PERMO-CARBONIFEROUS RIFTING IN EUROPE (PCR) is a multi-disciplinary research project, involving academic and industrial research groups in the UK, Norway, Germany, Spain and the Netherlands, whose aim is to further our understanding of the geodynamics of Permo-Carboniferous rifting and associated magmatism within the northern foreland of the Variscan orogenic belt. The project is funded by a European Commission TMR Research Network grant from 1997-2001.
During the Permo-Carboniferous northern Europe experienced a major episode of extensional tectonics, associated with widespread magmatic activity. Rifting and associated magmatism were associated with a fundamental change, at the Westphalian-Stephanian boundary, in the regional stress field affecting Western and Central Europe. This change was coincident with the termination of orogenic activity in the Variscan foldbelt, followed by major dextral translation between N. Africa and Europe. A regional hiatus, corresponding to the Early Stephanian is evident in much of the Variscan foreland, with Stephanian and Early Permian red beds unconformably overlying truncated Westphalian series. This coincides with the onset of voluminous magmatism across the region, suggesting that uplift may have been related to a widespread thermal anomaly within the upper mantle i.e. a mantle plume (or several plumes). The resultant elevated geothermal gradients would have had a significant influence upon the rheology of crustal extension and upon the thermal evolution of the lithosphere. This may have important implications for the thermal maturation of hydrocarbon source rocks within the region. Rifting and associated magmatism are also coincident with the onset of a major reverse polarity super-chron of the Earth's magnetic field which lasted from 310-265 Ma.
The most important objectives of the project are to:
* determine the relationship between the onset of magmatism, regional uplift and extensional tectonics.
* date by high quality Ar-Ar and U-Pb zircon methods, the chronology of magmatic events
* evaluate the role of thermally anomalous mantle plumes in the petrogenesis of the magmas and the excess mantle temperature via geochemical, He-Ne-Ar and Sr-Nd-Pb isotopic studies of the most primitive mafic magmas erupted within the province.
* constrain the onset and magnitude of basement uplift and thermal subsidence across the area, in order to evaluate the magnitude of any thermal pulse.
* integrate petrological/geochemical data with both onshore and offshore geophysical (seismic, gravity, magnetic) and geological data to understand the geodynamics of the rifting process and its associated magmatism.
* model the thermal evolution of the lithosphere in the northern foreland of the Variscan orogen during Permo-Carboniferous times.
During the Carboniferous and early Permian the Variscan foreland of northwestern Europe was characterised by widespread regional extension, often accompanied by magmatic activity. Throughout this period, volcanism, dyke and sill emplacement and intrusion of breccia pipes occurred at various stages in an area stretching from Ireland to England, the Midland Valley of Scotland and further east into the North Sea, the Oslo Rift, southern Sweden and the Northeast German Basin.
Although the geochemistry of some of the larger igneous complexes (e.g. Oslo Rift) is fairly well constrained, the crystallisation ages and geochemical characteristics of many important magmatic occurrences remain poorly known. These data are required in order to (a) establish the relationship between the onset of magmatism, regional uplift and extensional tectonics, (b) model the thermal evolution of the lithosphere in Permo-Carboniferous times, (c) determine the relative contributions of mantle and crustal source components in the petrogenesis of the magmas and (d) elucidate the role of extension-induced passive melting of mantle reservoirs versus the possible input of mantle plume material.
A program of Ar-Ar dating of stratigraphically and tectonically well-constrained samples is presently focused upon Dinantian volcanics and intrusions in southern Ireland (e.g. Limerick Basin, Croghan Hill), and Late Carboniferous to Early Permian sills and dykes in northern England (e.g. Whin Sill), Oslo Region, southern Sweden and sills from boreholes on Rügen island (Germany) and in NW Poland. Geochemical and Sr-Nd isotope studies are carried out on the most primitive magmas for mantle reservoir characterisation.
Careful selection and screening of samples is necessary for Ar-Ar dating to minimise the effects of alteration. During the Dinantian magmas were often erupted sub-aqueously or were intruded into wet calcareous sediments, causing appreciable alteration and element mobility. In addition, in SW Ireland these magmatic rocks were subsequently deformed and weakly metamorphosed during the Variscan orogeny. The K-Ar systematics of these samples are disturbed as is shown by the anomalously young K-Ar whole rock ages for a lower Visean trachyte flow in the Limerick Syncline (292 ± 9 Ma) and 245 ±7 Ma and 250±7 Ma whole rock ages for a phonolite and a trachyte intrusion, respectively, on the Beara Peninsula. In contrast, during the Late Carboniferous-Early Permian dykes and sills were intruded into consolidated sediments or Precambrian crystalline rocks. Although some evolved dykes carry abundant large plagioclase phenocrysts, there is a risk that these may contain excess Argon derived from partial outgassing of the Proterozoic host rocks. In many cases therefore, it is necessary to date acid leached whole rock or groundmass samples in combination with dating of the phenocrysts.
SHRIMP ages of 12 volcanic samples indicate intense magmatic activity to have occurred at the Carboniferous/Permian boundary throughout much of the NE German Basin. Rhyolitic crystal-rich samples have been taken from quarries in the Halle Volcanic Complex (HVC) and in the Flechtingen Ignimbrite, and from drill cores of the Kotzen, Mirow, Friedland and Penkun areas. 10 samples yielded 206/238 ages between 302 and 297 Ma (± 3 Ma, respectively) which indicate that the magmatic activity took place concentrated in a relatively short time span throughout much of the NE German Basin. Two HVC samples have ages of 307 and 294 Ma.This remarkably synchronous magmatic activity occurred during the initial phase of the basin development. The large volumes (approx. 40 000 km3) of predominantly calc-alkaline SiO2-rich magmas presumably formed during anatexis and magma mixing in an intra-continental transtensional setting (Benek et al. 1996). The basaltic magma which must have provided the thermal input into the lower crust probably formed during extension of lithospheric mantle fertilised by previous magmatic processes, a magma generation mechanism similar to that proposed by Harry and Leeman (1995) for the Tertiary Basin-and-Range Province in the W USA. The dated volcanic rocks formed in three geotectonic provinces, namely the (i) Mid German Crystalline Rise, which forms part of the Central Variscides, the (ii) External Variscides and the Variscan foreland which is considered as (iii) Eastern Avalonia. Many of the old zircons found in the HVC samples reflect the magmatic activity of the Mid German Crystalline Rise (325-400 Ma). However, Cadomian (500-650 Ma) and older Gondwanian elements (1254 and 2065 Ma) are also present. Allthough the Flechtingen, and Kotzen areas are located within the External Variscides, it seems more likely that the middle to lower crust, that experienced anatexis, formed part of the Avalonian Plate overriden by the Variscan Front. Thus the old zircons found in Flechtingen (350 and 538 Ma) and Kotzen (345 Ma), but certainly those present in the Penkun (1261 Ma) drill cores represent Eastern Avalonia geotectonic history. The anatectic magmas in the Friedland area probably formed partly from Baltic crust thrusted below Eastern Avalonia during the Caledonian Orogeny. Among the old zircons from the Friedland samples, in addition to Proterozoic ages (1456 Ma), we found testimony of Caledonian (443 Ma) and of clearly post-Caledonian (345-387 Ma) magmatism.
Benek R et al, Tectonophysics, 266, 379-404, (1996).
Harry DL & Leeman WP, J. Geophys. Res, 100, 10255-10269, (1995).
The association of mafic dyke swarms with large igneous provinces is well documented, with the distribution of the dykes often radial to the focus of magmatic activity e.g. Deccan. Magmatic activity in northern Europe during Permo-Carboniferous times is comprised of extrusive lavas, dyke swarms, and intrusive sill complexes. In this study we focus on a number of dyke swarms from Scotland, Norway and Sweden combining geochemical and geophysical data in order to ascertain whether the dykes are associated with a central plume. The geochemical variations evident are discussed in terms of possible source heterogeneities and/or higher level processes. In addition the influence of the prevaling stress fields is addressed in terms of effect- facilitating melting and affect - plume presence. The Permo-Carboniferous dyke swarms of Scotland number over 3000 individual dykes with local crustal extensions in the order of 3 - 4% (Morrison et al., 1987). British dykes vary in orientation from NE-SW during the early to mid Carboniferous to E-W in the late Carboniferous. NW/NNW trending dykes characterise the Permian. The chemical composition of the dykes is variable but overall temporal changes from alkaline to tholeiitic to alkaline are evident (early Carboniferous - late Carboniferous - Permian respectively). Norwegian dyke swarms are less well constrained but dykes from north and west of the country have been dated between lower Visean and Permian. These dykes are orientated NW-SE to N-S respectively. The geochemistry of the extensive dyke intrusions in both Norway and Sweden is not well known and so following field work 30 samples have been analysed for major, trace- elements and isotopic signatures. These samples originate primarily from southern Norway and Sweden and show similar stress patterns to those evident in Scotland, with the N-S oriented dykes cross-cut by E-W trending dykes. Characterisation of the possible source or sources of these dykes will be discussed together with the spatial, temporal and compositional variations observed in N. Europe.
Morrison, MA, Hendry, GL & Leat, PT, Trans. R. Soc. Edin. Earth Science, 77, 279-288, (1987).
Thousands of dykes of various thickness and length penetrate the granites and gneisses of the Precambrian basement and the overlying Lower Palaeozoic sediments in Scania, southernmost Sweden. They form a large WNW-ESE to NW-SE trending dyke swarm in the Sorgenfrei-Tornquist Zone and manifest an extensional tectonic period in the lithosphere at the southwestern margin of the Fennoscandian Shield. Most of the dykes are diabases, but there are also minor lamprophyric intrusions and some trachytic dykes. Palaeomagnetic studies (Bylund, 1974) and K-Ar dating (Klingspor, 1976) indicate a Permo-Carboniferous age.
In spite of a slight crustal contamination of the magmas, major and immobile elements clearly classify the diabase dykes as tholeiites. Fractional crystallization of the tholeiitic magmas from basalts to basaltic andesites is mirrored in bivariate element plots where elements sensitive for fractionation show strong correlation with the Mg#. In tectono-magmatic discrimination diagrams the dykes show an affinity to typical continental tholeiites, produced under conditions of an intra-continental rifting. Enrichment of the incompatible elements and REE further supports their legitimate appearance in such rift environment. The distribution of REE, Th, Ta and Y also shows that the tholeiitic magmas have been generated from an enriched garnet-lherzolitic mantle.
Geochemically the lamprophyres are very heterogeneous. They are alkaline and range from camptonites to basaltic camptonites and alkali-olivine basalts. In comparison with the tholeiites their content of the LREE is much higher indicating a low degree of partial melting. This property supports not only their alkaline character but it is also in harmony with the assumption that they represent a period of initial continental rifting. On the other hand, the very high concentrations of Ba, Sr, Nb and volatiles (CO2) indicate mixing processes with material from the lower crust. The hybridization of the mantle melts resulted finally in the formation of trachytic intrusions, the so-called kullaites.
The dyke swarm is intimately associated to the extension of the crust between S-Sweden and NE-Germany. The lithospheric stretching has induced pressure-release melting of the ascending mantle material. Evidently, the alkaline lamprophyric melts have been produced first at a very low degree of partial melting. Prolonged upwelling of the asthenosphere promoted increasing extension in NNE-SSW to NE-SW and generation of tholeiitic melts at a higher degree of partial melting. Subsidence south of the Fennoscandian border zone has led to the formation of the North-German basin and to contemporary rise of the asthenospheric mantle. Sills, dykes and effusive lava flows have been discovered in boreholes drilled on the German island Rügen. Interestingly, their geochemistry is very similar to those of ocean-floor tholeiites (Kramer, 1988). It can be concluded that they were generated from a depleted mantle source at relatively low depths and mark probably the rift-centre.
Bylund G, Geol. Fören. Förh, 96, 231-235, (1974).
Klingspor I, Geol. Fören. Förh, 98, 195-216, (1976).
Kramer W, Schriftenr. Geol. Wiss, 26, 136 pp, (1988).
When extensional stresses in the lower Carbonferous produced a series of faulted basins across the north-west of the British Isles, lithospheric attenuation and consequent mantle decompression triggered mantle melting and magma generation. Early eruptions were dominantly of transitional alkali basalts; trace element data suggest that these originated as small melt fractions (<5%) from garnet-facies peridotites. Subaerial lava successions up to 1 km thick accumulated in the Limerick basin (SW Ireland) and in the western part of the Scottish Midland Valley. Other notable, but thinner, successions erupted close to the margins of the Northumberland basin (northern England - southern Scotland) and in central and eastern parts of the Scottish Midland Valley (SMV). In these Dinantian successions there is a tendency for the lavas to become increasing differentiated (to hawaiites and mugearites) with time as magma productivity decreased and repose periods lengthened. Small volumes of trachytes (silica undersaturated and oversaturated) were produced late in the Limerick region, in the west and east of the SMV and close to the northern margin of the Northumberland basin. Comenditic rhyolites made a very minor appearance in the Clyde Plateau and Northumberland basin margin. Trace element and isotopic data indicate regional variations and derivation from heterogeneous mantle sources. 87Sr/86Sri values range from 0.7029 - 0.70545; 143Nd/144Ndi from 0.5122 - 0.5125. 206Pb/204Pb values are moderately radiogenic (17.65 - 18.29). The Dinantian lavas compositionally resemble OIB, with minimal crustal involvement. Evolution of the basalts is inferred to have been polybaric involving ol + cpx + sp fractionation at high pressures but with more differntiated compositions developed at lower pressures.
Alkalic magmatism persisted into the late Carboniferous on a subdued scale. The subsiding basins having generally accumulated thick sequences of sediment, surface eruption of lavas was the exception and sill generation more common. Interaction with wet sediments and/or surface waters gave phreatomagmatic eruptions yielding tuff-rings and diatremes. The post-Dinantian magmas are deduced to be derived by mixing from at least two different sources in heterogeneous asthenospheric mantle. Isotopic values (87Sr/86Sri of 0.7032 - 0.7049 and 143Nd/144Ndi of 0.5122 - 0.5126) are similar to those of the Dinantian rocks and suggest minimal crustal contamination.
A dramatic change marked the close of the Carboniferous with generation of quartz tholeiite magmas, intruded as a major E-W trending dyke swarm (ca. 150 km broad) across south and central Scotland and as sills in northern England and the eastern SMV. The location of the focus for this relatively large-scale melting event is not known but it may have lain east of the present coastlines of southern Scotland and northern England.
Minor alkali basalt magmatism continued into the Permian, presumably fed from mantle sources unaffected by the quartz tholeiite event. Stress fields were very different from those operating in the Carboniferous and a set of NNW-SSE and NW - SE faults and faulted basins developed from NW England to the Hebrides. A prominent basanitic lava field developed in the south west of Scotland. Basanites and ol-nephelinites and melanephelinites are mainly represented by dykes (and associated diatremes) in NW and extreme NE of Scotland and in the Orkneys. Camptonite and bostonite dykes in Orkney may be late Permian. Some Orkney melanephelinites represent < 0.002 melt fractions with La/Yb of up to 38. Isotopic data (87Sr/86Sri of 0.70302-0.70348 and 143Nd/144Ndi of 0.512470-0.512722) suggest sources similar to those for the SMV basalts.
Although small melt fraction (strongly silica-undersaturated) magmas erupted throughout the entire period, they are more characteristic of the upper Carboniferous and Permian. Rapid ascent permitted them to entrain a wide assortment of high pressure phenocrysts, mantle xenoliths and megacrysts and crustal xenoliths. The Carboniferous-Permian province in the British Isles provides a rare occurrence of this phenomenon in pre-Mesozoic basaltic magmatism.
New isotope and trace element data from both whole-rock samples and pyroxene separates are reported from the Skien basalts, which represent the earliest phase of magmatism in the Oslo Rift. This nephelinite-alkali basalt sequence represents the smallest-degree partial melts in the Oslo Rift system, and provides the best opportunity to study and identify the sub-lithospheric components present in the rift's magmatic system. Comparison of the bulk-rock trace element data and isotope data with back-corrected modern-day mantle reservoirs confirms the contribution of HIMU-type mantle to these 300 Ma magmas. The dominant sub-lithospheric component in the rest of the Oslo Rift basalts fits the PREMA signature of Stein & Hofmann (1994). We propose, therefore, that the Permo-Carboniferous igneous province seen across Europe was initiated by a lower-mantle plume, in keeping with the origin recently proposed for the modern, PREMA-dominated, Hawaiian plume (Brandon et al., 1998). Having identified these two sub-lithospheric components, work is underway to understand the contribution of the European lithosphere to the Permo-Carboniferous magmatism which it hosts.
A more detailed examination of the Skien basalts provides evidence for the incorporation of pyroxenes with xenocrystic Cr-diopside cores in several of the flows in the lower half of the ~70-flow sequence, with isotopic signatures out of equilibrium with their phenocrystic pyroxene overgrowths. New ion-probe trace-element data reveal Zr-Hf depletion in the cores, but Sr-(Ba)-depletion, uniform REE patterns slightly depleted in La and Ce, and Zr-Hf enrichment in the core overgrowths and in phenocrysts throughout the Skien sequence.
These results from the Skien basalts will be incorporated into an overall model of the formation of the Oslo rift and the European Permo-Carboniferous volcanic province.
Stein M & Hofmann AW, Nature, 372, 63-68, (1994).
Brandon AD, Walker RJ, Morgan JW, Norman MD & Pritchard HM, Science, 280, 1570-1573, (1998).
Structural, stratigraphic and isotopic data suggest that parts of the Late Paleozoic basin evolution in Laurasia needs revision in relation to age tectonic setting. New isotopic and paleomagnetic data from indicate that the upper part of East Greenlands classical Devonian basin partly formed after 330 Ma. On most time-scales is this age considered as part of the Carboniferous period. Three basalt flows are dated by 40Ar/39Ar furnace step-heating of plagioclase to c. 330 Ma. Detrital feldspar from interbedded deposits give c. 80 m.y. older ages suggesting that the Ar- clock was not reset by Carboniferous regional heating. The new ages and structural data furthermore suggest that sediments previously stacked in stratigraphic columns are correlative. Single zircon from extrusive rhyolite give a concordant age of 342 ± 1 Ma. The isotope data are interpreted to date extrusive events, and thereby the age of the basin. Further isotopic dating is in progress, however there is so far little evidence for error in the existing data. The new ages are particularly enigmatic as the basin is highly fossiliferous, including some of the most primitive tetrapods known (Ichthyostega and Acanthostega), which by some has been regarded as the "missing link" between fish and land animals. Paleomagnetic data cannot be used to give exact numeric ages, however the dated deposits plot at the ~ 330 Ma section of Laurentias apparent polar wander path. The data suggest, regardless of absolute time, that East Greenland moved considerable distances (c. 10° Southwards) during development of erosional hiati in the basin. That takes time. Contact tests and the zijderveld plot show little evidence of thermal resetting, and the deposits preserve a primary stratigraphically linked primary magnetic reversal pattern. The Paleomagnetic data furthermore suggest rotations probably related to major left-lateral wrench faulting. East Greenlands vast Late Paleozoic basins show parallel folds and extensional faults. Wrench faulting may explain these alternating phases of Devonian to Permian E-W shortening and extension, by strain partitioning. In Late Silurian Baltica docked obliquely from the South against Laurentia. Further amalgamation of Laurasia and finally Pangea continued through the Paleozoic, while the Caledonian orogen collapsed. The alternating shortening and extension of East Greenlands Late Paleozoic basins, reflect this apparent contradiction.
Permian volcanic rocks in the Sudetes, at the NE margin of the Bohemian Massif, occur within continental molasse deposits of late Palaeozoic intramontane troughs, two major of which are the Intrasudetic Basin (ISB) and the North-Sudetic Basin (NSB). The basic to intermediate lavas erupted from several volcanic centres, including small shield volcanoes and multivent central volcanoes, and significant volumes of magma were also emplaced as laccoliths and sills. Two geochemically and geographically distinct volcanic suites were recognised. High-K calc-alkaline suite comprises basaltic andesites and trachyandesites and it is found in the NSB. Mildly alkaline suite comprises basaltic trachyandesites and trachyandesites and occurs in the ISB. Compared to the mildly alkaline suite, the calc alkaline suite shows higher #Mg numbers and Cr and Ni contents, as well as lower REE, HFSE and LILE abundance. Normalised trace element patterns of both suites are similar, generally typical of within-plate lavas (relatively high trace element abundance and LREE, K, Rb, Ba, and Th enrichment), but with some features gradational towards plate-margin lavas (e.g. relative Nb, Ta and Ti depletion).
Parental magmas of both suites could have originated from similar mantle sources at moderate depths corresponding to the spinel-lherzolite stability field. Parental magmas of the mildly alkaline suite (ISB) must have formed at a lower degree of partial melting compared to the calc-alkaline suite (NSB). The parental melts underwent variable differentiation processes on their way towards the surface. Fractional crystallisation at relatively deep crustal levels produced augite-plagioclase-phyric basaltic trachyandesites (ISB) and basaltic andesites (NSB), found in rather small amounts within the earliest eruptive products. These were followed by more voluminous magmas, probably resulting from an open system differentiation at shallower crustal levels. These include augite-plagioclase-phyric trachyandesites of the ISB that could have formed due to fractional crystallisation coupled with assimilation of crustal rocks, and enstatite-olivine-phyric basaltic andesites and trachyandesites of the NSB that originated within shallow level magma chambers due to fractional crystallisation and mixing with more primitive (picritic) magmas.
The Skagerrak graben is the offshore continuation of the well-known Oslo graben. The structure of this failed rift is remarkably preserved. However, very little is known about the kinematic and the geodynamic conditions prevailing during the Permo-Carboniferous rifting in that area: the uplift and subsequent erosion in the Early Mesozoic erased the Palaeozoic sedimentary cover apart from the tilted blocks, so that neither the vertical motions nor the amount of stretching are well resolved. Moreover, the fine structure and history of the extension are themselves poorly constrained, while the magmatic activity is still debated.
We present a study of the lithospheric deformation during the Permo-Carboniferous rifting in the Skagerrak sea based on the complete reprocessing of deep multi-channel seismic reflection profiles crossing the graben axis (R/V Mobil Search, 1987). The resolution of reflectors at depth is improved with f-k filtering in the shot domain, while the mapping of faults in the upper crust strongly benefits from pre-stack depth migration technics.
The rift structure appears to be mainly asymmetric, featuring from NE to SW two half grabens overlapping in a pseudo-symmetric graben. However, distributed normal faults and smaller peripheral half grabens suggest that the deformation affected a wider area, maybe in relation with pre-existing weaknesses in the upper crust. The depth-migrated sections allows to discuss the geometry of the graben and the amount of stretching, but also the distribution of deformation between brittle and ductile regimes.
There is no obvious relationship between the shallow, brittle deformation and the reflectivity at depth. The estimated base of the crust features a very strong reflectivity below the eastern side of the rift, with continuous reflectors rising up toward the NW below the rift axis. This lower crustal high reflectivity, correlated with the positive mantle Bouguer gravity anomaly along the north Skagerrak sea, is interpreted as the signature of magmatic intrusions related to rifting. In that perspective, an attempt is made to relate the location and extent of the upper crustal faults to the distributed deformation of the lower crust.
The subsidence and uplift history documented by the post-Variscan paleogeographic and stratigraphic evolution of Central Europe reflects a sequence of tectonic and magmatic events mirroring the complex post-orogenic evolution of the lithosphere. In order to constrain vertical tectonic movements during this evolution, integrated numerical subsidence analyses were carried out for the Paris and South German basins. Special attention was paid to the paleo-waterdepth evolution, including sea-level fluctuations, as inferred tectonic subsidence rates are very sensitive to these factors. Results were integrated with previous subsidence analyses published for Central Europe. Subsidence mechanisms are discussed in the frame of the post-Variscan lithospheric evolution. During the initial phase of post-orogenic isostatic and thermal re-equilibration of the lithosphere with the asthenosphere, the Permo-Carboniferous tectono-magmatic event played a very important role. Stephanian and Autunian continent-scale wrench faulting caused partial destruction of the Variscan orogen, detachment of subduction slabs, delamination of lithospheric roots and the development of narrow transtensional troughs. This, together with a probable impingement of a perhaps not very vigorous mantle plume or an upwelling branch of the mantle convection system on the base of the lithosphere, introduced new thermal anomalies. Subsequently, cooling and contraction of the thermally destabilized lithosphere governed its overall subsidence that persisted well into the Mesozoic. As the crust subsided below the erosional base-level depositional areas expanded progressively during the Triassic. However, additional overprinting effects are indicated by local variations in the Mesozoic subsidence history. Far field intra-plate stresses, related to rifting in the Tethys and Arctic-North Atlantic realms, appear to have partly reactivated inherited Permo-Carboniferous fault systems, as reflected by minor pulses of accelerated subsidence. Wrench faulting, coupled to crustal extension, could be responsible for transpressional regimes causing local uplift. Towards the end of the Cretaceous, the lithosphere had apparently stabilized on a regional scale. During the latest Cretaceous and Paleocene build-up of compressional intraplate stresses, related to early phases of the Alpine and Pyrenean orogeny, induced basin inversion in large parts of the area, prior to the onset of Tertiary rifting and magmatism, that was associated with renewed thermal destabilization of the lithosphere.
We analyse the tectonic subsidence of the Paris basin as resulting from the decay of a thermal anomaly due to thecollapse of the Variscan belt. The modelisation of the thermal evolution of the lithosphere begins at c.a. 300 Ma BPwith a thickened crust underlained by a relatively thin mantle lithosphere (until a depth of about 100 km). We assume that thermal reequilibration took place, leading to a partially molten lower crust. The crust then returns to its present thickness (30 km) by Permo-Carboniferous extension. The thermal evolution of the lithosphere is computed using two kinds of boundary conditions at the base of the lithosphere, either a constant temperature at depth (PLATE model), or a constant heat flow at asthenospheric temperature (CHABLIS model). The CHABLIS model yields a slightly better fit to the tectonic subsidence data, and present lithospheric thickness and heat flow in better agreement with geophical observations, than the PLATE model. The effect of phase transitions in the mantle (spinel/garnet) and crust (granulite/eclogite) is studied. A good fit to the subsidence data (from 230 Ma) can be obtained by different end-members scenarii, with either short (Stephano-Autunian), or long (Stephano-triasic) extension durations, with or without strong initial delamination of the lithosphere. The implications of these scenarii in terms of palaeogeography, type and timing of magmatism, retrograde P-T-t paths are discussed and compared with published observations in surrounding areas.
Permo-Carboniferous magmatism in northern Europe extends from Scandinavia across mainland Europe and the British Isles. The individual provinces have been studied in some detail but an overview of the related magmatic activity has not yet been provided. The majority of the lavas in the Oslo rift, and in southern Scotland, northern England and parts of Ireland are inferred to be the result of small degrees of partial melting of one or more sub-lithospheric sources, subsequently affected by higher level processes. In order to determine whether these magmas were linked with a hot-spot, as is common for many provinces associated with continental rifting, an area in southern Norway was chosen to initiate a helium isotopic study. The earliest and most alkalic magmatism in the Oslo region associated with the initiation of rifting occurs at Skien. The sequence at Skien consists of nephelinites, basanites and alkali basalts (MgO = 5-13 wt%). The mineralogy varies but clinopyroxene is an ubiquitous phenocryst phase. The isotopic characteristics of these lavas place them within the HIMU array. Helium isotopes have proved sensitive tracers of magma origins. The observation that MORB have 3He/4He ratios of 8 times the atmospheric ratio (RA = 1.4 x 10-6) whilst some OIBs have high 3He/4He ratios ~ 30RA (Loihi, PREMA, Kurz et al., 1987) whilst others (Tristan da Cunha -EM, St. Helena, HIMU) are lower. Clinopyroxene phenocrysts from 10 samples from Skien were separated for 3He/4He analysis. The results from all samples analyzed vary between 0.2 and 1RA. Low 3He/4He ratios are potentially a result of (i) crustal contamination; (ii) increased 4He production over time from a decay of U and Th. Crustal contamination is probably not important in these samples from Sr, Nd and Pb isotope data. However in the case that the host magma degassed prior to eruption radiogenic ingrowth of 4He within the magma chamber could have occurred and have masked the original mantle signature. Moreover, the production of 4He from radioactive decay of U-Th (0.01 - 0.1 ppm U, and 0.04 - 0.4 ppm Th) is 0.5 - 5 x 10-9 mol/g over the last 300 Ma. This is up to 200 times the amount of 4He we released by crushing of the samples. Although crushing experiments are designed to distinguish between fluid inclusion hosted and lattice hosted noble gases, it is known that crushing might release a significant proportion of the lattice component (Hilton et al., 1993). In this case some of the radiogenic helium might be explained by this effect. Likewise <alpha> particles will have been implanted into fluid inclusions over the last 300 Ma, further lowering the 3He/ 4He ratio. As the data is inconclusive for the presence of a hot-spot further studies are currently underway.
Kurz, MD, Garcia, MO, Frey, FA & O'Brien, PA, Geochim. Cosmochim. Acta, 51, 2905-2914, (1987).
Hilton, DR, Hammerschmidt, KTeufel, S & Freidrichsen, H, Earth. Planet. Sci. Lett, 120, 265-282, (1993).
In this study, an integrated geochemical and geophysical approach is being applied to the Midland Valley of Scotland (MVS) in an attempt to constrain the behaviour of the lithosphere during rifting. An initial study area around the Firth of Forth has been chosen as it exhibits several igneous events during the period which should allow temporal effects to be isolated. About 4 km of Carboniferous deltaic sandstones and shales were deposited. There is a wealth of published data on the stratigraphy, partly due to associated economic deposits of coal, oil-shales, limestone and ironstones. Decompaction and backstripping of this data has been used to produce water-loaded subsidence curves. Syn-rift subsidence started at the base of the Dinantian (~ 363 Ma) and continued into Namurian times (~ 328 Ma). Some post-rift subsidence is seen but in the study area strata younger than about 305 Ma are missing. Coal vitrinite reflectance suggests that up to 3 km of sediment was eroded during later basin inversion and uplift, probably including considerable amounts of the post-rift sequence. Preliminary inverse modelling of the subsidence curves shows that extension was modest, with ß ~ 1.4. This amount of lithospheric thinning would not be sufficient to promote melting by local upwelling of normal temperature asthenospheric mantle. The length of the rifting period (~ 35 Ma) means that conductive cooling effects must be included when modelling subsidence or melting during rifting.
New ICPMS analyses are presented for contemporaneous basaltic lavas from the Firth of Forth area, originally sampled and described by Smedley (1986) and Wallis (1989). These lavas are mildly alkaline and 'OIB like' in character. Modelling of melting by inversion of the REE concentrations includes a source which has been both depleted, during the formation of crust, and enriched by small degree metasomatic melts, which freeze in the lower part of the Mechanical Boundary Layer. Best fit melt models show that the Carboniferous melting was largely in the spinel - garnet transition zone. Moderate lithospheric stretching (ß >= 1.3) would be sufficient to decompress and remelt the source region. Variations in trace element abundances between different age igneous events can be modelled by small changes in the depth of melting. Variations within individual lava successions can be modelled by differences in degree of melting. If the source of the metasomatic melts is asthenospheric mantle (i.e. MORB source), then observed isotopic ratios in the MVS (<epsilon>Ndt ~ 3.0 - 5.0) imply that enrichment took place at least 800 Ma before eruption of the basalts. Pre-Grenvillian (?) subduction represents a possible mechanism of introducing volatiles into the mantle and hence promoting melting on the wet solidus.
Smedley P.L., Unpublished PhD Thesis, University of Edinburgh, (1986).
Wallis S.M., Unpublished PhD Thesis, University of Edinburgh, (1989).
In the present study we focus on Permo-Carboniferous rifting and related magmatic activity in northwestern Europe. We are especially interested in the distribution of in- and extrusives in relation to rift geometries. A large database containing seismic, gravity, magnetic and well data has been put together and analysed to constrain these objectives.
The continuation of the Oslo Rift into Skagerrak has been a starting point for this regional study of Permo-Carboniferous rifting and magmatism. Permo-Carboniferous rift structures (with characteristic half graben geometries) and the distribution of magmatic rocks (intrusives and extrusives) have been mapped using integrated analyses of seismic, gravity and magnetic data. A similar approach has been used to map the Sorgenfrei-Tornquist zone and the North Sea, where the interpretation is calibrated with well data.
The Permo-Carboniferous rift structures in Skagerrak can be followed into the Sorgenfrei-Tornquist zone where similar fault geometries have been observed. Both in Skagerrak and Kattegat, thick lava sequences have been deposited which are mainly parallel with the underlying Palaeozoic strata. This volcanic episode therefore predates the main fault movements, which caused the half graben geometries filled with Permian volcano-clastic material.
Permian extrusives and intrusives have also been found in wells in Kattegat, Jutland and in the North Sea (Horn and Central Grabens). Especially in the last area, the dense seismic and well coverage has allowed us to map out similar late-Palaeozoic rift geometries, although the presence of salt often conceals the seismic image of the underlying strata and structures. Our results show that the pre-Jurassic structures below large parts of the Norwegian-Danish Basin and northwards into the Stord Basin on the Horda Platform most probably belong to the same rift system, where the apparent age of faulting is late-Carboniferous(?)-early Permian.
Tectonic deformation, as well as igneous and hydrothermal rock alteration and mineralisation features, occur in the western part of the East European Craton, adjacent to the Permian Oslo and Mid-European rifting area. These features are observed in the southern part of the Fennoscandian Shield, in the Baltic Sea and the East Baltic countries.
Faults of post-Devonian, more specifically post-Early Carboniferous to pre-Late Permian, age are well documented in the southern part of the Baltic sedimentatry basin by deep drilling as well as by seismic surveys. Geological data suggest that the half-graben structures north of Lake Vättern in south-central Sweden, and the Bothnian Sea half-graben filled by Lower Paleozoic sediments are of pre-Late-Permian age. In the Baltic countries onshore, located east of the main pre-Late Permian faulting area, regeneration of mainly Late Caledonian faults occurred.
Permian mafic volcanic rocks are known from Kinnekulle in south-western Sweden, located around 150 km east of the Oslo Rift. In the southern part of the Baltic Sea, and in the adjacent eastern onshore area, mafic rocks identical in composition to those in the Oslo rift area form sills in the Silurian sedimentary sequence. These sills have been penetrated by deep wells during oil exploration. The mentioned occasional findings of mafic sills across this wide area suggest a high igneous activity there. Further northwards in the Baltic Proper, and in the Bothnian Sea, dikes crosscutting the Ordovician sequence have been recorded by shallow seismic surveys. Their age is unknown, but no other igneous bodies than Permian have been reported in the sedimentary cover of the region.
Along the Baltic Sea coasts, and elsewhere in the Fennoscandian Shield, lead sulphide mineralisation in calcite-bearing veins in basement fractures has been found. Isotopically, a substantial part of them are of late Palaeozoic age. The richest low-temperature hydrothermal sulphide mineralisation occurs in the Ordovician and Silurian, but occasionally also in the Devonian, bedrock of Estonia and in the neighbouring areas of north-western Russia and northern Latvia. This mineralisation occurs either in dolomitised bodies or in calcite-sulphide veins in extensional fractures. They occur in different areas. Mineralisation in dolomitized bodies are often found along the Narva-Pärnu north-east to south-west trending regional fault belt. However, all the occurrences are subeconomic. The large-scale secondary dolomite bodies of multistep formation suggest long-lived and voluminous hot-water migration in fault zones and layered aquifers.
Areally, the tectonic, igneous and hydrothermal activity of the presumed pre-Late Permian epoch covered hundreds of thousands sq. km. Our conclusion is, that the processes of the pre- Late Permian rifting, which intensely reworked the Caledonian and Variscan crust in Central Europe, penetrated deep into the western part of the East European Craton, far away from the location of the known rifts. The Oslo rift, located near the craton margin, is just the most apparent manifestation of a much larger reworking of the East European Craton.
The South Iberian Shear Zone (SISZ) of the Iberian Variscides, which separates the northern Ossa Morena from the southern South Portuguese Zone, was created during a period of compression followed by sinistral convergent strike-slip movements. The deformation was associated with pervasive HT-LP metamorphism and diatexis.
Magmatic melts of a variety of chemical compositions were injected into the SISZ contemporaneous with the deformation. Intrusive magmatic rocks from areas north, within and south of the Aracena Metamorphic Belt in the central part of the shear zone were investigated by isotope methods to describe the timing of evolution of the SISZ and the sources and genetical relationships of the melts.
The intrusive rocks comprise gabbro-norites, hornblende-gabbros, diorites, quartz diorites, granodiorites, hornblende tonalites, monzogranites and two-mica granites. The basic, intermediate and acid rocks are characterized by low TiO2 contents (< 1.8 wt.%) and enriched incompatible trace elements including the LREE while the ultrabasic rocks show a significantly different element pattern.
Initial 87Sr/86Sr-ratios and initial 143Nd/144Nd ratios range from 0.70547 ± 0.00004 to 0.70721 ± 0.00003 and 0.51209 ± 0.00001 to 0.51192 ± 0.00002, respectively, for basic, intermediate and acid rocks while the gabbros vary from 0.70728 ± 0.00006 to 0.70792 ± 0.00007 for 87Sr/86Sr and 0.51203 ± 0.00001 and 0.51202 ± 0.00001 for 143Nd/144Nd.
The isotope data show that the magmatic melts which crystallized to form the basic, intermediate and acid rocks were not differentiated from primary magmas nor derived from the ultrabasic gabbroic melts by mixing. Isotopically the highly deformed tholeiitic Acebuches Amphibolite, however, can serve as a mixing component together with crustal lithologies to form the basic, intermediate and acid melts. This is corroborated by mixing calculations. The isotopic compositions of the gabbroic rocks can be modelled using AFC processes.
The magmatism crosscutting the SISZ can be reduced to a mantle-derived component and a secondary crustal component which was generated by heat transfer from the mantle magma into crustal gneisses. Pulses of tholeiitic melts initiated the generation of the secondary component and caused magma mixing and mingling with this component by repeated injections. Also the gabbroic melts are products of such process.
Rb-Sr mineral isochron dating of these intrusive rocks yielded ages ranging from 328 ± 3 Ma to 344 ± 3 Ma. U-Pb analyses of zircons from a hornblende tonalite (Aroche pluton) within the Aracena Metamorphic Belt constrain a discordia with an upper intersect of 347 -12/+51 Ma. Magmatites of the Beja Massiv and the Sevilla Ranges Batholith are characterized by already published similar Rb-Sr- and Ar-Ar-ages.
The Permian Seui Basin in central Sardinia shows a sequence of depositional, magmatic and tectonic events recontructed also on the ground of many boreholes (PROGEMISA 1983/87). 1) The 400 m thick sequence, begins in a subsiding basin with the deposition of a Basal Conglomerate reworking the Variscan metamorphic basement. The thickness variation of the conglomerate suggests an uneven morphology of the floor. Thin layers of rhyolite pyroclastite and reworked rhyolite pebbles support a coeval volcanism. 2) A fine-grained alluvial-to-lacustrine sedimentation follows, characterized by Autunian floras and coal deposits (presently anthracite). 3) Andesitic magmas were emplaced as small plugs intruding the basement and the sediments, or filling a large tectonic trough. Both the eastern border of the basin and the andesitic bodies show a NE-SW trend. 4) At the northern boundary of the basin, large diorite dikes feed dacitic domes intruded at shallow leves and ballooning the overlying basement and/or sediments; thermal metamorphism developed in the pelites at the contact. The diorite and dacite domes aligned along a NNE-SSW direction. As a consequence of dome emplacement and/or associated tectonics, slices of crystalline basement locally overlapped the Autunian volcano-sedimentary sequence. 5) The sequence is topped by rhyolitic ignimbrites some tens of meters thick, originated north of the basin where they attain > 500 m thickness.Andesites have average (sum)REE = 181; REE fractionation is moderate (LaN/YbN = 8.42), with LREE fractionation (LaN/SmN = 3.39) higher than HREE (GdN/YbN = 1.62). Eu/Eu* is 0.75. Dacites have average (sum)REE = 152; REE are poorly fractionated (LaN/YbN = 7.44), with LREE fractionation (LaN/SmN = 3.31) higher than HREE (GdN/YbN= 1.52); Eu anomaly is pronounced and Eu/Eu* is on average 0.82. Almandine-pyrope rich garnet xenoliths occur in andesite and dacite lavas and suggest interaction with subcrustal sources. On the whole, a calc-alkaline affinity characterizes the sequence.The Seui and the smaller adjacent Seulo basins likely formed along a NE-SW present direction during Autunian times, south of a major transcurrent lineament and bounded to the north by a structural high of basement, marked by magmatic activity.Afterwards, probably still during the Early Permian, a large area (Barbagia di Seui) was affected by an important explosive acid volcanism, widely occurring in Sardinia, (Ogliastra, Nurra, Gallura, Iglesiente) likely associated with a later, widely occurring extensional phase. Subsequently, the tectonic alignments of the Seui - Seulo basins turned to a ESE-WNW direction, and the deposition within the basins was sutured by one or more regional unconformities. The structural evolution can thus be envisaged as switching from transtensional to extensional tectonic regime.The origin for intermediate-acid calc-alkaline volcanism in a transtensional-transpressive regime can be associated with partial melting in the lower continental crust previously thickened by stacking of crustal nappes.
In SE Sardinia, Late Palaeozoic intramontane basins and coeval volcanism in the Variscan basement developed after the mid-Carboniferous collisional phase, as a consequence of transtensional movements along ancestral lineaments presently NE-SW aligned. An Early Permian block-faulting tectonics produced fault-bounded basins with a present NNW-SSE direction, and a horst-graben paleomorphology that controlled the sedimentation within the troughs. Extensively in SE Sardinia, calc-alkaline magmatism occurred as i) intrusives in the metamorphic basement, ii) intermediate and acid effusives on the basement, and iii) effusives within basins. The biostratigraphic record (macro- and microfloras, ostracods, stromatolites, amphibians) indicate an Early Permian age (Middle Autunian) of the lacustrine basins.
The Perdasdefogu basin (Ogliastra) shows a 250-300 m thick volcano-sedimentary sequence composed of: i) A Basal Conglomerate unconformably on the basement. ii) Alluvial-to-lacustrine black shales, sandstones, rare conglomerates, up to 100 m thick, iii) Intercalated andesite lavas and intermediate to acidic breccias, iv) Lacustrine coal-bearing carbonates, dolostones and cherts followed by silicified tuffs and hyaloclastites, up to 70 m thick. v) Decameter thick dacite flows. Subvolcanic rhyolites occur as dikes and sills across the basement and cut the sedimentary units. The PROGEMISA boreholes (1986/87) evidenced the continuity of Permian sediments under the Mesozoic covers and several buried faults. Andesites have (sum)REE = 191, moderate REE fractionation (LaN/YbN = 8.20), LaN/SmN = 3.16 and GdN/YbN = 1.67. Average Eu/Eu* is 0.75. Dacites have (sum)REE = 170 and moderate fractionation (LaN/YbN = 9.42), with LaN/SmN = 3.79 and GdN/YbN = 1.60; average Eu/Eu* is 0.73. Rhyolite dikes have (sum)REE = 166 and low fractionation (LaN/YbN = 5.34), with LaN/SmN = 2.33 and GdN/YbN = 1.33; average Eu/Eu* is 0.44.
The Escalaplano basin (Gerrei) shows a 150-200 m thick sequence with prevailing volcanic rocks and subordinate lacustrine sediments. The sequence includes: i) a Basal Conglomerate ii) a lower volcano-sedimentary sequence, characterized by fine-grained epiclastites, cinerites and rhyolitic ignimbrites iii) an upper volcano-sedimentary sequence characterized by a coarse clastic horizon, a lahar with andesite fragments, a conglomerate made by well rounded andesite-dacite pebbles, fine-grained epiclastites with calcite-siderite nodules and andesite lavas at the top. In ii) and iii), likely separated by a minor unconformity, 9 lithostratigraphic units are distinguished. Lower rhyolites have average (sum)REE = 165; REE fractionation is moderate (LaN/YbN = 10.30), with LaN/SmN = 3.82 and HREE (GdN/YbN = 1.49); Eu/Eu* is 0.56. Andesites have (sum)REE = 161; REE fractionation is low (LaN/YbN = 7.31), LaN/SmN = 3.17 and GdN/YbN = 1.64. Eu/Eu* is 0.72.
E of the basins, several rhyolite dikes cut the basement, whereas ignimbrites directly overlye it. Further east, a conspicuous acid activity directly resting on the basement, is at places reworked in explosive breccias, and locally cut by basaltic dikes of probable upper Permian age.
The early/mid-Carboniferous Variscan collisional phase is followed in many areas of the Southern Europe by Early Permian transtensional tectonics that controls the development of small intramontane basins. The associated widespread calc-alkaline magmatism is represented by intrusives in the metamorphic basement and by intermediate and acid effusives within basins or on the basement. The Collio Basin in the Central Southern Alps exposes i) a basal rhyolitic non-welded ignimbrite and associated tuffs, ii) a volcano-sedimentary formation (Collio Fm.), iii) a thick fluvial conglomerate (Dosso dei galli Cgl.), iv) an upper rhyolitic welded ignimbrite. The oldest volcanites are high-K andesite dikes across the basement; later extensional structures reactivating the dike are filled by acid products. Rhyolite dikes feeding domes occur along major faults to the east of the basin.
A combined petrological-volcanosedimentological study was focused to the reconstruction of the tectonic, magmatic and depositional development of the volcano-sedimentary formation. In the lower part of the Collio Fm., a 20 m thick volcaniclastic composite mass flow deposit (Dasdana Bed) is intercalated. It is extensively exposed over 10 km along a discontinous E-W aligned chain of outcrops. Field relations and preliminary granulometric results indicate that the mass flows originated from the eastern margin of the basin, where a prominent fault zone associated with a (sub-)volcanic complex occurs, and flowed towards the west along the lake floor.
The Dasdana Beds consist of a lower 10-12 m thick sub-unit which contains amalgamated coarse-sandy to fine-gravelly crystal-rich turbidites, and an upper well-bedded fine sandy unit which becomes thicker towards the distal outcrops in the west. The lower part comprises fragments of plagioclase (20-30%), K-feldspar (7%), quartz (20%), biotite (3-10%), and of porphyritic SiO2-rich lava (20-30%) as well as metamorphic basement clasts (5%). Uncompacted porphyritic lava clasts show granophyric groundmass (mainly albite) and irregular-ragged shapes typical of explosive fragmentation of viscous magma. The phenocryst content of the fragmented lava was probably in the order 20-30%. The lower Dasdana Bed revealed some systematic proximal-distal trends, such as a higher plagioclase and biotite content together with a diminishing maximum particle size of porhyritic lava fragments towards the W. The upper well-bedded part of the Dasdana Bed is rich in matrix supported, strongly compacted porphyritic lava clasts of 1-2 cm in size.
Presumably, a strongly porphyritic SiO2-rich lava(-dome) which formed in a sublacustrine environment near the eastern margin of the Collio Basin has been fragmented by phreatomagmatic explosions and/or seismic fluidization. The resulting fragments were transported into deeper parts of the lake by mass flows, whereby the crystal fragments became concentrated in the massive lower units of the Dasdana Bed and the irregularily shaped and probably vesicular porphyritic lava fragments have been winnowed out and later deposited with fine sandy low-energy turbiditic flows, which comprise the upper Dasdana Bed.
Main aim of the study is a characterisation of the Variscan deformation history of the Orobic Alps in terms of plate-tectonic setting, convergence angle, geodynamic processes and characterisation of the type of basin which developed during the Permian. The pre-Variscan crustal framework needs consideration for this study. A major accident, say a proto Tonale line, has been developed during the Ordovician, when a narrow chain of terranes, including the Austroalpine and Penninic terranes accreted against and close to the future Southalpine (Orobic) zone at the NW Gondwana margin. Ordovician sedimentation at this Orobic zone occurred in a passive margin setting narrowing due to mentioned convergence and accretion. During Palaeotethys opening, the Hun superterrane, a wider continent, which then included the previously accreted crustal fragments, drifted away from Gondwana and collided with Laurussia. Variscan dextral synaccrertionary shearing caused a.o. the former Austroalpine and Pennine terranes to move along the southalpine (Orobic) crust. Whilst locally oceanic crust subducted and accreted (Ivrea zone) through gaps between the Austroalpine, Pennine crustal fragments a.o., these fragments were dextrally sheared inbetween the docking Hun superterrane and Laurussia. The Ordovician terrane boundary between the Austroalpine terrane and the Southalpine block reactivated as a Variscan Tonale crustal shearzone. The southern part of this zone has been studied in the Orobic Alps.Orientations of structures, like folds, stretching lineations, ductile shearzones, brittle-ductile shearzones and brittle faults, which subsequently developed in the exhuming rocks indicate that this shearzone is chacterised by oblique convergent deformation with a very small convergence angle during its Variscan to late Permian activity. The deformation regime within this intraplate shearzone is very well compatible with critical wedge models. Transpression occurred in the lower to middle crust, causing underplating by ductile folding, mainly along and oblique to the steep inwards dipping margins. Related uplift along these relatively steep boundary zones of this "positive flower" caused collapse in the brittle suprastructure. This collapse and related exhumation of intermediate P assemblages rocks was very much enhanced by the relatively large strike slip component. Whilst rocks, ramped and folded upwards along the edges, inward collapse occurred inside the "flower", causing magmatic underplating and gabbro intrusion above the zones of reverse faulting. Melts of the typical augengneises formed along developing transtensional detachments, which excellerated the process of collapse. This material flow, with rise of intermediate P assemblages to surface and inward collapse resulted in a collapse graben with higher pressure assemblages at the edge and higher temperature assemblages in its center. The transtensional Permian Collio basins developed. Those along the edge of the shearzone started to develop as supradetachment volcanic basin, whereas in the middle of the collapsing structure others developed. After the phase of calcalkaline rhyolitic volcanism, characterised by major ignimbrites and lavadomes, high angle transtensional block faulting occurred a second member, with lacustrine and fluviatle deposits, was layed down syntectonically.
The Austroalpine basement units experienced metamorphic imprints during the Hercynian orogeny at about 370 to 300 Ma and/or during the Alpine orogeny. However, gabbroic and granitic intrusions (Thöni & Jagoutz, 1992) as well as pegmatites have been emplaced since the Middle Permian. Recent geochronological and petrological data from the Austroalpine crystalline basement support an additional widespread Permo-Triassic HT/LP metamorphism:
Lower greenschist facies assemblages of paragonitic mica + chlorite + albite + quartz with Ar-Ar ages of c. 240 Ma have been presented by Müller (1994) for the Wechsel Unit. Ages in the same range are also reported from the Innsbruck and Katschberg Quartzphyllite, but the relevance of these data is still under dispute (Rockenschaub & Kolenprat, 1998). Pebbles with a thermal overprint at about 245 Ma can be found within the upper Gosau Group of the Weyerer Bögen (Frank et al., 1998). In the Wölz Complex of the Niedere Tauern Permian garnets (Sm-Nd 269±4 Ma) proof a greenschist facies LP/HT metamorphic imprint with temperatures of more than 450°C at about 2.5 kbar. In southern parts of the basement (former) andalusite bearing assemblages and andalusite-quartz veins are characteristic. Permian age data (240-275 Ma) of related pegmatites, granitic gneisses and some Sm-Nd data of garnet from metapelites suggest a coeval formation of the andalusite bearing parageneses. Well preserved upper greenschist facies assemblages occur in the southernmost Gailtal crystalline basement near Jenning. Andalusite, overgrowing Hercynian staurolite and garnet, developed by the reaction chlorite + garnet + muscovite = andalusite + biotite at about 500°C and 3 kbar. In the Strieden Unit (Kreuzeckgruppe) amphibolite facies conditions of more than 550°C are indicated by the prograde breakdown of the Hercynian staurolite by the reaction staurolite + muscovite = andalusite + biotite + garnet. Ar-Ar and Rb-Sr cooling ages of muscovite respectively biotite scatter between 220 and 200 Ma. Further to the east a similar evolution reaches the sillimanite stability field. For the disthenflasergneisses of the Saualpe 590±20°C at 3.8±0.1 kbar (Habler & Thöni, 1998) and for the Strallegg Complex conditions of about 580-650°C at 3-3.5 kbar (Berka et al., 1998) were determined. In the southernmost part of the Strallegg Complex migmatites and granites of Permian age occur.
The Permian HT/LP metamorphism can be found at least over a distance of more than 300 km from Sopron (Hungary) in the east to the Kreuzeckgruppe (Carinthia) in the west. It is characterized by a geothermal gradient of ca. 45°C/km, over different metamorphic grades. Coeval magmatic rocks such as gabbros, granites and pegmatites are common but volumetrically subordinate relative to the metapelites. Peak metamorphic conditions of the HT/LP metamorphism were reached at about 270 Ma, preliminary data suppose a slow cooling until 200 Ma.
Berka R, Schmidt K, Schuster R & Frank W, Mitt. Österr. Mineral. Ges, 143, 242-245, (1998).
Frank W, Schuster R & Faupl P, Mitt. Österr. Mineral. Ges, 143, 273-275, (1998).
Habler G & Thöni M, Mitt. Österr. Mineral. Ges, 143, 291-292, (1998).
Müller W, unpub. Dipl. Arb. Formal- u. Naturwiss. Fak. Univ. Wien, 267pp, (1994).
Rockenschaub M & Kolenprat B, Freiberger Forschungshefte, C471, 179-180, (1998).
Thöni M & Jagoutz E, Geochim. et Cosmochim. Acta, 56, 347-368, (1992).
Lamprophyre dykes occur widespread throughout the Fichtel- and Erzgebirge and in the Frankenwälder zone which are parts of the eastern Saxothuringian zone of the Variscan orogen. These occurrences include the area where Gümbel coined the term "lamprophyre" in 1874.
The Fichtel- and Erzgebirge represent an antiformal pile of Devonian nappes of varying provenance, some of which contain eclogites. This metamorphic complex underwent a multi-stage uplift history from the mid Devonian into the early Carboniferous that ended with Flysch deposition and intrusion of granites. Field relations clearly show that some lamprophyre dykes post-date the post-kinematic granite intrusions. Lamprophyric dykes also occur close to co-magmatic shoshonite lavas in the southern margin of the Erzgebirge molasse basin.
The kersantites, minettes and shoshonites are rich in Mg ,Cr, have low oxygen isotope ratios, high concentrations of Ba, Th, La, Ce (LILE), fluids (OH, CO , F), and negative Nb, Ta and Ti anomalies. Such features are characteristic for mafic magmas in convergent plate-margin settings. Fluids released during possible earlier subduction and dehydration of oceanic crust might have mobilised LILE and concentrated them in the lithospheric mantle. Following continental collision and crustal thickening, intrusion may have occurred after detachment and removal of sub-continental lithospheric mantle and its replacement by asthenospheric mantle material. The emplacement of hot asthenosphere may have triggered the generation of lamprophyric and shoshonitic partial melts in the remainder of the old lithosphere. Alternatively, the fluids released from detached lithospheric mantle may have triggered partial melting within the upwelling asthenosphere. During their ascent these magmas must have undergone a variety of changes, as reflected in their heterogeneous textures and alteration mineral assemblages. Existing K-Ar ages on phlogopites and Mg-biotites from the lamprophyres often contradict the field relations. Therefore, Ar-Ar dating is carried out on dykes from widely spaced localities in order to establish whether these have similar crystallisation ages, and to bracket the time span in which the generation of lamprophyric and shoshonitic melts occurred.
Rotliegend metavolcanic rocks in the Gorzow Wielkopolski region (NW Poland, Fore Sudetic Monocline) overlie Carboniferous sandstone and siltstone and are covered by thic Zechstein evaporitic sequence. The volcanic rocks are highly affected by very low grade metamorphism; the scarce relict minerals: clinopyroxene, ilmenite and Cr-spinel can be found only in vesicle poor varietes. Metamorphic assemblage comprises pumpellyite, laumontite, corrensite-type mineral, quartz, chalcedony, albite, smectite, calcite, solid bitumens, and minor potassium feldspar, anhydrite, titanite, hematite, pyrite, and celadonite. The metamorphic alteration of Rotliegend volcanic rocks is ascribed to to penetration of descending sea water trough Zechstein evaporite sequence; however, bitumens seem to be a product of ascending fluids release during metamorphism of clastic rocks underlying volcanic Permian unit. Despite metamorphic overprint REE signature still records unequivocal continental basalt characteristics; Metamorphic alteration did not significantly affect REE concentrations. However, LREE contents in samples adjacent to evaporite rocks is slightly hihger that in distant ones. Moreover, rocks dismenbered into fragments due to hydraulic fracturing and cemented by younger anhydrite, quartz, and chalcedony recorf only somewhat lower REE concentrations that can be interpreted as simple dilution by newly formed LREE-poor minerals. Both mineralogy, e.g. abundance of laumontite and calcite, and low REE mobility suggest alkaline conditions during alteration of the Rotliegend volcanic rock. Fixed Rb/Ta ratio seems to reveal potassium feldspar content in the pristine volcanic rocks, whereas different potasium content and K/Rb ratio can be tentatively assigned to scarce celadonite in altered rocks. Uniform Ti/Eu ratio (R2=0.92) in the studied rocks is probably related to the incorporation of Eu2+ during titanite crystallization under moderate reducing conditions.
Permo-Carboniferous sedimentary rocks in the SE part of the Bohemian Massif are preserved only as relics in three parallel furrows of NNE - SSW orientation ("the Rhine orientation"). These sediments are of continental (limnic, fluvial, proluvial) origin. The Blanice and Jihlava Furrows are developed in the area of strongly metamorphosed rocks (the Moldanubicum) and since the Upper Permian they were intensively denudated. A large intrusion of Upper Carboniferous two-mica granites (NNE - SSW branch of the Moldanubian Pluton) is situated just between them together with many lamprophyric dikes and acid subvolcanics. The third Boskovice Furrow masks the transverse contact of the Brunovistulicum with next four crystalline units of the Bohemian Massif. Distances among the three furrows are the same - about 65 km.
The best preserved Boskovice Furrow is a narrow (3-10 km) but long (about 130 km) continuous depression filled by Stephanian and Autunian clastics (conglomerates, breccias, arcoses, sandstones, mudstones) of characteristic brown-red colour. There are also present coal-bearing sediments with thin layers of rhyolitic tuffs and tuffites (Stephanian C). Total thickness of sediments is estimated about 2000 - 3000 m. Besides it, there exists an isolated occurrence of Stephanian - Lower Permian clastics 65 km to the SSW at Zobing (Austria) which lies at the same fault forming a margin of the Boskovice Furrow.
In the Boskovice Furrow, subvolcanic dikes and sills of probably Permian age cut its sedimentary filling. The magmatic rocks are usually represented by basaltic trachyandesites or trachyandesites but there were ascertained trachytes and dacites as well. Strikes of the dikes are often similar to the orientation of the Furrow with their dips both to the west and east. Pebbles of non-metamorphosed (Permian?) volcanic rocks have been found in the Lower Permian conglomerate at Zobing.
No magmatic rocks are known in the Blanice Furrow. A few isolated occurrences scattered in the length 125 km are built of the Stephanian - Lower Permian sediments with lithology similar to that of the Boskovice Furrow. According to boreholes, total thickness of local sediments is about 1000 m. In the most denudated Jihlava Furrow, only one small occurrence of non-metamorphosed Carboniferous - Permian conglomerates represents a sedimentary filling. The course of the Furrow can be followed as an intensive mylonite zone in underlying Moldanubian gneisses and migmatites.
Al three furrows together with lamprophyric and subvolcanic dikes among them including subvolcanic and volcanic rocks in the Boskovice Furrow give evidence for Stephanian - Lower Permian extension in the SE part of the Bohemian Massif. This extension is very probably a consequence of the gravitational extensional collapse of thickened continental crust which originated after the subduction of the Brunovistulicum beneath the Moldanubicum.
An occurrence of juonniite, a new phosphate of Sc was discovered recently in the Kovdor alkaline-ultrabasic massif. This mineral formed with posterior phosphate associations in cavities and fissures of mineralized dolomite carbonatites during hydrothermal alteration of these rocks. Sr isotopic composition was studied in phosphates, which are paragenetic to juonniite, and in carbonate-fluorapatite, which is a cement-forming mineral of apatite- "francolite" breccias. These rocks conventionally were believed to be the crust weathering of the phoscorite-carbonatite stock and were studied by us in respect to possible high Sc concentrations (similar to apophoscorite-apocarbonatite crusts on Tomtor massif, kolbekite-bearing apocarbonatite crusts on Mrima-Hill, etc).It has been found that the hydrothermal phosphates associated with juonniite (i.e. from the Sc-bearing late associations) have low 87Sr/86Sr (0.7033 - 0.7038). These values are much lower than those of host dolomite carbonatites (0.7047 - 0.7051), and are close to those of the Kovdor calcite carbonatites (Fig. 1) and unmineralized massive dolomite carbonatites (0.70330 - 0.70363, according to Zaitsev & Bell, 1995), which are cut by mineralized dolomite carbonatites.Hydrotherms could not trap Sr from the host rocks (including calcitic, with low 87Sr/86Sr), because hydrothermal alterations and the redistribution of elements were most intensive in the mineralized late dolomite carbonatites and did not raise the 87Sr/86Sr values of hydrothermal phosphates. These data suggest that the solutions had a juvenile origin and were derived from a reservoir of calcite-dolomite melts. The hydrothermal Sc-bearing mineralization had no genetic relation with host late mineralized dolomite carbonatites, which in this case had experienced posterior alterations. The hydrotherms, which undoubtedly have a crustal origin, however, have a mantle composition of Sr, owing to their genetic relation to the mantle-derived calcite and dolomite carbonatites. 87Sr/86Sr value in carbonate-fluorapatite of apatite-"francolite" breccia is unusual: if primary fluorapatite from the clasts of the breccia shows 87Sr/86Sr =0.7049 (Landa et al., 1982), late crustified cement-forming blue carbonate-fluorapatite from the epicenter of the brecciated zone has 87Sr/86Sr =0.7039. At the same time, light-colored carbonate-fluorapatite, forming cement in apatite- "francolite" breccia from the periphery of the brecciated zone (close to apogneiss fenites) has 87Sr/86Sr=0.7057° (Landa et al., 1982). The facts confirm an explosive nature of the apatite-"francolite" breccia, rather than an exogene one (what they are in agreement with empirical conclusions of Krasnova, 1979; Bulakh, Ivannikov, 1984), and testify to a hydrothermal alteration of the clastic material with solution that were genetically conected to the reservoir of calcite melts. The absence of accumulation of not mobile elements that are typomorphic for apocarbonatitic crusts (such as Sc, REE, Nb, Zr, Sr et al) in the Kovdor apatite-"francolite" breccias support the conclusions.
Krasnova, NI, Composition of phosphorites, 164-172, (1979).
Landa, EA, Murina, GA, Shergina, YuP & Krasnova, NI, Bull. of AS of USSR, 264, 16, 1480-1482, (1982).
Bulakh, AG & Ivanikov, VV, Problems of mineralogy and petrology of carbonatites, (1984).
Zaitsev, AN & Bell, K, Contrib. Mineral. Petrol, 324-335, (1995).
It has become commonplace in apatite fission track (FT) analysis for track length and age data to be quantitatively modelled to extract a samples temperature history information. In contrast, similar temperature history information derived from zircon FT analysis is not yet produced. This is because a suitable annealing model has not been available. Neither were there boreholes whose present day temperatures penetrated the zircon partial annealing zone (ZPAZ) in its entirety, for which annealing predictions derived from the model could be matched to observations. Recently a quantitative model of zircon annealing kinetics has been published which predicts a ZPAZ of ~200° to ~350°C for geological time scales. Where present day temperatures have been exceeded in the past, an estimate of this palaeo-temperature and its timing can be obtained by adopting the method of Gallagher [1995, EPSL 136; 421-435]. The results of modelling zircon FT data in this way, obtained from a borehole in the North German Basin, yield systematic variations in peak palaeo-temperature that allows an estimate of the palaeo-geotherm to be made. This work suggests peak temperatures in the Palaeozoic strata of this basin were reached during the Permian and are not the result of the much later Tertiary burial.
The Lower Permian in the Southern Alps is characterized by extensional sedimentary basins with volcanic inliers (Tregiovo, Collio, Orobic basins), as well as by large caldera-collapse structures in the Bolzano and Lugano areas. The Collio basin between Val Trompia and Val Caffaro (Brescian Prealps) is bounded by extensional faults and structural highs consisting of pre-Permian metamorphic basement. Alluvial fan deposits and lacustrine sediments form the bulk of the basin fill and locally reach a thickness of almost 1000 meters. Palynomorphs suggest a late Early Permian age for sediment deposition in the Collio basin (late Artinskian or younger; Cassinis & Doubinger, 1991). Rhyolitic lavas to ignimbrites occur at the base as well as above the sediment succession and are suitable candidates for numeric dating of basin formation. U-Pb dating of zircons from the lowermost and topmost rhyolite layers yielded preliminary results of 283 ± 1 and 281 ± 2 Ma. This suggests a time span of 1 to 4 million years for sediment accumulation in the Collio basin. This short lifetime of a continental basin is in agreement with findings from Lower and Upper Carboniferous sedimentary basins of the Variscan Belt (Schaltegger, 1997). However, the preliminary ages from the Collio basin are significantly older than numeric values assigned to the corresponding time interval in current time-scales. In their chronostratigraphic subdivision of the Permian, Jin Yu-gan et al. (1997) indicate an age of 272 Ma for the top of the Artinskian.
Precise isotopic ages exist for a part of the Bolzano volcanics (Th-Pb allanite: 276 ± 3 Ma; Barth et al., 1994) to the east of the Collio Basin. The precise age of the emplacement of volcanic rocks and shallow intrusives in the western part of the Southern Alps (Orobic Basin and Lugano area) is as yet unknown. Additional high-resolution U-Pb dating of old and young members of magmatic successions at Bolzano and Lugano should help establishing the chronology of basin formation in the Southern Alps. The Lower Permian basins and their volcanic rocks probably formed during short-lived, regional events of extension. The basins are not necessarily coeval but contributed in the accommodation of large-scale strike slip movements close to the southern limit of the Variscan chain. High-K calc-alkaline magmas presumably formed through extension-related reactivation of crustal and mantle sources.
Barth S, Oberli F & Meier M, Earth Planet Sci Lett, 124, 149-159, (1994).
Cassinis G & Doubinger J, Atti Tic Sc Terra, 34, 1-20, (1991).
Jin Yugan, Wardlaw BR, Glenister BF & Kotlyar GV, Episodes, 20/1, 10-15, (1997).
Schaltegger U, Terra Nova, 9, 242-245, (1997).
Two main magmatic cycles are known in southern Europe during Late Palaeozoic: the first calc-alkaline to transitional in composition dated Late Carboniferous to Early Permian; the second alkaline, presumably developed between the Late Permian and the Early Triassic. Volcanites mainly emplaced in lacustrine intramontane basins or as dikes and sills in the Variscan basement. At the considered regional scale, the volcanic events show a common stratigraphic setting: a) early rhyolite activity, b) andesitic extrusions c) a main rhyodacitic to rhyolitic, with subordinate dacites, activity.Andesites, occurring as dikes, flows and magmatic breccias, are porphyritic (P.I. 5-20) with plagioclase, biotite and hornblende phenocrysts; clinopyroxene is subordinate and occurs in basaltic andesites; orthopyroxene is rare and often replaced by hornblende. Biotite is widespread and can represent the prevailing femic phase. Fe-Ti oxides (ilmenite and Ti-magnetite) are common as microphenocrysts or in the mesostasis. Zircon and allanite occur rarely. Quartz xenocrysts and xenoliths and rare garnet xenocrysts occur. The andesites show a marked compositional affinity with orogenic andesites; in Ligurian Briançonnais a rough zoning in the spatial distribution of K-normal and K-high andesites occurs; in Sardinia the two types can be found within the same eruptive centre, whereas in the Southalpine andesites emplaced as early dikes across the basement.Rhyolites and rhyodacites mostly occur as pyroclastic, ignimbrite products, and less commonly as dikes (Sardinia) or domes (Collio Basin). They are characterized by phenocrysts of embayed quartz, often rimmed by eutectic alkali-feldspar + quartz. K-feldspar, plagioclase, biotite, ilmenite and zircon are common constituents in a pumiceous and glass shard matrix. Dacites occur as pyroclastics and domes, showing porphyritic textures, with plagioclase, quartz, hornblende and biotite phenocrysts. Quartz and garnet xenocrysts are relatively common. Dacites have a medium- to high- K character, and show high average concentrations of Rb, Ba, K, Zr and lower Nb and Ti. Locally, dacites show high SiO2 contents (>70 wt%), due to occurrence of quartz xenocrysts. Rhyolites and rhyodacites show calc-alkaline, high-K chemical parameters with high average FeOtot/MgO, and a significant negative Eu anomaly.The development of Permian-Upper Carboniferous basins was coeval with: 1) an extensional tectonic regime, possibly with an important component of tectonic unroofing; 2) high thermal flow.Compared with calc-alkaline volcanism in a subduction environment, the Permian activity shows both analogous and contrasting features, e.g. in space and time distribution. A genetic environment under significant crustal thicknesses has been suggested for the Ligurian Briançonnais andesites. Early crystallization stages under hydrous, relatively high P conditions are supported by the presence of biotite and hornblende phenocrysts. Equilibrium with garnet during the melting process, suggested by garnet xenocrysts and by the Zr-Y and Ce-Yb correlations, also matches the HREE fractionation for Ligurian Briançonnais and Sardinian andesites.
The Salvan-Dorénaz basin is a NNE-SSW trending intracontinental fluvial basin, which formed during the end of the Westphalian within the Aiguilles Rouges crystalline massif (Southwestern Switzerland - Eastern France). Today only a part of the basin is preserved and a maximum thickness of 1.5 - 1.7 km of continental sediments represented by alluvial fan and river deposits is recorded. In response to intrabasinal differential subsidence and tectonic movements four different lithological units formed. In particular Unit I, at the base, corresponds to the emplacement of the basin and is composed of alluvial fan and braided river deposits; Unit II consists of palustrine and anastomosed river deposits; Unit III comprises meandering river deposits; and Unit IV consists of alluvial fan deposits interfingering with Units II and III and spreading into the fluvial basin from the northwestern margin. A synsedimentary volcanism coeval to the emplacement and development of the basin is documented by a basal rhyodacitic pyroclastic flow and by several volcaniclastic and volcanic ash-fall deposits interbedded with the alluvial plain sediments of Unit II and III. A dacitic to rhyodacitic composition for these volcaniclastic and ash-fall deposits is suggested by geochemical analysis. The basal pyroclastic flow yielded a zircon U-Pb isotopic age at 308 ± 3 Ma, while an ash fall deposit from the uppermost part of the sedimentary record yielded a zircon U-Pb isotopic age at 295 ± 3 Ma. Thereafter a time frame for basin development of 10-15 Ma is presumed and suggests rather fast average subsidence rates during basin development. The 308 Ma basal pyroclastic flow can be correlated to a major magmatic pulse documented in the AR/MB massifs by several peraluminous granites intruded syntectonically as vertical sheets at 307 Ma (e.g. the Vallorcine and Montenvers granites) (Bussy, '98). On the other hand, the 295 Ma ash fall deposit might originate from a remote volcanic edifice, possibly located in the Aar/Gotthard area, where magmatic rocks of similar age have been identified (Schaltegger & Corfu, '95).
Bussy F, Conference "The Alps and Their Variscan Framework", (1998).
Schaltegger U, Corfu F, Geodinamica Acta, 8-2, 82-98, (1995).
In the South Alpine unit, the northern part of the Adriatic microplate, extensional structures accompanied with marine transgressions, widespread magmatism and high temperature/low pressure metamorphism (HT/LP) (Diella et al., 1992) indicate an extensional regime in Permian-Triassic time (Bertotti et al., 1993). Also from the Austroalpine, which represents the northern margin of the Adriatic microplate, Permian to Triassic magmatic activity (e.g. Thöni & Jagoutz, 1992) is known. Recent geochronological and petrological investigations document a widespread contemporaneous HT/LP metamorphic imprint in large areas of the basement units from the southern margin of the Austroalpine Unit (Schuster et al. this volume).
The HT/LP metamorphic imprint can be found over a distance of more than 500 km from Sopron (western Hungary) in the east to Lake Como (Switzerland) in the west. It is characterized by an average geothermal gradient of c. 45°C/km form lower greenschist to high amphibolite facies conditions. Related magmatic rocks are gabbros, granites and pegmatites. These rocks are common but volumetrically subordinate with respect to the country rock. The peak of the metamorphism was reached at about 270 Ma. Preliminary data suppose a slow cooling until 200 Ma, when the steady state geotherm was reached again.
The geothermal gradient during the Permian metamorphic peak is much higher than the relaxed geotherm after the Carboniferous Hercynian orogeny, wich is about 35°C/km, based on data from the retrograde part of the Hercynian pressure-temperature path (Tropper & Hoinkes, 1996; Diella et al., 1992). Therefore an additional heat source is needed to create the Permian geothermal gradient of more than 45°C/km. Thinning of the lithospheric mantle is taken as the most probable mechanism. Stretching of approximately 25% within a short time span is necessary to create the observed thermal structure.
The distribution of the Permo-Triassic thermal structure suggests extension in the Adriatic microplate between the South Alpine and the Austroalpine. From palaeomagnetic data (e.g. Mauritsch, 1992) it is known, that the Austroalpine separated from the Adriatic microplate in upper Permian time by an anticlockwise rotation and involveing a dextral strike slip component. The extension continues with less velocity until middle Triassic time. The result was the passive continental margin at the western end of the Neotethyan ocean (Meliata-Hallstatt ocean). It is remarkable, that the separation of the Austroalpine from the South Alpine starts at the time, when extension within the European plate died out.
Bertotti G, Picotti V, Bernoulli D, Castellarin A, Sed. Geol., 86, 53-76, (1993).
Diella V, Spalla MI, Tunesi A, J. metamorphic Geol., 10, 203-219, (1992).
Mauritsch HJ, [IN] Neubauer F, Raumer JF, The pre- Mesozoic Geology of the Alps, 41-51, (1992).
Schuster R, Scharbert S, Abart R, Mitt. Österr. Miner. Ges, 143, 383-385, (1998).
Thöni M, Jagoutz E, Geochim. et Cosmochim. Acta, 56, 347-368, (1992).
Tropper P, Hoinkes G, Mineral. Petrol, 58, 145-170, (1996).
In the Permian and Triassic sediments of the Northern Calcareous Alps (Austria) high coalification and illite aggradation gradients are related to a pre-Alpine metamorphism. A anthracite stage of rock maturity and epizonal illite crystallinity (IC) values in Skythian sediments were related to a post-volcanic heating (Ferreiro Mählmann, 1994). The knowledge about the age of hyperthermal heating between the Hercynian and the Alpine orogeny is limited. Recently fingerprints of a Permian to Triassic high temperature / low pressure metamorphism are also find out in Austroalpine basement nappes (Schuster et al. 1998).
For a better understanding of the thermal conditions between the Upper Carboniferous and the Late Cretaceous (Alpine nappe stacking) the Silvretta nappe in Graubünden (Switzerland) was selected. In this unit, basement and sedimentary rocks are in a stratiform succession. Graben structures are filled with Permian acid volcanoclastic material and ignimbrite flows. Also in the basement Upper Carboniferous diabas dykes are giving evidences for volcanism. Alpine metamorphism did not cover all traces of the retrograde path of the Caboniferous andalusite-facies metamorphic peak. During the Alpine orogeny the sedimentary rocks did not exceed diagenetic to low anchizonal conditions of 250°C and 1.8 kbar (Ferreiro Mählmann, 1996). Therefore, the burial pattern can be used to reconstruct the paleo-geothermal evolution.
The samples analyzed by XRD, optical microscopy and microprobe come form the pelitic and psamitic Triassic and Jurassic. On the basis of optical microscopy and microprobe analyses, followed by fluid inclusion thermo-barometry and rock maturity modeling, P-T conditions of the metamorphism were calculated. Mica and illite K/Ar-model ages and zircon fission track ages determine the thermal evolution of Variscian cooling, Permian to Jurassic burial and Alpine orogenic heating. Coalification gradients, illite aggradation gradients, and the illite-smectite relation (amount of expandability) are used to fit Arrhenius type time-temperature-pressure models based on empirical T, t and P rate equations (Sweeney and Burnham, 1990; Hillier et al., 1995; Dalla Torre et al., 1997).
The results indicate an increase of metamorphic grade from the Jurassic to Permian sediments in the sedimentary section and a steep increase in the Carnian to Permian part. The hypothermal gradient in the Jurassic sediments is the result of Alpine diagenetic to anchimetamorphic heating. The increase to a hyperthermal gradient in the Triassic section and at the contact with the volcanic rocks may be inherited from a Permo-Triassic diasthathermal event prior to nappe stacking. Mica ages of 300 to 260 Ma from the basement are giving the upper and lower limits of the Permian metamorphism. A subsequent period of varying high heat flow from Permian to Jurassic times is necessary to explain the metamorphic pattern in the Silvretta nappe.
Dalla Torre M, Ferreiro Mählmann R & Ernst WG, Geochim.Cosmochim.Acta, 61/14, 2921-2928, (1997).
Ferreiro Mählmann R, Frankfurter geowiss. Arb., C/14, 498, (1994).
Ferreiro Mählmann R, Schweiz.mineral.petrogr.Mitt., 76/1, 23-46, (1996).
Hillier S, Matyas J, Matter A & Vasseur G, Clays and Clay Minerals, 43/2, 174-183, (1995).
Sweeney J & Burnham A, Amer.Assoc.Petrol.Geol.Bull., 74/10, 1559-1570, (1990).
Schuster R, Scharbert S & Abart R, Mitt.Österr.Miner.Ges., 143, Abstract, (1998).
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