstones and shales overlain by Silurian graphitic slates and black cherts. These rocks are thrust over one another forming a WNW-directed thrust stack and were metamorphosed first to blueschist-facies, later overprinted by a greenschist-facies assemblage. The Kaczawa Mélange locally appears to seal these thrusts, yet is itself pervasively deformed by top-to-the-WNW thrusting. The above rocks are all thrusted over detrital limestones, shales and fine sandstones that contain conodonts of upper Visean age and are interpreted as highly deformed turbidites foreland-ward of the Sudetes thrust belt. Subsequent to this top-to-the-NW thrusting the area has been deformed by strike-slip faulting related to the Intra-Sudetic fault isolating a series of tectonic units that can otherwise be tectonostratigraphically related. Unconformably overlying Stephanian sandstones of the North Sudetic basin provide an upper age for deformation.
The Kaczawa mélange is interpreted as a subduction-accretion deposit formed within the Rheic ocean prior to, and coeval with, collision between Laurorussia and Bohemia during the Variscan orogeny.
The northern and northeastern margins of the Bohemian Massif are complex mosaic of tectonometamorphic units or suspected terranes, respectively. The terrane juxtaposition is the result of multiple Variscan collisions of Gondwana derived microplates with Baltica and/or East Avalonia, and subsequent late Variscan large-scale shear movements.
The South Krkonose Complex forms the southern rim of the suspected terrane of central West Sudetes. The South Krkonose Paleozoic sequences experienced Variscan polyphase metamorphism; the succession of the tectonometamorphic events was precised by Ar-Ar age determination method. The results distinguished several main clusters of ages: (1) ca. 360 Ma, cessation of subduction related blueschist facies event, (2) ca. 345 Ma, greenschist up to lower amphibolite facies overprint (probably related to the Early Carboniferous tectonometamorphic and igneous activity), (3) 325-320 Ma, late Variscan shearing, (4) 314-313 Ma, upper limit of magmatism and metamorphism, including late-tectonic granite intrusions.
The cooling age of 465 Ma (Middle Ordovician) of the detrital muscovite from the Ordovician (-Silurian?) quartzite (Ponikla Group) indicates that the mineral isotopic system was undisturbed by the polyphase tectonothermal history. However, two blueschist samples from the underlying(?) lithostratigraphic subunit (in the midst of the South Krkonose Complex) provided ages ca. 320 Ma which are the evidence of complete resetting of the earlier blueschist event. On the contrary, in the easternmost part of the Krkonose Complex, the LP-HT overprint at ca. 340 Ma simply followed the HP-LT event (terminated at 360 Ma), and left its record quite undisturbed. That is, within one lithostratigraphic unit are present undisturbed Ordovician (-Silurian?) quartzites together with the Late Devonian blueschists which are showing late Variscan overprint.
The distribution of the determined Ar-Ar plateau ages suggests that the Variscan polyphase tectonothermal development - involving the above mentioned events (1) to (4) - affected individual lithostratigraphic units of the South Krkonose Complex in different ways. The differences mirrored propagation of the orogenic wedge towards NW which is evidenced by both inverse metamorphic pattern (from chlorite zone on the northwestern side up to garnet zone on the eastern side) and reversed stratigraphic ages (the Late Devonian to Early Carboniferous units are overthrust by several Early Paleozoic crustal slices). This structural pattern was modified by late postorogenic extension and shearing. The present position of the South Krkonose Complex units is the result of late Variscan (Late Carboniferous) juxtaposition.
This contribution is involved in the European PACE-project.
In order to understand the Palaeozoic Amalgamation of Central Europe (PACE) with respect to the East European Craton (EEC), it is necessary to solve the controversy that centres on the Sudetes Mountains of Poland and the Czech Republic. Arguments regarding the lithologically, structurally, and metamorphically different crustal blocks in this area concentrate on i) provenance (i.e., correlation with the Variscan zones west of the Elbe line versus separate terranes that sequentially accreted to Baltica), ii) the pre-Variscan geotectonic environment (extensional versus compressional), and iii) the accretion history (Caledonian and Variscan tectonothermal events or single-cycle Variscan orogeny).With the objective to decipher the pre-Variscan and Variscan evolution and accretion history of various crustal blocks in the Sudetes, we have obtained U- (Th-) Pb dates on accessory minerals that grew during high-grade metamorphism and deformation in the allochthonous Gory Sowie block, southwest Poland. Here, previous geochronology (Oliver et al., 1993, Kröner and Hegner 1998) implied an early Palaeozoic compressional tectonic setting that, however, is contradicted by structural and petrological interpretations (e.g., Zelazniewicz 1997, Kryza et al. 1998).Long- to short prismatic euhedral zircons of various sizes from a syntectonic granite at Walim, Gory Sowie block, show distinct homogeneous to magmatically zoned cores, and variably thin rims of secondary magmatic overgrowth. These inherited zircon cores are interpreted to originate from a sediment fed by sources such as ca. 480 Ma granitoids and other lithologies (Kröner and Hegner 1998). Partial melting of the sediment during high temperature metamorphism and subsequent crystallisation led to additional zircon growth around the inherited cores, producing the current euhedral to multifaceted external crystal forms. Conventional U-Pb geochronology on magmatic monazites from the Walim granite yield an age range from 394 ± 2.6 Ma to 400 ± 2.8 Ma, interpreted to reflect the timing of Variscan high-grade metamorphism and anatexis. These new data contradict previous isotope studies that proposed a ca. 473 to 440 Ma age for anatexis (Oliver et al., 1993, Kröner and Hegner 1998). Instead, these older ages are interpreted to reflect the age of source rocks of the partially molten metasediment. Furthermore, the new data support tectonic models for pre-Variscan major rifting and a single cycle Variscan orogenic event as opposed to models that propose a Caledonian orogeny for the Sudetes.
Kröner A & Hegner E, J Geol Soc London, 155, 711-724, (1998).
Kryza R, Aleksandrowski P & Mazur S, PACE meeting, Keele University, Program with Abs, (1998).
Oliver GJH, Corfu F & Krogh TE, J Geol Soc London, 150, 355-369, (1993).
Zelazniewicz A, Geol Mag, 134(5), 691-702, (1997).
The Izera gneisses in northern part of the Izera-Karkonosze Block, West Sudetes, have had granitic protolith whose isotopic and geochemical characteristics inconclusively point to within-plate, calc-alkaline, syncollisional, post-orogenic, peraluminous granitoids of S-type. These metagranites yielded U-Pb and Pb-Pb zircon ages of 515-480 Ma (Korytowski et al., 1993; Oliver et al., 1993; Kröner et al., 1994; Philippe et al., 1995) and 480-450 Rb-Sr whole rock ages (Borkowska et al., 1980), all indicating intrusion and tectonothermal event during Late Cambrian-Ordovician times. The Izera metagranite body contains small yet ubiquitous enclaves/xenoliths derived from biotite paragneisses, amphibolites, leucogranite(gneisses) and from PT-sensitive metapelites. The latter in the form of diffused xenoliths of fine- to medium-grained rocks have been found in central and NE part of the Izera massif. Some gneissic xenoliths contain an assemblage of phengite (Si=3,55-3.6 pfu)-garnet (Grs=52-54% mol)-biotite-K-feldspar-albite-zoisite-quartz with symplectitic garnet-quartz intergrowths probably formed at the expense of pyroxene and plagioclase. This assemblage developed at PT minimum conditions of 17 kbar, 680°C. Other xenoliths of dark, high-P metapelites preserved a metastable assemblage of garnet (Alm = 94% mol)-biotite-kyanite-jadeite (?) which was produced at minimum 12 kbar, 680°C. A replacement of this assemblage by staurolite (XFe = 0.9)-paragonite-kyanite-biotite was achieved at 8 kbar, 640°C, possibly during decompression triggered by the upward transfer of the Izera granite magma. Accordingly, rocks of the upper amphibolite/ granulite facies were sampled by the Izera magma. Yet other xenoliths are dark, medium-grained, cordierite-bearing rocks. Their early assemblage of K-feldspar-cordierite-sillimanite-biotite-quartz was partially replaced by a garnet-cordierite-K-feldspar assemblage which documents heating from the K-feldspar-cordierite zone conditions up to those of the garnet-cordierite zone at c. 800°C, 4 kbar. All these means that the Neoproterozoic Cadomian crust, which became partially molten to produce the Izera granite, must have been strongly tectonized and diversified, with high-P and low-P rocks tectonically intermingled prior to the 515 Ma onset of the Palaeozoic rift-related granite magmatism.
Contribution to 'Europrobe' TESZ and 'Orogene Prozesse...' projects.
Borkowska M, Hameurt J & Vidal P, Geol Sudetica, 30, 1-27, (1980).
Korytowski A, Dörr W & Zelazniewicz A, Terra Nova, 5, 331, (1993).
Kröner A, Hegner E & Jaeckel P, J. Czech. Geol Soc, 39, 60, (1994).
Oliver G J H, Corfu F & Krogh T E, J. Geol Soc Lond, 150, 355-369, (1993).
Philippe S, Haack U, Zelazniewicz A, Dörr W & Franke W, Terra Nostra, 95/8, 122, (1995).
The Intra-Sudetic Basin, a Variscan intra montane trough in the central Sudetes (NE part of the Bohemian Massif) is filled with Carboniferous, Permian, Triassic and Upper Cretaceous deposits. The Carboniferous sediments display considerable lateral and vertical facies variation which reflects intense contemporaneous tectonic and volcanic activity. The sedimentary material was transported from various sources, a good indicator of which, apart from the lithology of pebbles, appear heavy minerals.
To recognise the variation of heavy mineral assemblages, both lateral and in the stratigraphic column, all Carboniferous formations along two profile lines across the eastern and western part of the basin were sampled. Sixteen transparent and six opaque minerals were identified optically and using the microprobe. In the oldest Lower Carboniferous (Upper Tournaisian) sediments, the most abundant are clinozoisite-epidote, chlorite and grossularite-rich garnet. This indicates that the neighbouring basement HP-LT complexes of the Kaczawa Mts and Rudawy Janowickie were alimentary areas during the earliest stage of the molasse sedimentation in the basin. The younger Lower Carboniferous sediments comprise mainly chlorite with minor garnet (Alm 50-70, Grs 5-30, Prp - 5-10, and Sps 5-25%) and Ti-minerals (rutile, brookite and sphene), amphibole, pyroxene and less abundant clinozoisite-epidote. Zircon is suprisingly rare.
The heavy mineral spectra of the Upper Carboniferous deposits are generally similar but they show greater variation both laterally and in the stratigraphic column. Therefore, direct links with the surrounding basement complexes are less evident and, usually, it is more difficult to point a single and well defined source of the detrital material. These sediments appear to have derived not only from the basement complexes in the neighbourhood of the basin but also, probably, from more distant areas and from re-deposited older sediments of the Intra-Sudetic Basin.
The heavy mineral analysis confirms previous interpretations of transport direction based on sedimentological criteria, with one important exception: it excludes the Sowie Mts as an alimentary area at that time. There are no clear indications that the Góry Sowie Block played an important role as a source area for the Carboniferous molasse (syllimanite, kyanite and garnets typical of gneisses in that area are lacking in the studied heavy minerals assemblages).
The research supported from Grant KBN no 6 P04D 052 12
The syn-collisional exhumation of deep crustal rocks and the mechanisms involved are still a matter of debate. In this context, the Saxo-Thuringian Belt of the Variscan orogen provides an interesting case study as several datasets are available to constrain fundamental principles of exhumation of HP/HT rocks.
Seismic data acquired across the Saxonian Granulite complex and its western continuation allow the correlation of surface structures with those at depth. The reflection seismic lines show prominent reflections in the upper crust, with a dome-like structure along strike the Saxonian Granulite antiform. Refraction profiling reveals, that the main reflector corresponds to a high-velocity layer with Vp ranging from 6.2 to 6.6 km/s, probably a sheet of meta-basic rocks. The felsic granulites exposed at the surface overlie this refractor, and form a cap that wedges out towards the SW. It appears that the entire NW part of the Saxo-Thuringian belt is underlain, at shallow level, by rocks of very high metamorphic grade. They extend in the shallow subsurface to the SW at least as far as the SW margin of the Bohemian massif. In the Erzgebirge, HP rocks metamorphosed under medium temperature conditions form thin sheets alternating with LP rocks.
Exhumation of the Granulite Massif occurred beneath a shallow marine basin thus ruling out exhumation by erosion of extensional collapse of overthickened crust. Subsequent to rapid exhumation and cooling, the Saxo-Thuringian domain was incorporated into the accretionary belt at the NW margin of the Tepla-Barrandian block, and later affected by SE-facing deformation in the retro-arc of the Mid-German Crystalline High. The structural high occupied by the Saxonian granulites was hardly affected by the deformation fronts impinging from both these margins.
The timing of these events, seismic, petrologic and geochronologic data are used to constrain quantitative geodynamic scenarios for the evolution of the Saxonian Granulite complex. In particular, thermo-mechanical simulations are used to model the rapid exhumation of crustal rocks. One promising approach is the extrusion of hot, low viscosity material from a crustal root located to the SE.
Gabbros occurring along the western margin of the Tepla-Barrandian Unit (TBU) and its contact with the Marianske Lazne Complex (MLC) show variability in geochemical composition within the MORB field and in metamorphic grade. The group of MLC metagabbros comprises types distinct both in primary mineral composition and in metamorphic record: gabbros (i) with the simple magmatic mineral pl-cpx-ilm-ap±hbl assemblage affected by late penetrative amphibolization resulting in hbl blastesis and replacement of primary minerals, in places with the reaction garnet coronas, (ii) more complex and variable cpx-opx-pl±ol-ilm-ap primary assemblage showing in places different types of metamorphic coronas of orhopyroxene, amphibole, and garnet around primary mineral grains and late amphibolization, and (iii) fine-grained coronitic metadolerite. The three types occur in three roughly parallel southwest-northeast trending zones and show systematic variations in mineralogical composition and metamorphic overprint; they underwent at least upper amphibolite facies metamorphism. In addition, (j) microdolerite with coronitic orthopyroxene and dendritic garnet represents an exotic rock of the core of the MLC, and coarse-grained gabbro was the protolith of the intensely reworked rock composed of the assemblage pl-hbl-grt-ky-zo-Nacpx-ru with apparent textural relics of gabbroic fabric and of eclogite and granulite events (jj), that is relatively common in the north-eastern part of the MLC. Minor occurrences of unmetamorphosed coarse-grained hbl-pl melt of gabbroic appearance (jjj) surround in places the basal serpentinite body; these rocks were probably derived in situ. Some garnets from omp-grt eclogites from the core of the MLC seem to trace the original gabbroic texture and to verify thus the result of geochemical studies proposing gabbros to be also the protolith of most of MLC omphacite-garnet eclogites.
The types of gabbroic rocks from the MLC are compared to gabbros found in the only directly comparable unit - the Zone of Erbendorf-Vohenstrauss in Germany, where only some analogues were described from the KTB drillholes (e.g., O'Brien et al. 1997).
Due to the effect of ductile tectonics and multiple metamorphism, a relict gabbroic fabric and mineral components of some types are visible mostly in microscale. New detailed field work provides new data on recent distribution of gabbro analogues within the MLC and its surroundings. Metamorphic imprint of gabbroic rocks increases from amphibolite facies to the SE to eclogite and granulite facies in the northern margin of the MLC. The question is whether there is one source or more protoliths of distinct provenance. The problem of magmatic and tectonic evolution is solved by means of geochemical and isotopic study of gabbros.
O'Brien P, Duyster J, Grauert B, Schreyer W, Stöckert B & Weber K, Journ. Geophysic. Res, 102/B8, 18203-18220, (1997).
The western part of the Bohemian Massif exposes one of the major Variscan tectonic boundaries. Carboniferous high-temperature gneisses, migmatites and granites (the Drosendorf or Monotonous Unit of the Moldanubian Zone) are in contact with a complex unit consisting mainly of a low-grade metamorphic Cadomian basement with granitoid and mafic intrusions and a Paleozoic sedimentary cover (Teplá-Barrandian Zone). At the southeastern border of the Teplá-Barrandian with the Moldanubian Zone, gabbroic rocks and amphibolites of the former are in contact with metamorphic rocks of the latter. The Moldanubian metamorphic rocks comprise a sequence of mainly WNW-ESE trending lithologies. From north (adjacent to the Teplá-Barrandian) to south these are garnet-bearing micaschists, andalusite-bearing micaschists, sillimanite-bearing schists and gneisses, K-feldspar and cordierite-bearing gneisses and migmatites and garnet-cordierite-migmatites. The whole sequence is thought to display a tilted metamorphic pile displaying increasing metamorphic grade from north (greenschist facies) to south (amphibolite facies). Recent investigations have shown that metamorphism and partial melting in the cordierite-bearing migmatites occured at higher temperatures than previously thought (approximately 850°C at 0.4-0.5 GPa) and that there is no increase in metamorphic grade from K-feldspar and cordierite-bearing gneisses and migmatites towards garnet-cordierite-migmatites. The investigations presented here are new petrostructural studies on the northernmost part of the K-feldspar and cordierite-bearing gneisses and migmatites and their relation with the sillimanite-bearing schists and gneisses and with the andalusite-bearing schists. Preliminary petrological data suggests that the high-temperature cordierite-bearing gneisses and migmatites were subject to a retrograde hydrothermal overprint leading to extensive formation of muscovite. Despite the hydrothermal overprint, temperatures close to peak metamorphic conditions (approximately 800°C) are still retrievable. The data also suggest that andalusite-bearing rocks in the north are characterised by significantly lower temperatures of metamorphism. The formation of fibrolitic sillimanite in some andalusite-bearing rocks near to the migmatites is best explained by a short period of heating. Meso and microstructures in all lithologies are complex and suggest that only the last deformational event was common to all of the studied lithologies.The preliminary results are consistent with petrostructural studies on the Czech part of the Moldanubian / Teplá-Barrandian boundary, indicating that the Moldanubian consists of two units with different metamorphic and structural evolution: a lower-temperature metamorphic unit of micaschists and a high-temperature metamorphic gneiss and migmatite unit.
We combine experimental work and numerical simulations to reconstruct the thermal history of the Frankenwald Transverse Zone, which was formed by a granitic intrusion into a fault zone. Illite crystallinity, vitrinite reflectance, and geobarometric investigations reveal a metamorphic and paleotemperature anomaly associated with the granitic intrusion. Results of numerical simulations adequately explain paleo-temperatures in that area. In order to be able to obtain a quantitative comparison between numerical model results and paleotemperature as observed in the field, we propose an empirical relationship between illite crystallinity and the maximum paleotemperature based on literature data of illite crystallinity and a combination of other temperature-dependent parameters like vitrinite reflectance, phase petrology and smectite-to-illite transformation. Application of this strategy to the Frankenwald Transverse Zone yields the following results: (1) the paleo-temperature anomaly can be explained by the cooling of a number of plutons which intruded into the center of the zone. No additional heat sources are required to explain the observed anomaly. (2) The diapiric shape of these plutons could be confirmed because, in contrast, dike-shaped bodies would produce much smaller paleo-thermal anomalies. (3) The resolution of paleo-temperatures obtained from the illite crystallinity data is not good enough to discriminate precisely between advective and conductive modes of heat transfer. According to our preferred model, conductive heat transport is more likely than fluid driven advective heat transport.
The Ruhla Crystalline Complex (RCC) is part of the Mid-German Crystalline Rise (MGCR) and is additionally transected by the NW-trending Franconian fault system. The RCC consists of four structural-metamorphic units: Truse Formation in the SE, Ruhla Formation in the W, Brotterode Formation in the NE and the Central Gneiss Unit. These four units were affected by granitic and dioritic magmatism at different times as evident from 207Pb/206Pb dating, using the evaporation method on single zircons with igneous morphology (all errors given are 1 sigma standard deviation).
The oldest magmatic event in the RCC is indicated by zircons from orthogneisses intercalated within greenschist facies metasediments of the Ruhla Formation. These zircons yielded ages of 425.6 ± 4.5 Ma (Silbergrund Gneiss, 6 zircons) and of 423.2 ± 6.0 Ma (Erbstrom Gneiss, 5 zircons), inferred to be the Silurian emplacement ages of the granitic protoliths.
Orthogneisses from the Central Gneiss Unit points to a second, significantly younger magmatic event during Late Silurian to Early Devonian times: 413.4 ± 4.5 Ma (Liebenstein Gneiss; 7 zircons), 409.0 ± 8.6 Ma (Dorngehege Gneiss; 8 zircons), 408.5 ± 6.1 Ma (Schmalwasserstein Gneiss; 6 zircons) and 399.8 ± 6.1 Ma (Steinbach Augengneiss; 6 zircons).
Notably, nearly all orthogneisses contain a few zircons, which yield significantly older ages: 2286.7 ± 5.2 Ma (Silbergrund Gneiss); 1329.2 ± 2.7 Ma (Schmalwasserstein Gneiss); 895.2 ± 8.4 (Erbstrom Gneiss) and 713.4 ± 15.8 Ma (Steinbach Augengneiss). These indicate assimilation of older crustal material during the intrusion of the orthogneiss protoliths and/or derivation by partial melting of Proterozoic crust.
A third magmatic event, assumed to be related to the compressive phase of the Variscan Orogeny, is represented by the intrusion of the Thuringian Hauptgranite. This granite, which postdates the penetrative structural-metamorphic overprint in the RCC, gives a zircon age of 337.3 ± 4.8 Ma (6 zircons).
Caused by Late Carboniferous to Early Permian extensional tectonics, the RCC was again affected by voluminous magmatism, representing a fourth magmatic event. This is evident from zircon data for the Trusetal Granite (two samples: 297.3 ± 3.1 Ma, 7 zircons; 295.4 ± 3.2 Ma, 3 zircons), the Ruhla Granite (295.2 ± 4.1 Ma, 9 zircons) and the Brotterode Diorite (289.3 ± 4.6 Ma, 6 zircons). Finally, these plutonic rocks were transected by dikes of basaltic and rhyolitic composition, related to the Rotliegend volcanism in the Thuringian Forest.
In the Ardenne, Eifel, Brabant, and Campine regions an omnipresent thermal overprint of the sedimentary rocks can be observed, which comprises diagenetic to epizonal/beginning mesozonal metamorphic conditions. From field and microstructural observations, the review of own and existing data (mineral paragenesis, illite crystallinity, vitrinite reflection, conodont alteration index, fluid inclusions), and their relationship to small and regional scale geological structures the following thermal evolution for the central Variscan Rhenohercynian basin can be deduced :
1)A pre-Variscan metamorphism, which reached up to anchi- to epizonal conditions, affected the pre-Devonian basement rocks of the Brabant Massif and possibly some of the Lower Palaeozoic Massifs further south (Stavelot Massif). The probable correlation with the age of the rocks and herewith their depth points to a burial metamorphism.
2)The main regional metamorphism affected all Devonian and Carboniferous cover rocks and the Lower Palaeozoic basement highs of the Ardenne and Eifel areas. This metamorphism does not exceed diagenetic conditions in some areas, but reaches epizonal to beginning mesozonal conditions at the southern borders of the Lower Palaeozoic basement highs. It is pre- to synkinematic to the penetrative deformation and correlates well with the age and thickness of the sedimentary cover. The metamorphism is associated with the pre-Carboniferous preorogenic rifting stage of the Rhenohercynian basin, documents the peak of subsidence and sediment accumulation, and must therefore be interpreted as burial metamorphism (= diastathermal metamorphism after Robinson & Bevins 1989).
3)This regional metamorphism is overprinted in the central part of the Ardenne and Eifel areas by an anchi-/beginning epizonal metamorphism and is replaced in their northern respectively northwestern foreland areas by diagenetic to lower anchizonal conditions. This metamorphism is mainly documented in the Middle Devonian to Carboniferous rocks from the Dinant synclinorium and the foreland coal basins north of the Midi fault zone. It documents the presumed thicknesses of the synorogenic middle to upper Carboniferous clastic wedge related to the Carboniferous contraction of the Rhenohercynian basin and is itself affected by the deformation of the advancing orogenic front.
Robinson, D & Bevins, RE, Earth Planet. Sc. Lett, 92, 81-88, (1989).
Dep. of Geology and Paleontology, Masaryk University, Brno, Czech Republic.
The Brunovistulicum (BC) is a crustal segment regarded as a continuation of Avalonian group of terranes (KALVODA 1995). It was derived from Gondwana probably in lower Cambrian and later amalgamated to Baltica along the TESZ. The northern continuation of BC, which is known only from deep boreholes, is called Upper Silesian Block (USB). The strong geochronological, petrological and geochemical evidence suggest, that the magmatic sequence of BC were derived from Arabian Nubian shield - NE Gondwana (LEICHMANN 1996, FINGER et al. 1998).
The lithological record of the Cambrian marine sediments (sandstones, siltstones) may indicate the extension and rifting connected with fragmentation of northern Gondwana margin. The clastic material was probably derived mostly from local basement which is well in accord with an observation from the USB, where clastic material were derived as well from the BC basement. Nevertheless the trilobite fauna as well acritarch fauna contain taxa typical for Baltica province (Jachowicz, Pøichystal, 1997, Belka et al. 1998).
The second phase of extension is assumed to be connected with intraplate stresses due to the slab pull which accompanied the subduction of the Moravosilesian plate beneath Moldanubian group of terranes. The faults of approximately NW-SE direction paralleling the Teisseyre-Tornquist Zone and Kraków-Lubliniec Zone were reactivated. The remnants of the extensional zones can be distinguished in Nesvaèilka, Jablùnka, Jablùnkov and other synclinal zones in the east, while more to the west detached sediments and their basement were incorporated in the complicated mosaic of the Moravo-Silesian shear Zone.
The extension started with the deposition of Devonian basal clastics (conglomerates + sandtones, Old Red Facies) in continental to marine environment. The petrological and geochronological analyses identified again the brunovistulian basement as a main source for this deposits. They are overlain by marine siliciclastic and carbonate sediments. The foraminiferal fauna shows close similarity with the assemblages described from the East European platform. Several halfgraben basins which development shows a distinct polarity from Moravia to Poland were formed. The deepening upward sequences ended mostly in shales with radiolarites or distal calciturbidites and a narrow belt of oceanic crust in the Famennian and Tournaisian is assumed.
In Moravia the compression started in the Tournaisian (increased sedimentation of lithoclasts and siliciclasts) while in southern Poland extension still took place. During the compression former extensional faults served as overthrust zones in the east while the rotation and perpendicular clockwise displacement took place in Moravo-Silesian shear Zone in the west.
Paragneisses and migmatites of pelitic to psammitic compositions are the most abundant rocks in the internal part (Moldanubian Zone) of the Variscan belt in central Europe. These rocks are, therefore, the key to understanding the sedimentary, metamorphic and deformational evolution of a major part of the Variscan orogenic belt. However, high-grade metamorphism and deformation during Devonian and Carboniferous collision have largely obliterated features of the pre-Variscan geological record. Nevertheless, palynological studies on metamorphic rocks indicate that organic-walled microfossils such as acritarchs and chitinozoans can be preserved under favourable circumstances up to amphibolite-facies conditions und thus are a promising tool in combination with petrological and isotopical data to elucidate the very early geological history of the high-grade metasediments.
The Moldanubian part of the Schwarzwald consists of three distinct tectonometamorphic units that have shared the last HT-LP overprint at approximately 330 Ma, but are characterised by diverging earlier metamorphic histories. (1) Monotonous gneisses and migmatites with numerous intercalations of eclogites and ultramafic high-pressure rocks; (2) Heterogeneous gneisses with more variegated lithologies that are devoid of high-pressure rocks; (3) Gneisses with relics of a granulite-facies metamorphic stage.
Vase-shaped microfossils in a grt-crd-gneiss from unit (1) point to the local presence of Late Proterozoic sedimentary protoliths. However, the first single zircon 207Pb/206Pb evaporation data merely record the intense Variscan metamorphism. The occurrence of deformed and graphitized bottle-shaped chitinozoans and galeate acritarchs in amphibolite-facies paragneisses from gneiss unit (2) point to an Early Palaeozoic sedimentation age for the gneiss protoliths in a marine basin, presumably at the northern margin of Gondwana. Detrital zircons from a microfossil-bearing paragneiss of gneiss unit (2) record Lower Cambrian zircon 207Pb/206Pb evaporation ages (maximum ages), supporting an Early Palaeozoic sedimentation and pointing to the erosion of Cadomian basement rocks. A zircon subpopulation in the same rock indicates minor Archean components.
The present data suggests sedimentation of the gneiss protoliths of the Schwarzwald during different episodes, namely the Neoproterozoic and the Early Palaeozoic.
The gneisses of the Moldanubian zone of the Black Forest are intersected by a zone of low-grade greywackes and volcanic detritus of late Devonian to early Carboniferous age. The zone has been interpreted by some authors as a Variscan suture. The basement to the south of the greywacke zone consists of paragneisses, subordinate amphibolites and felsites intruded by late Variscan granites. It is overlain by a nappe consisting of paragneisses showing variable degrees of anatexis (the rocks have been described as diatexites) with cross-cutting and little deformed granitic dikes that are absent in the basement. Consequently, dating of the nappe should provide time constraints on the final stage of Variscan collision in the southern Black Forest.
We present single zircon 207Pb/206Pb ages obtained by the evaporation method for two sam-ples of the diatexites and a single sample from a cross-cutting granitic dike. In addition, we present ages for metamorphic zircons from two amphibolite samples of the basement that provide an upper age for HT-metamorphism. Magmatic zircons of the diatexites indicate crystallization at 348.1 ± 3.7 Ma and 346.5 ± 0.8 Ma (2<sigma>). We interpret these ages as dating crustal thickening due to plate collision. The ages of zircons from the granitic dike of 342.5 ± 1.5 Ma (2<sigma>) provides an upper limit for nappe emplacement. If these granitic dikes were pro-duced by shear-melting of the base of the nappe, a model which is supported by identical initial <epsilon>Nd-values in the granitic dikes and the diatexites, the 342.5 Ma age would represent a best estimate of nappe emplacement. The metamorphic zircons of two amphibolite samples yield 342.2 ± 1.5 Ma (2<sigma>) and 342.6 ± 1.1 Ma (2<sigma>) identical to the age of the granitic dikes. The fact that the formation of metamorphic zircons in the basement coincides with age of the granitic dikes suggests a common thermal history.
Our data support a model for the Variscan collision in the Moldanubian zone of the Black Forest with onset of collision shortly before 348 Ma and culmination of crustal thickening at ca. 342 Ma. The geochronological constraints as outlined above are similar to the time frame for nappe emplacement and accompanying crustal melting in southern Bohemia (e.g. Gföhl nappe). This finding suggests a similar collisional history in this part of the Moldanubian zone.
The Variscan orogenic belt in Central Europe is a very complex conglomerate of terranes, sutures, basins, and accretionary wedges. An important question is the missing Variscan orogenic crustal root, crustal thickness being about 30 km (except in regions of Tertiary tectonism). We apply numerical modelling to investigate the geodynamic processes that could have removed the crustal root. Our aim is to understand the principal features and we concentrate ourselves on the Erzgebirge around the Saxothuringian-Moldanubian boundary. A general feature is extensive high-pressure-metamorphism during the Lower Carboniferous, followed by rapid uplift, exhumation of metamorphic core complexes and HT/LP metamorphism with increasing volcanic activity. This indicates anomalously high heat flow and extension of the upper crust. Important tectonic aspects are:- Reversal of horizontal stress from compression to extension between 340 and 330 Mabp,- high temperatures in upper lithospheric levels after the orogenic peak- exhumation and sedimentation within the orogen at the same time
2-d numerical FD-calculations have been carried out with the routine FDCON (H. Schmeling). The approach is that of convection modelling (conservation of mass, momentum and energy) in a dynamically, rheologically and thermally as consistent a fashion as possible, e.g. viscosity and density depending on temperature and therefore sensitive to thermal effects as shear heating and radiogenic heating. We assume for the geodynamic evolution a delaminating mantle lithosphere within a compressive continental regime. Several lower crustal rheologies have been tested, as isoviscosity, pseudo-plasticity and thermally activated creep.
It is shown, that models of lithospheric thickening as well as models of root detachment require weak zones, e.g. vertical regions of decreasing strength in the lithospheric mantle. The rheological characteristics of the lower crust play a key-role in this process: upper crust and mantle decouple in a compressional orogenic setting if lower crustal effective viscosity is <1022 Pas. On the crustal scale deformation is governed by uplift with erosion and denudation, extension of the upper crust and uplift of lower crustal units. We observe a complex interplay of heat conduction, temperature-dependent rheology and convergence rates.
Critical problems are the boundary conditions, neglect of oblique convergence (transpression) in 2-d modelling, erosion and denudation, the crustal response to buoyant instability and thermal effects as a result to the lithospheric detachment, and generally, the geometrical resolution.
Paleomagnetic investigations carried out in various massifs of the European Variscides, demonstrate that at least 90% of the investigated Early Carboniferous units have undergone magnetic overprinting during Carboniferous-Permian times. Thanks to the overprinting phenomenon, a nearly continuous paleomagnetic record, interpretable in terms of geotectonic evolution, was obtained. After 340 Ma, which is the age of most of the investigated units, the geotectonic evolutions of the Armorican Massif, Massif Central, Vosges, Black Forest, Odenwald, Spessart, Bohemian Massif and Sudetes were nearly the same. In the light of recent geochronological data and of geological interpretations the following story is proposed.- From 340 Ma up to 330-328 Ma, the presently SW-NE striking Variscides were oriented SE-NW (Cn-Cp magnetic components) and underwent a possible N-S drift along the southern margin of Baltica.- A prominent compression phase (the Sudetic phase), probably resulting from blocking of the eastern Variscides against the south-western margin of Baltica, marked the end of the N-S drift and led to magnetic overprinting (Cp components) in relation with uplift, folding and wrenching within the belt. - Around 328 Ma, this compression became strong enough to give rise to a 70-80° clockwise rotation (Cp to B change in declination) that closed the Lizard-Giessen oceanic basin. - After the closure, the convergence continued and high pressure metamorphism was recorded along the Saxothuringian - Rhenohercynian boundary. During the relaxation that followed the rotation, large scale magnetic overprinting (B components) in relation with low-grade metamorphism affected all areas of the internal Variscides.- In the time range 320-305 Ma, the whole Variscides, east of the Iberian-Armorican arc, rotated clockwise by 40-50° (B-A1) and the Rhenohercynian zone was thrusted over the Caledonian Brabant block. After acquisition of the primary and secondary A1 magnetizations, the clockwise rotation continued together with Laurussia, up to the end of the Alleghanian convergence in the Middle Permian.- In the Latest Carboniferous- Early Permian (300-270 Ma), the transpressive dextral wrenching affected mainly the southern part of the belt and led to a clockwise rotation by more than 100° of the Maures-Esterel-Corsica-Sardinia block. Up to 300 Ma, the metamorphic zonation within this block was parallel to the zonation of the eastern branch of the Variscides (NE-SW in the present configuration).
The present work points out that the relative location of the Variscan belt with respect to Avalonia and Baltica has significantly changed during the Carboniferous and that geological models are too much influenced by the present situation.
New structural-geological, sedimentological as well as illite "crystallinity" data reveal a new interpretation of the tectonometamorphic evolution of the Southalpine Variscan basement of the Carnic Alps (Austria, Italy) located at the southern border of the Variscan orogen. From Ordovician to early Carboniferous times, the Carnic Alps were part of the northern passive continental margin of Gondwana. Contraction started in early Carboniferous times as the result of underplating of the thinned continental crust in the S under the uplifted Austroalpine crystalline complexes in the N. The Carnic Alps were deformed into a S-verging fold-and-thrust belt with southwards prograding flysch sedimentation, deformation and metamorphism. Our data prove the existence of three major thrust sheets respectively nappes differing in their stratigraphic, environmental, sedimentological, deformational and metamorphic histories. The three nappes are the low-grade metamorphic Fleons nappe (FN: western Carnic Alps), which was thrust over the very low-grade metamorphic Cellon-Kellerwand nappe (CKN: central Carnic Alps) and the Hochwipfel nappe (HN: central-eastern Carnic Alps). Both FN and CKN reveal a continuous stratigraphic sequence of Ordovician to early Carboniferous sedimentary rocks of passive continental margin origin and early Carboniferous flysch deposits. The HN is characterized by wildflysch deposits, which contain all Ordovician to early Carboniferous pre-flysch units as olistolites and hence does not show a continuous stratigraphic sequence in contrast to the other two nappes. A first ductile deformation event (D1, presumably Viséan/Namurian) and epizonal metamorphism (Tmax 300-450°C) affected only the FN. A roughly N-S oriented stretching lineation of quartz and calcite lies in penetrative foliation planes. Shear sense indicators in the XZ plane of the finite strain ellipsoid indicate top-to-S thrusting. A second deformation event (D2, presumably Namurian/Westfalian) affected FN, CKN and finally the external HN under high-anchizonal metamorphic conditions (Tmax 260-300°C). Corresponding structures are S-verging thrusts and tight folds associated with an axial planar cleavage. In the FN, incompetent layers additionally reveal small-scale folds in the order of some cm. A stretching lineation, as during D1, did not develop. Both deformations are of Variscan age, because their styles and metamorphic grades differ from the ones in the less intensely deformed post-Variscan cover rocks. The nappe contacts between FN and CKN on the one hand and CKN and HN on the other hand are marked by two klippen zones, where remnants of the higher-metamorphic and older nappes rest on top of the lower-metamorphic and younger nappes. The already weakened zones in the crust were repeatedly reactivated in late-Variscan and Alpine times by extensional and contractional tectonics.
The Paleozoic core of the Eastern Carnic Alps (NE Italy-S Austria) represents the outermost part of the Southern Alpine Hercynian basement. Its almost continuous and fossiliferous stratigraphic sequence ranges Late Ordovician through middle Carboniferous. Both Hercynian and Alpine orogeny affected this sequence, thus originating a quite complex structural frame, given by a thin-skinned fault-fold style, characterized by reactivation and interference among structures. The Hercynian succession is prevalently non- and anchi-metamorphic. However, close to the Periadriatic Lineament green-schist facies (Eder Unit) occurs.
The metamorphic zonation from the diagenetic up to the anchizone and until the epizone has been investigated through a detailed geological mapping (150 km2 wide) combined with conodont color alteration index (CAI) analysis (Epstein et al., 1977; Rejebian et al., 1987). More than 200 CAI data, from Hercynian substratum, late-Hercynian and post-Hercynian cover units, were integrated into an isograde map.
CAI patterns reveal a progressive trend towards an increase of CAI values from older to younger units. Nevertheless, there is no considerable break among Hercynian, late-Hercynian and post-Hercynian units. This suggests that both Hercynian and Alpine orogeny would have involved the same crust sector. The same thin-skinned fault-fold tectonic style, in fact, is shown both in the substratum and in the cover units, even if geological evidences allow a strong Hercynian event to be distinguished.
The outermost part of the Hercynian substratum shows a continuos and very gradual increase of CAI values from 3.5-4 to 5 towards the inner side of the chain. According to Läufer (1996), metamorphism reached high-anchizonal conditions. This trend is probably inherited from the Hercynian orogeny.
In the innermost epi-metamorphic part of the substratum, CAI values from 5.5 to 8 are registered. In the westernmost part of the study area, near the very important Val Bordaglia Line, each sample shows quite different CAI values (from 5.5 to 8). In this very area, moreover, CAI values are observed to change from 4.5 to 6.5 in almost coeval rocks only few hundreds meters apart, without important tectonic features in the way. Very close to this area, inside anchi-metamorphic units, two samples with CAI values ranging 6,5 through 8 have been found. Some kilometers easterly, near the Periadriatic Line, an inversion in CAI values trend is documented by higher values in more outer areas.
All these data suggest the presence of a hydrothermal metamorphic overprint probably connected with the well-documented Oligocene intrusions located at the Periadriatic Line. These effects are superimposed on a pre-existing regional metamorphic zonation of Hercynian age.
Epstein AG, Epstein JB & Harris LD, US. Geol. Survey Prof. Paper, 995, 1-27, (1977).
Läufer AL, Tübinger Geow. Arb, A26(102S), 1-40
Rejebian VA, Harris AG & Huebner JS, Geol. Soc. Am. Bull, 99, 471-479, (1987).
The Northern Apennines mostly consist of cover sedimentary sequences of Mesozoic-Cainozoic age, whereas outcrops of Palaeozoic basement are rare and scattered. In the Cerreto area (Tuscan/Emilian Apennine, NW Italy), the presence of metamorphic rocks belonging to the Tuscan basement of the Northern Apennine has been known since the end of last century. However, their age, tectono-metamorphic history as well as their relationships with the basement outcrops in the nearby areas (Alpi Apuane, Punta Bianca and Monti Pisani), are still poorly understood. This contribution presents some new structural, petrological and radiometric data which allow to put new constraints on the history of these rocks. The metamorphic rocks (metasediments and amphibolites) are associated with Triassic evaporites and quarzites ("Verrucano Formation") and are interposed between two anchimetamorphic/nonmetamorphic tectonic elements: the overlying Tuscan nappe, here represented by shales and sandstones (Scaglia and Macigno fm.), and the underlying Cervarola Unit (shales and sandstone of lower Miocene age). The Cerreto basement rocks consist of a sequence of interlayered metapsammites (biotite or biotite + muscovite paragneisses) and metapelites (micaschists) with lenses of amphibolites. The metasedimentary rocks are characterized by a pervasive mylonitic foliation with dynamic recrystallisation of muscovite + biotite + oligoclase + quartz followed by retrograde development of muscovite + albite + chlorite + quartz. Relics of an older assemblage are represented by porphyroclasts of biotite + Alm-rich garnet + muscovite + plagioclase (An34-24). Well developed mica-fish and biotite-fish, oblique shape preferred orientation in dynamically recrystallized quartz grain, C-axis orientation of quartz and well developed shear bands indicate involvement of these rocks in a mylonitic shear zone. The widespread asymmetric shear sense indicators point out noncoaxial deformation although locally conjugate systems of shear bands suggest partitioning of the strain and more coaxial shear. The mylonitic foliation is folded by metric- to decimetric-scale SW verging open folds associated with a crenulation cleavage in which a chlorite + muscovite + quartz assemblage is stable. The amphibolites are fine grained rocks with a grano-nematoblastic texture. The mineral assemblage consists of plagioclase (An38-24) + green amphibole (Mg-hornblende or tschermakite) + quartz ± ilmenite ± sphene; garnet porphyroblasts (Alm58-65Grs21Sps19-12Py2-5) with hornblende + plagioclase coronas and rutile crystals rimmed by ilmenite can be locally observed. The foliation defined by horneblende+quartz is locally cross-cut by retrograde millimetric to decimetric wide shear zones in which chlorite + epidote + albite + sphene develop. Whole-rock geochemistry of the amphibolites suggest that their protoliths were mafic rocks crystallized from ocean-floor basaltic liquids with T- to P-MORB affinity. Preliminary 40Ar-39Ar dating of hornblende show a complex pattern suggesting, as a whole, an Hercynian age (~ 330 Ma) for the amphibolite-facies metamorphism with a Permo-Triassic overprint (~ 240 Ma), possibly related to the development of retrograde shear zones. These results, together with the lack of any evidence of mylonitic foliation in the Verrucano quarzites, should imply a pre-Alpine age for the development of the Cerreto mylonites.
Sardinia belongs to the southern part of the Variscan chain. Low- to medium grade metamorphic rocks, outcropping south of the Posada-Asinara suture zone, belong to the internal nappes, translated toward the SW during the Carboniferous collisional event (Carmignani et al., 1994). The metamorphic rocks, represented by micaschists, quartzites, paragneisses, Ordovician orthogneisses and amphibolites, are affected by a collision-related Barrovian-type metamorphism in which metamorphic zones rapidly follow one another especially from the Grt+Bt zone to the Ky+Bt zone.
New meso and microstructural investigations in the low and medium-grade metamorphic rocks identified a thickened zone (15 Km) of plastically deformed rocks within a shear zone. Shear planes strike almost E-W and have a steep to moderate dip toward the S, whereas stretching lineations trend NW-SE and plunge toward the SE. Kinematic indicators show a top-to NW sense of shear. Strain intensity increases from south to north as testified by grain size reduction by dynamic recrystallization, fold axis parallel to the stretching lineations and presence of sheath folds. The geometry of the shear zone and the kinematic indicators show the presence of a transpressional deformation. Vorticity analysis performed on plagioclase porphyroclasts, using the stable orientation analysis, confirmed the transpressional character of the deformation.
Microtextural relations indicate that shear deformation occurred during and after the reaching of peak metamorphic conditions at low and medium-grade rocks respectively (M1 metamorphic episode; Franceschelli et al., 1989). Temperature and pressure estimates and P-T path suggest that deformation caused by the transpressional shear zone was responsible for the decompression and exhumation of the low to medium-grade metamorphic rocks and could explain the observed telescoping of the metamorphic isograds.
After the exhumation of the medium-grade micaschists and gneisses up to higher structural levels, they finally underwent extensional tectonics, characterized by collapse folds and brittle low-angle shear zones.
The transpressional shear zone characterized the collisional history of this segment of the Variscan chain. It was probably active between 350-340 Ma (age of the amphibolite facies metamorphism) and nearly 300 Ma (age of extensional collapse). The activity of a transpressional shear zone in northern Sardinia fits well with the reconstruction of the indentation between Armorica and Gondwana of Dias & Ribeiro (1995). According to these authors during the Carboniferous, a dextral transpression begins to predominate in the Armorican branch of the arc, to which Sardinia was connected (Matte, 1986).
Carmignani L, Carosi Ret al, Geodinamica Acta, 7, 31-47, (1994).
Dias Rand Ribeiro A, Tectonophysics, 246, 113-128, (1995).
Franceschelli M, Memmi Iet al, Geol. Soc. London Spec. Publ, 43, 371-375, (1989).
Matte P, Bull. Soc. géol. Fr, 8, 9-24, (1986).
U-Pb age data are presented for zircon, monazite and rutile obtained from igneous (metagabbro, metagranite and orthogneiss) and metasedimentary rocks of the uppermost allochthon of the Ordenes Complex (Variscan belt of NW Spain). The new data indicate that this terrane, located in the hangingwall to the Variscan suture, was involved in a pre-Variscan tectonothermal event.
Zircon from metagabbro (Monte Castelo Gabbro, MCG) indicate a protolith age of 499 ± 2 Ma (this and the following ages are reported at 2<sigma>). The MCG presents field evidence of a post-intrusion granulite facies dynamic metamorphism, and the U-Pb system in zircon remained unaffected during this event, as is indicated by the concordant character of the analysed mineral fractions. Igneous zircon and monazite from a metagranite and orthogneiss yielded similar ages of 500 ± 2 Ma, indicating that felsic and mafic magmatism are contemporaneous in the uppermost allochthonous unit.
Concordant analyses of monazite from three samples of granulite and amphibolite facies metasedimentary rocks (Ordenes Series) yielded ages at 493 ± 1 Ma, 496 ± 3 and 498 ± 2 Ma, very close to the magmatic ages presented previously. Rutile from the same metasedimentary rocks indicate cooling below 450º C at ca. 390 Ma. Monazite ages from metasedimentary rocks are interpreted as the age of a regional metamorphism immediately following the igneous intrusions, in accordance with two facts: 1) metasedimentary rock-samples were collected, in some cases, far away from the Ordovician plutons; therefore monazites do not seem relics of contact metamorphism; 2) temperatures of granulite facies metamorphism are clearly above the closing temperature of the U-Pb system in monazite and, therefore, the age of monazite from metasedimentary rocks of the granulite facies can not be older than metamorphism.
The Barrovian character of the metamorphism clearly points to crustal thickening and, consequently, reveals a compressional tectonic event. Taking into account the chemical signature of the gabbros and the flyschoid character of the metasediments, and also that magmatism and metamorphism are almost coeval, the uppermost units of the allochthonous complexes in the Iberian peninsula, occurring above the ophiolites marking the suture, are considered pieces of an Early Ordovician accretionary complex. This implies a relationship with a convergent plate margin, and suggests a volcanic arc as the more probable setting for these uppermost units.
In the Iberian Massif (Spain), the Tormes Gneissic Dome is a Variscan metamorphic complex that comprises an upper amphibolite facies core (Lower Unit) separated of the overlaying lower grade rocks (Upper Unit) by a major extensional detachment (Escuder Viruete et al., 1994, 1997). Despite high metamorphic temperatures during thermal peak (near 700°C), metapelites from the highest levels of the Lower Unit contain garnet with well preserved growth zoning. These rocks were used for reconstruction of quantitative P-T paths based upon interpretation of the microstructural evolution of fabrics, thermobarometry, and thermodynamic modelling of garnet zoning. The results are consistent with a two-stage tectonothermal evolution under high-grade conditions: (a) an early D1 compressional phase of deformation that led to upper amphibolite facies barrovian-type metamorphism and to P and T increase during pre-kinematic garnet core growth, from 7±0.5 kbar and 600°C to 9±0.5 kbar and 700-725°C, approximatively; (b) a subsequent major D2 extensional phase of deformation that led to quasi-isothermal decompression from 8-9 to 3±0.5 kbar at T conditions between 700-740°C, registered in syn-kinematic garnet rim. Several lines of structural, textural and chemical evidence suggest that up to 15-20 km of overburden was removed from the Lower Unit by tectonic exhumation while these rocks were still at upper amphibolite facies conditions. A final stage of quasi-isobaric cooling to greenschist facies conditions is locally recorded in late D2 low-grade detachments. The changes of P-T segments can be interpreted as documenting the burial and exhumation history of the Central Iberian Zone during Upper Paleozoic.
Viruete J, Arenas R & Martinez Catalan JR, Tectonophysics, 238, 117-138, (1994).
Escuder Viruete J, Indares A & Arenas R, J. Metamorphic Geo., 15, 645-663, (1997).
The northen boundary of the Central Iberian Zone in the Truchas basin (TB) is controlled by a synsedimentary extensional fault, the Villavieja Fault (VF). This fault worked as normal fault at least during the Upper Ordovician, deepening the TB and developed a crest-rollover anticline along which a carbonate platform was deposited, the Aquiana limestone; moreover the siliciclastic Agueira Fm. was deposited in the half-graben basin.
The geometry of the VF was easily reactivated at depth during the Variscan Orogeny, but the steeper part of the fault could not reactivate, leading to the development of the Valdueza short-cut thrust.
Microstructural analysis points to a strong rheological contrast between limestone, quartzites and conglomerates, and suggests that the inversion occurred at the brittle-ductile transition in greenschist metamorphic conditions. Axial strain ellipse ratio was calculated by the Fry and "aspas" normalized methods using a GIS and values close of 3 were found in limestone with coarse grain-size (<sigma>= 16,154-22,088µm2). Differential stress of ca. 290 Mpa was estimated using the calibration of Rowe and Rutter (1990). At this strain conditions intracrystalline plasticity was the most important deformation mechanism in limestones, in which a well-defined c-axis fabric and a strong e-plane orientation is found. The inverse pole figures (IPF) present a maximum close to the e and r position and confirms the low temperature strain conditions during the inversion process. On the other hand, quarzites and conglomerates were mainly strained by pressure solution and microcathaclasis and the girdles of the c-axis fabrics are not well defined. The internal strain ratio at these lithologies has values of only 1.5 or less.
The strain intensity and conditions change progressively to the structural prolongation of the VF to the North, following the Asturian Arc macrostructure, in the zone when the VF is known as Viveiro Fault. In this zone, the fault has a more important vertical normal slip (>8 km) and in the hanging wall presents a middle-high metamorphism that exhibits a mainly plastic rheological behaviour and homogeneous strain, with its involved dynamic recrystallization processes.
Rowe, KG & Rutter, RH, J. Struct. Geol, 12, 1-17, (1990).
The Lora del Rio metamorphic core (LRMC) is located at the southeastern border of the Ossa-Morena Zone of the pre-Mesozoic Iberian Massif. It is overlain by unconformable Tertiary successions of the Guadalquivir basin.Its Variscan tectonic evolution began with the first Variscan deformation phase (±380 Ma ago), during which large nappes were generated that involved the Cadomian basement and led to notable crustal thickening. Later, a complex extensional event (±340 Ma) occurred (Apraiz, 1998). It comprised generalized ductile deformation related to the development of a E-W-trending, low-angle normal shear zone with burial of the N block. After a short time span during which the footwall block underwent limited uplift, a second low-angle N-S trending extensional ductile structure was generated with the W block being the hangingwall. This feature and the lateral inestabilities related to differential uplift of the footwall, splitted the latter block into two. Thus, the development of the two low-angle extensional shear zones led to partition of the Lora del Rio extensional region into three blocks representing different structural levels and recording contrasted tectonometamorphic histories. The uppermost block exhibits structural and metamorphic features that vary depending on the footwall block (intermediate or lowermost) with which it is in contact. Higher grade metamorphism and generalized deformation are widespread along the eastern area, in the contact with the LRMC (lowermost block). The intermediate block exhibits a typical hangingwall block metamorphism due to thermal heat conduction from the lowermost unit across the second low-angle shear zone, but most of the extensional structures recognized relate to the first low-angle ductile shear zone. Finally, the lowermost block, affected by high-grade metamorphism (high-T-amphibolite and low-P granulite facies), experienced a rapid ascent favored by the simultaneous operation of the two low-angle shear zones.Passing of the Lora del Rio core across progressively shallower structural levels led to deformation localization within gradually thinner bands. These bear gentler dips and cut across previous extensional structures that were verticalized in the course of doming associated to uplift of the LRMC. The extensional process was still active when the LRMC reached relatively shallow crustal levels, as demonstrated by the nucleation of brittle-ductile normal faults dipping 60-70° that postdate the low-angle extensional features and accommodate the final ascensional processes in the LRMC.
Apraiz, A, Unpublished thesis, 575 pp, (1998).
The Aracena metamorphic belt (AMB), southern Spain is a subduction related HT/LP complex developed by plate convergence during the Paleozoic. The AMB is subdivided into two major zones: an oceanic domain (OD) to the south and a continental domain (CD) to the north, which includes high grade metamorphic rocks (tonalite gneisses, migmatites, nebulites, amphibolites, calc-silicates and marbles) and igneous rocks (granodiorites, trondhjemites and boninites) related to the subduction of an oceanic ridge (Castro et al, 1996). We have noticed in The Aracena metamorphic belt the presence of granodiorites and trondhjemites whereas peraluminous granites are very scarce. Melting experiments have been performed on tonalitic gneiss of the Fuente de Oro formation from the AMB. These tonalitic gneisses (Bt, Qtz, Pl ± Kfs ± tourmaline) appear migmatised and are the most probable source rock for surrounding anatectic granodiorites of the AMB. Wet and dry melting experiments have been performed. Experiments with added H2O were performed at 650, 675, 700, 725 and 750ºC, and 2 and 6 kbar whereas experiments with no added H2O were performed at 850, 900 and 950ºC, and 3, 6 and 10 kbar. All experiments were done in solid-medium piston cylinder apparati at the University of Huelva (Spain), in 12,7 mm cell assemblies. Melting experiments with no added H2O generate peraluminous granitic melts, whereas melting of the same material with 5% added H2O generates granodioritic melts that coexist with neoblastic amphibole (hornblende) crystals (amphibole does not appear in the starting material). Thus, we can conclude that the generation of granodioritic rocks in the AMB needs of free H2O in the system. The predominance of granodioritic rocks over the peraluminous granites in the AMB indicates that H2O is largely present in the system. It's very likely that the source of this H2O is the subduction of an oceanic plate. The subsequent ridge subduction (Castro et al, 1996) and the associated thermal anomaly would generate the heat responsible for the partial melting of the tonalitic gneisses.
Castro AD, Fernandez CR, De la Rosa JD, Ventas IM, El Hmidi H, El Biad M, Bergamín JF & Sánchez N, Geol Rundsch, 85, 180-185, (1996).
The ophiolitic units in the allochthonous complexes of NW Iberia represent a suture inside the Variscan belt (Martínez Catalán et al., 1997). Their analysis may inform upon the geological evolution and dynamics of the Rheic oceanic realm, and also upon the tectonic evolution of the orogen. The Careón ophiolitic unit outcrops in the SE of the Ordenes complex, in NW Spain, and its study has revealed new data concerning the earliest subduction-related tectonothermal events, that were obscured by subsequent collision tectonics, and also about the exhumation of the ophiolite.
The most complete ophiolitic section consists of a mantle section composed of harzburgites and dunites, above which lies a crustal section formed by numerous small intrusions of coarse-grained gabbros and gabbroic pegmatoids. The whole has been intruded by a great number of gabbroic dikes, pegmatoid gabbros and diabase dikes. These features would be characteristic of ophiolites of the LOT type, formed in slow spreading environments with episodic magma chambers (Nicolas, 1989).
The U-Pb zircon age of the Careón ophiolite (395 Ma) indicates that the oceanic crust was still being formed by the Early Devonian. Convergence processes took places 5-10 Ma later, giving rise to a mantle-rooted thrust system, synthetic with the westward polarity of the Variscan subduction. In the ultramafic sheets, it can be recognized a mylonitization progressive toward the thrust plane, and operating under decreasing temperature conditions. Metamorphic soles developed at the footwall to some sheets, and are characterized by a sharp decrease of metamorphic gradient downward, from garnet amphibolites attached to the thrust plane to epidote-amphibolite facies rocks below. Corundum-rich metabasites have been locally observed and attest to heat transfer from the overriding ultramafic sheet. Thermobarometric estimations at the metamorphic sole (T=650°C; P=11,5 kbar) point to a subduction setting for thrust generation.The subsequent flattening of the subduction plane allows the exhumation of the ophiolite and the HP rocks situated below. It and can be produced through the continuous development of extensional detachments in the upper part and understacking of the continental margin of Gondwana in the lower part of the Variscan accretionary wedge.
Martínez Catalán, J.R. et al., Geology, 25, 11, 1103-1106, (1997).
Nicolas, A, St. of Ophiolites. Kluwer Academic Publishers, 4, 367, (1989).
The Cantabrian Zone constitutes the northern foreland of the Iberian Massif. This is a Variscan, arcuate fold and thrust belt of non-metamorphic Paleozoic rocks. Variscan magmatism in this zone is scarce and is restricted to small, discrete, post-tectonic, shoshonitic to calc-alkaline plutonic complexes and post-orogenic shoshonitic volcanic rocks. Preliminay U-Pb dating of representative rocks types suggest the presence of two separate magmatic pulses.
The first pulse at c. 303 Ma has shoshonitic character. It is restricted to the volcanic rocks of the Villaviciosa Basin (303+7/-6 Ma andesite, Niao Unit) and the neighbouring sub-volcanic intrusive complex of Infiesto (c.302 Ma age; Opx-Cpx-Bt quartz monzodiorite), in the central portion of the Cantabrian Zone.
The second pulse covers the 297-290 Ma interval. It is present in both ends of this zone (100 km apart): the eastern Palentinian Zone (292+2/-3 Ma Zrn and Ttn, Peña Prieta granodiorite) and the Salas-Belmonte belt (293±2 Ma Mnz, sub-volcanic Opx-Cpx-Bt quartz monzodiorite, Courio; 297±6 Ma Zrn and 292-294 apparent U-Pb Ttn ages, Arcellana granodiorite). This magmatism is H-K calc-alkaline to calc-alkaline in character. This pulse has also been dated 80 km west, in the neighbouring West-Asturian Leonese Zone (c. 290 Ma Salave granodiorite). All these rocks are closely associated with most of the gold-bearing mineralization in the Asturian gold belt.
Zircon inheritance in the Courio Opx-Cpx quartz monzodiorite indicates assimilation of rocks with Late Precambrian zircons. This suggests a complex petrogenesis for these rocks possibly involving variable assimilation-crystal fractionation of modified, mantle-derived potassic and high-aluminous mafic magmas with upper crustal rocks. The late Variscan magmatism in the Cantabrian Zone coincides with widespread post-tectonic granitic plutonism at c.295 Ma in the hinterland (e.g., Central Iberian Zone). The trigger of this magmatic pulse is uncertain but appears to be a relatively large scale process involving partial melting of mantle lithosphere (lithospheric delamination ?).
Acknowledgements: Research carried out as part of CICYT project PB-94-1338.
A new computer method was developed to document 3D preferred orientation patterns of internal porphyroblast foliations, and was applied in a Variscan thrust complex exposed in five, synformal high-grade metamorphic massifs (tectonic klippen) of NW-Iberia. An ophiolitic unit in these massifs separates an upper Laurentia-derived domain from a basal unit thought to represent the subducted and subsequently exhumed margin of Gondwana (Martinez Catalan et al., 1996). Mineral inclusion trails in albite and garnet porphyroblasts of this unit are the only relicts of a prograde subduction-related metamorphic assemblage (Arenas et al., 1995). Computer analysis of over 6000 inclusion trail orientation data collected in different planes reveals a particular spatial arrangement of internal foliations in two regional fans with horizontal fan axes. Both fans correspond to porphyroblasts populations of different age and are inferred to record a change in tectonic transport direction, from early N-S (overlapping the subduction event), to later SE-NW. Porphyroblast orientations are spectacularly consistent in a strongly curved segment of the Variscan belt, despite the younger age of this orocline, and indicate that the involved large-scale rotations were induced by internal crustal distortions, not by rigid-block rotations. In such a context, stable positions of rigid porphyroblasts relative are predicted by general deformation-partitioning models for metamorphic rocks. Consistent with the pre-oroclinal trends recorded by porphyroblasts, a younger NE-SW trending orogenic zone is envisaged, which overprinted a previously E-W trending orogen, and induced heterogeneous rotations proportional to newly accumulating compressive or transpressive strain. Detailed structural mapping in the Morais massif and its immediate autochthonous basement, provides evidence for a second major change in bulk compression direction, in the form of previously unrecognized fold-interference patterns, different generations refolded thrusts and/or detachments, and preferred orientations of younger porphyroblast fabrics than the one in the allochthonous massifs. The microstructural and field data combined witness a complex succession of Variscan folding, thrusting, and foliation development in NW-Iberia, rendering inadequate the traditional three deformation scheme. The significance of this polyphase deformation might be elucidated by the pronounced preference of internal foliations for subvertical or subhorizontal positions evidenced in the analyzed samples combined. Similar orthogonal orientation patterns have been demonstrated systematically in other orogenic belts as well, are thought to reflect orogenic histories characterized by alternating crustal compression and extension (gravitational collapse). In NW-Iberia three major kinematic stages are characterized by different bulk compression- or plate-motion directions. However, each of these stages embraced multiple generations of axially-arranged foliations, folds and thrusts, which are thought to reflect dynamic interactions between plate forces (compression and folding) and gravity (extension and gravitational thrusting) during constant plate motion.
Martinez Catalan JR, Arenas R, Diaz Garcia F, Rubio Pascual F, Abati J & Marqunez J, Tectonics, 15, 106-121, (1996).
Arenas R, Rubio Pascual FJ, Diaz Garcia F & Martinez Catalan JR, J. Metam. Geol, 13, 141-164, (1995).
The calc-alkaline granitoïds from Oranie bear evidence of spectacular mingling/mixing mecanisms. Their genesis has been investigated through petrological and geochemical studies on the enclaves and the host rocks of the Tifrit complex (Saïda). This late-Hercynian intrusives, emplaced within Silurian to Visean sedimentary series, include mainly biotite + amphibole granodiorite with minor monzogranite and cordierite bearing granite and with some late quartz microdiorite, rhyodacite and rhyolite dykes. No related ultramafic and mafic were found. The biotite+ amphibole dominant facies display I-type calc-alkaline and metaluminous features. They enclose numerous mafic microgranular enclaves which show many evidences of mingling/mixing processes: biotite-hornblende clots, acicular crystals of apatite, patchy zoned plagioclase and poïkilitic K-feldspar. Their modal composition vary from quartz diorite to quartz monzonite composition reflecting various stages of mixing between mafic and felsic poles. In the scope, two successive stages of crystal growth have been observed : (i) a pre-mixing assemblage with An-rich plagioclase (An70-82), core of K and Ti-rich of zoned hornblende, and biotite-hornblende clots (after Cpx transformation). This suggests that the mafic melt was below the liquidus conditions at the time of mixing; (ii) a syn to post-mixing assemblage with An19-49 plagioclase, magnesiohornblende, biotite, K-feldspar and quartz. The mineral chemistry is similar for both this late assemblage in enclaves and that of the host granodiorite, this attests that the post-mixing thermal re-equilibration was reached.Compared to host rocks, enclaves display similar or higher contents in Na2O, K2O, Rb, Nb, Y, Zr and REE. The low Sri (0.7043 to 0.7065) and high <epsilon>Nd (-2.5 to -2.97) in the enclaves and their host rocks respectively, indicate that isotopic equilibrium was attained.These features reflect either a primary magmatic characteristic and secondary processes related to hybridization.These results suggest that the late to post-collisional Hercynian magmatism involved both (probably enriched) mantle and crustal sources and very complex evolution through mixing and mingling mechanisms.
A thick passive margin series at the NW edge of the Saharan craton is involved in Late Variscan (post Visean) collision tectonics. Paleozoic cover rocks of up to 10 km thickness are folded with deformation intensity fading out toward the SE. Thrust faults and major detachments are conspiciously absent and folds lack any systematic craton- oriented vergence. Roughly concordant fold-trains of distinct wavelenghts and amplitudes are developed within Devonian ("Richs"), Ordovician ("Bani") and Cambrian series. Precambrian basement crops out in large scale basement domes, or so called "plis de fond", apparently "folded" at a wavelength of several 10's of kms and with amplitudes of up to 10 km. The classic interpretation of a thick skinned folding of the entire series, including basement, can be viewed as a pure shear deformation of most if not all of the continental crust, with an estimated 30% horizontal shortening and essentially vertical extension. In this interpretation, buckle-folds of different wavelengths would have developed in response to the relative thickness, competency and alternation of different layers in a "multilayer". Alternatively, the cover series may have been folded in a thin skinned manner above a hitherto undiscovered basal décollement and multiple splays ramping up through the thick Paleozoic series from NW to SE. The most external deformation of the Antiatlas belt is seen in the Jbel Ourkziz, a rectilinear monocline dipping SE-ward into the Tindouf-basin foreland. We interpret this monocline as a frontal triangle structure, riding above a blind thrust probably within Silurian shales. This view is corroborated by the observation of tectonic stylolites indicating a horizontal shortening on the order of 10% within the gently tilted Carboniferous carbonates of the Jbel Ouarkziz. The high angle of such small scale deformation structures with respect to the fold axes trend, the cylindricity of folds, their gently dipping fold axes and lateral continuity, together with the lack of any positive evidence in form of outcrop scale faults are at odds with a postulated major strike slip deformation zone along the NW border of the Jbel Ourkziz (The Bas Drâa valley). In a thin skinned interpretation of the Antiatlas fold belt, the Precambrian basement inliers represent a slightly later, second phase of thick skinned deformation superimposed on the former thin skinned fold belt. The Antiatlas of Morocco has a strikingly high thickness/width ratio. In terms of critical taper geometry this points to a high wedge angle, probably due to the scarcity or absence of suitable décollement horizons within the Paleozoic sedimentary series.
Many studies have shown that geochemical and Nd isotope data are a powerful tool in provenance analyses. Especially mantle additions of juvenile material to older crust can be recognized. We applied these methods to elucidate the evolution of the western Paleozoic margin of Gondwana which is still under debate. If juvenile or exotic material was added to that margin, the sedimentary record should preserve this signature. The Late Proterozoic to Early Cambrian Puncoviscana Fm. and the Upper Cambrian Meson Group are used to establish a record of the margin prior to the Ordovician. Their <epsilon>Nd values are fairly uniform ranging from -10.0 to -14.8 (mean -11.6). Calculated average crustal residence ages range from 1.7 to 1.6 Ga. The Cambrian Snt. Rosa de Tastil granite has an <epsilon>Nd of -9.4, only slightly less negative than the <epsilon>Nd of the Puncoviscana Fm. into which it intruded. The northern Puna is divided in an eastern (Sierra del Cobre) and western part. Early Ordovician sedimentary rocks of the eastern part of the Northern Puna have <epsilon>Nd values indistinguishable from the underlying units (-10.9 to -11.7). A pillow basalt that intruded these sedimentary rocks has a juvenile signature with <epsilon>Nd of -1.2. We explain these pillows as intruded in an extensional intraplate setting. Two samples of the Faja Eruptiva de la Puna Oriental have <epsilon>Nd values of -9.8 and are identical within analytical error to the older Sta. Rosa de Tastil granite. Middle Ordovician samples of the eastern part have <epsilon>Nd values of about -13 indicating derivation from slightly older sources rather than the addition of juvenile material. Volcanoclastic rocks were deposited during the Arenig in the western part of the N-Puna. These Volcanosedimentary Successions are overlain by the Puna Turbidite Complex. Only the rocks of the Volcanosedimentary Successions show clear input of a juvenile component (<epsilon>Nd -7.1 to -10.8). The samples of the Puna Turbidite Complex do not show this juvenile component. They appear to be derived from old crustal sources indistinguishable from the one(s) of the Puncoviscana Fm. or Meson Group (<epsilon>Nd -11.9). Furthermore, we analyzed Devonian and Permian rocks of N-Chile. They can also be mostly explained by reworking of older crustal material of Gondwana origin. The presence of a juvenile source is documented for only a restricted area and time interval. No addition of an exotic component is recognized as may be expected if a terrane had been added to the Gondwana margin.
The Dabie Mountains (China) contain the largest distribution of ultra high pressure metamorphic (UHPM) rocks in the world. Coesite-bearing metamorphic rocks are found as disrupted lenses, layers or blocks within apparently low-pressure gneisses. The lack of clear pressure indicators in the gneisses has fueled a long-standing controversy regarding the "foreign" or "in-situ" tectonic relationship between coesite-bearing metamorphic rocks and their country gneisses. Multi-chronometric approach was employed to tackle this issue for two prominent localities at Bixiling and Shuanghe.
The mineral parageneses of the gneisses correspond to the retrograde metamorphism under the amphibolite facies. No UHPM mineral relics are observed. Ion probe U-Pb zircon ages for the granitic gneisses and coesite-bearing eclogites indicate a Triassic age (ca. 230 Ma) for the UHPM event at both localities. The zircon ages coincide with our Sm-Nd garnetages of 229 to 234 Ma for the gneisses. The Rb-Sr phengite-WR and biotite-WR ages (170 to 210 Ma) of the gneisses are slightly younger than or marginally overlap with the Sm-Nd garnet (= 220 Ma) and Rb-Sr phengite ages (198 to 223 Ma) obtained on coesite-bearing eclogites from the Bixiling Complex (Chavagnac and Jahn, 1996). Thus, the host gneisses underwent both a peak event and a retrograde evolution at the same time as the coesite-bearing eclogites. Consequently, the coesite-bearing eclogites have an "in-situ" relationship with the country gneisses.
However, the Ar-Ar phengite and biotite data show three complications due to the effects of alteration, excess Ar and possible disturbance of the Rb-Sr system. Ar-Ar mica ages (212 to 194 Ma) are found older than Rb/Sr ages on the same sample for all cases. The conflicting Ar-Ar and Rb-Sr mineral ages mean that a simple estimate of the cooling paths of these UHPM rocks in terms of thermochronology is not accurate, as chemical reactions and mass transfer played a key role during the retrograde exhumation path.
Chavagnac V, Jahn B-M, Chem. Geol, 133, 28-51, (1996).
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