From Saudi Arabia, south to India, East and West Gondwana are separated by a complex, anatomising string of oceanic sutures separated from each other by blocks of pre-Neoproterozoic continental crust. The overall architecture is similar to Cenozoic suture zones as in the Tethysides.
In Saudi Arabia, the 680-640 Ma Nabitah suture separates an upper crustal collage of Pan-African arc-dominated rocks on its western side (the Asir terrane) from late Archaean-Mesoproterozoic higher-grade crust to the east (the Afif terrane). It occupies a critical boundary between Nd and Pb isotopic provinces and it is marked by a belt of ophiolites and island-arc rocks. The eastern margin of the Afif terrane is marked by a further Neoproterozoic suture (the Al Amar suture). Further east juvenile Pan-African rocks also occur in Oman suggesting a wide region of Neoproterozoic accretion in Arabia.
The Nabitah suture extends to Hajjah in north-western Yemen, where it contains a 30-km long vertical ophiolite belt separating the presumed continuation of the Asir and Afif terranes. It extends into eastern Ethiopia, and southwards to the Adola-Moyale area on the Ethiopia-Kenya border, where there are 700 Ma ophiolites. The suture continues into southern Somalia west of the Buur massif.
In Madagascar a suture zone is traced from the north-west to the south-east of the island, separating high-grade Pan-African orogen to the west (U-Pb ages from metamorphic zircons ~550 Ma) from the Masoala Archaean craton (no high-grade metamorphism since 2.5 Ga) on the east which is a continuation of the Dharwar craton of India. The suture zone is dominated by high-grade paragneisses with lenses of quartzite-marble (from a shelf still partly overlying the Masoala craton), mafic and ultramafic rocks, and graphite schist. This is one of the finest examples of a suture preserved in the deep crust and marks the easternmost site of Neoproterozoic reworking in Gondwana.
The Malagasy suture continues south-eastwards into South India as the Palghat-Cauvery shear zone, which separates the northern Archaean Dharwar craton from the southern high-grade Pan-African orogen.
Where definable (i.e. Madagascar and India), the most eastern suture separates Archaean-Mesoproterozic crust to the east from Pan-African orogen to the west. Within the Pan African, a remarkable transect through the continental crust is preserved with upper crustal rocks in Arabia and Yemen contrasting with deep crustal rocks in Madagascar and southern India.
The Pan-African basement of the Estern Desert in Egypt, is subdivided by NW trending sinistral strike-slip zones. Within two branches of the Nadj Shear Zone, the area west of Marsa Alam exhibits various magmatic features from syn- to postorogenic magmatic rocks.
The most spectacular field evidences are circular structures of bimodal igneous suites. Ductile shear zones mark off these structures. These shear zones are partly intruded by the first representative of the voluminous Pan-African post-orogenic granites. The interpretation of the structural data and of the age determinations give clear evidence for an exhumation of the middle crust of a Pan-African island-arc. The exhumation style corresponds to the formation of metamorphic core complexes.
Petrological and geochemical data of the various igneous rocks of the Gabbro-Diorite- Granodiorite-Association show clear island-arc affinities. The postorogenic, mainly granitoid, magmatism started with the intrusion of metaluminous Biotite-Amphibole-Granites into the above described shear zones. The geochemical character of these granites (e.g. the Igla-el-Ahmar-Granite) is very ambiguous with coupled island-arc and postorogenic characteristics. The next stage is the formation of Rare-Metal bearing Albite-Granites by remelting of the granulitic lower crust.promoted by the intrusion of mafic rocks into the lower crust and the uplift of the magmatic core complexes. The increasing influence of these mafic magmas and the mixing with crustal melts from different educts, yielded to the formation of sodium- and potassium-dominated postorogenic granite types.
The examination of the multitude of different dike-rocks allows the reconstruction of these complex magmatic activities. The geochronological data point at a duration of the postorogenic magmatism in the area from 620 to 530 Ma.
With respect to the absence of high-pressure metamorphic rocks in the Pan-African sutures, the authors will discuss a model of the tectono-magmatic evolution in Panafrican collisional zones.
The Meatiq metamorphic core complex (MMCC) formed as a result of multiple deformation and polymetamorphic events during Pan-African orogeny within the Eastern Desert of Egypt. The basement dome comprises amphibolite-facies metamorphic and magmatic rocks which are covered by greenschist-facies metavolcanics and island arc rocks. On the basis of new geochronological data (U/Pb single zircon evaporation method and 39Ar/40Ar-dating), as well as from structural analyses and fluid inclusion studies we present a new and detailed model for the geological history of the MMCC.
Extension tectonics: between 800 -780 Ma ago the Um Ba´anib granitoid intruded older (approximately 1.15 Ga), highly metamorphosed amphibolites, which exhibit old (Rodinian?) structures. Rifting caused the formation of a basin SE to the Um Ba´anib (related to today's geographic position) in which continental quartz- and mica-rich sediments were transported. Contemporary, ophiolites formed further to the SE of the Meatiq metamorphic basement.
Subduction tectonics: between 660 Ma and 640 Ma magmatic activity happened in the eastern part of the dome. The magmatic events might have been caused by subduction processes during convergent movement of East and West Gondwana. During compression tectonics the sediments experienced amphibolite-facies metamorphism due to subduction (to a depth of c. 20 km) and, subsequent, they were thrusted NW-ward over the Um Ba´anib granitoid, which synchronously deformed to gneiss. Minor crustal thickening was very likely in this time. It caused rapid uplift of the dome from a depth of c. 20 km to around 12 to 15 km. Contemporary, accretion of island arcs took place and continued until the end of the Proterozoic.
Transpressional tectonics and exhumation: between 640 Ma and 570 Ma ophiolitic nappes were thrusted under greenschist-facies conditions NW-ward over the amphibolite-facies metamorphic basement. Subsequently, major sinistral NW-SE-trending strike-slip faults formed under east-west compression, accompanied by the development of low-angle normal faults in the north and in the south of the dome. Contemporary, large volumes of magma intruded into the basement dome and caused enhanced heat input as well as contact metamorphism within the MMCC.
The MMCC experienced a polymetamorphic and a polydeformational history during formation and exhumation. This led to complex and contrasting P-T-t paths. The deformation history dates back to the late Proterozoic and continued at least until the Precambrian/Cambrian transition. The Pan-African orogenic processes (in s. str.) started at c. 640-660 Ma in the MMCC area. The long-time orogenic history in addition to a lack of high-pressure mineral assemblages, as well as the occurrence of synkinematic low-density fluid inclusions (indicating high-T low-P deformation) suggest thin-skinned transpressional tectonics during orogeny and exhumation. Prominent crustal thickening as reported for the southern parts of the East-Pan-African orogen can be unequivocally excluded for the Meatiq metamorphic core complex.
On the basis of lithology, metamorphism and structures, the geology of the Wadi Kid area, Sinai, Egypt can be divided in two parts. The upper crustal rocks consist of low-grade metasediments and metavolcanics. Lower crustal rocks consist of a thick sequence of metavolcanic and metasedimentary rocks and were metamorphosed at LP/HT conditions. Foliated and lineated tonalites, diorites and granodiorites also represent lower crustal rocks. The lower crustal rocks were intruded by A-type granites and composite felsic and mafic dykes. The composite dykes also intruded the upper crustal rocks. The upper crustal rocks were dated at 800-650 Ma (Priem, unpubl. data). The deformation and metamorphism of the lower crustal rocks were dated at approximately 620-560 Ma (Bielski, 1982). The little to non-deformed granites and dykes were dated at 590-530 Ma (Bielski, 1982; Stern and Manton, 1987).
A D1 phase is recognized in the upper-crustal rocks. This phase is represented by F1 open- to closed folds, a S1 axial planar foliation and L1 intersection lineation. These features indicate WNW-ESE compression during D1.
The D2 phase is recognized in the lower crustal rocks. D2-features include the S2 foliation, a L2 lineation, dykes and indicators of non-coaxial deformation. The foliation is predominantly flat-lying. The L2 stretching lineation trends subhorizontally to NW-SE and the dykes trend NE-SW. The synchronous dykes and stretching lineation indicate NW-SE extension. Non-coaxial strain indicators indicate a top-to-the-NW movement with a reversal to the SE. The described D2 features together with HT/LP metamorphism and the intrusion of composite dykes and A-type granites are indicative of a core complex in an extensional setting in the Wadi Kid area.
The D1 phase indicates WNW-ESE compression. A similar regime is observed in the Late Proterozoic of NW Saudi Arabia. This is related to arc-accretion (Quick, 1991). I thus interpret the WNW-ESE compression to be a result of the arc-accretion. The features of the D2 phase indicate the formation of a crustal-scale normal shearzone in a NW-SE extensional setting. Doming of A-type granites caused the formation of a core complex.
The transition from compression to extension is best explained by extensional collapse. The thickened lithosphere was formed during arc-accretion. Conductive heating of the lower lithosphere decreased its strength. The collapse was initiated and extension started. A crustal-scale normal shearzone was formed. Doming caused the formation of an actual core complex.
Throughout the Arabian-Nubian Shield relicts of lithospheric thickening were found in the form of accreted terranes. Structures resembling the Wadi Kid core complex were described in the Eastern Desert of Egypt. Further relicts of extension throughout the ANS are found in the form of post-orogenic molasse basins and NE-SW trending dykes. The process of extensional collapse appears applicable for the entire ANS.
Bielski M, Unpubl. PhD thesis, Hebrew Univ, Jerusalem, (1982).
Quick JE, Precambrian Res, 53, 119-147, (1991).
Stern RJ and Manton WI, J. Geol. Soc, 144, 569-575, (1987).
The Adobha region in northern Eritrea consists of Neoproterozoic volcanosedimentary units belonging to three Terranes. Geochemical data on igneous suites assert that lithologies which belong to the Nakfa Terrane represent a calc-alkalic island arc setting (Woldehaimanot, 1995; Teklay, 1997). Rocks exposed in the Hager Terrane and its extension in the Sudan also show a calc-alkalic geochemistry (de Souza Filho and Drury, 1998; Kröner et al., 1991). The intervening area between the two Terranes is characterized by high strain that resulted in the formation of extraordinary E-directed thrusts which evolved to transpressive, sinistral strike-slip shears. The main lithologies in the intervening zone, basalts and gabbros, are distinguished by their unique tholeiitic T-MORB signatures (Woldehaimanot, 1995). Similar tholeiitic MORB chemistry has also been reported from further south around Kerkebet (de Souza Filho and Drury, 1998). The Kerkebet basalts bear evidences of high P-low T (14.5 kbar, 550°C) assemblages indicative of subduction zone environment. In the presence of such MORB-type ophiolitic sequence bounded on either side by calc-alkalic island arc Terranes that extend for several hundreds of kilometers along strike, I interpret the high strain zone as a major suture, named here the "Adobha Suture Zone".
The Adobha Suture Zone runs in a N30°E direction within Eritrea and continues outside the Eritrean border in a more easterly direction until it is covered by recent sediments near the Red Sea coast. Reconnaissance geological traverses by Kröner et al. (1991) in the area SE of Tokar between Karora and Aqiq indicate that the rocks show similar structural features as those of the Adobha Suture. In addition, geological and geochemical studies carried out in the Al-Lith area of SW Saudi Arabia reveal that amphibolites of the Baish group (Reischmann et al., 1984) bear evidences of MORB affinity and strong shearing. I propose that the highly deformed SE Tokar rocks in the Sudan and the AfAf belt/suture in the Al-Lith area of SW Saudi Arabia are possible continuations of the Adobha Suture.
de Souza Filho C R, Drury S A, J. Geol.Soc. London 155, 551-566, (1998).
Kröner A, Linnebacher P, Stern R J, Reischmann T, Manton W, Hussein I M, Precambrian Res. 53, 99-118, (1991).
Reischmann T, Kröner A, Basahel A, IGCP Project 164, Proceedings, King Abdulaziz University 6, 365-379, (1984).
Teklay M, Department of Mines, Memoir No. 1, (1997).
Woldehaimanot B, Ph. D. Thesis, University of Giessen, Germany, (1995).
The ultramafic-mafic (UMM) complexes from the panafrican belt of Togo are generally considered as a unique lithotectonic unit, commonly interpreted as representative of the suture zone. In the southern part of Togo, the UMM complexes outcrop as a linear trend between Agou and Atakpamé. Recent field survey, petrological and geochemical studies bear some new data on their magmatic and metamorphic evolutions. In that scope, the UMM complexes have been distinguished into four mains types with regards to their primary magmatic and subsequent metamorphic features. This abstract deals with two very spectacular igneous complexes: 1) The Agou type: it seems to be directly intrusive within the pre-panafrican gneissic basement. It corresponds to HT-HP granulites (Opx+Cpx+Pl+Ilm+Ru+Qtz: 925±50°C, 13±0.5 kb) showing a polyphase retromorphic evolution from coronitic granulites (Opx+Cpx+Grt+Pl±Ky+Qtz : 700-790°C, 11±0.5 kb), to high to low grade amphibolitic assemblages (700-750°C, 8-10 kb; 700-650°C, 7-9 kb ; 650-600°C, 5-6 kb) and finally greenschists facies conditions (550-500°C, 3-4 kb). The inferred P-T path defines an anti-clockwise loop in the kyanite stability field. This evolution suggests intrusion and equilibration of mafic magmas in the lower crust and exhumation processes during the Pan-African orogeny. Igneous protolites display composition quite similar to LREE enriched tholeiites (E-MORB). 2) The Lato type: the mafic rocks are always associated with panafrican metasediments (phengite bearing quartzites). They correspond to more or less extensively retrogressed eclogites that occur in different tectonic slices (750±50°C at 19±2 kb and 630±50°C at 13±2 kb). Several generations of amphiboles (pargasite : 750-700°C, 8-10 kb; hornblende: 580-540°C, 4-6 kb and actinolite: 500-450°C, 2-4 kb) record the retrograde transformations down to greenschist facies. The related P-T path displays a clockwise evolution compatible with subduction and collision processes during the panafrican orogeny. Metabasites of the Lato type arise from volcanic and subvolcanic protoliths with MORB affinities ; they marked a lithospheric extension in a continental environment or a narrow ocean.The UMM complexes of southern Togo are probably emplaced in various geological settings, moreover they suffered contrasted P-T evolution suggesting very distinct locations in the crustal section, during the collision. They only share the latest stages of recrystallisation, under amphibolite and greenschist conditions, corresponding to the nappe stacking that put together all the UMM complexes.
Pressure-temperature paths from two granulite-facies areas, the Uluguru Mts and the Lelatema Mts. are constructed. Gneissic garnet-two pyroxene granulites from the Uluguru Mts. suffered isobaric cooling from about 900°C to 750°C at 11-9 kbar, while metaigneous anorthositic and ultramafic rocks show decompressive cooling from as much as 15 kbar and 1100°C to 10 kbar and 750°C. In the Lelatema metasedimentary gneiss-area an isobaric cooling path from about 850°C and 6.5-7 kbar to 450°C and 6 kbar can be constructed based on both textural, paragenetic and thermobarometric data on different metapelitic, calc-silicate and ultramafic rock types.
Combined with the data derived by Coolen (1980) from the Furua complex, it is evident that in large areas and different crustal levels (from 20 to 40 km) an isobaric cooling path is prominent in the Pan-African domain of Tanzania.
Generation and underplating of anorthositic to ultramafic magmas at dry mantle solidus, their crystallization, cooling and decompression during emplacement and subsequent heeting of country rocks can explain the evolution. After that, when the PT-paths of igneous rocks and country rocks joined each other, the rocks underwent nearly isobaric cooling with slight amount of decompression, approaching stable continental geotherm. At least in the beginning of this stage, there is evidence of some extensional or transtensional deformation but the later stage of cooling took place without significant structural reworking. At the end, the rocks were quite rapidly exhumed to upper crustal levels combined with focused deformation and water-rich fluid flow.
Coolen, JJMMM, GUA Papers of Geology, Ser 1, 13, 1-258, (1980).
The Kaoko belt of northwestern Namibia consists of pre-Pan-African basement rocks (about 1.9 Ga and 2.6 Ga) covered by a Pan-African volcano-sedimentary sequence (sandstones, greywackes, pelites, carbonates, felsic and basic volcanics). The complete sequence was subjected to polyphase deformation and probably two phases of metamorphism during Pan-African orogenesis at about 650 and 560 Ma ago (e.g. Seth et al., 1998; Franz et al., 1998).
The metamorphic grade of the second phase increases from the greenschist facies in the east of the Kaoko belt to the granulite facies in the west. The peak of metamorphism coincides with the main deformation D2. Based on investigations of metapelites, evaluated along a transect in the Gomatum and Hoarusib valleys, different metamorphic zones can be distinguished:
garnet zone: garnet + biotite + muscovite + chlorite + plagioclase + quartz staurolite zone: staurolite + garnet + biotite + muscovite + plagioclase + quartz kyanite zone: kyanite + staurolite + garnet + biotite + muscovite + plagioclase + quartz ky-sill-mu zone: kyanite + sillimanite + garnet +biotite +muscovite + plagioclase + quartz sill-mu zone: sillimanite + garnet + biotite + muscovite + plagioclase + quartzsill-ksp zone: sillimanite + garnet + biotite + k-feldspar + plagioclase + quartz g-crd-sill zone: garnet + cordierite + sillimanite + biotite + k-feldspar + plagioclase + quartz
Using the observed mineral assemblages in the various zones, conventional geothermobarometers, and phase diagrams calculated for specific bulk compositions (pseudosections), the following peak metamorphic temperatures and pressures were obtained: 500-550°C / 6-8 kbar for the eastern, 550-650°C / 7-9 kbar for the central, and 650-730°C / 4-6 kbar for the western Kaoko belt.
As examples, PT pseudosections for a kyanite-staurolite-mica schist from the kyanite zone and a garnet-cordierite-sillimanite gneiss from the g-crd-sill zone are shown. These are based on a PT grid for the KMnFMASH system. Using these pseudosections in conjunction with the observed peak assemblages, the prograde growth zoning of garnet and the formation of retrograde minerals enables a detailed reconstruction of PT path segments experienced by these rocks.
Our data indicate that the Pan-African sequence underwent a medium to high T / medium P metamorphic evolution of Barrovian type in the eastern and central Kaoko belt, and a high T / low P evolution of Buchan type in the western Kaoko belt.
Franz L, Romer RL & Dingeldey DP, Eur. J. Mineral., in press, (1998).
Seth B, Kröner A, Mezger K, Nemchin AA, Pidgeon RT & Okrusch M, Precam. Res, in press, (1998).
The Kaoko belt of NW Namibia is part of the late Neoproterozoic mobile belt system of western Gondwana, and its geodynamic evolution is considered to be the result of collision between the Congo (Africa) and Rio de la Plata (South America) cratons. The coastal region of the Kaoko belt consists of high-grade metasediments, migmatites and extensive granitoid intrusives some of which were investigated by the U-Pb, Pb-Pb, Nd and Sr isotopic systems. Two discrete magmatic events can be distinguished on the basis of zircon ages: early igneous activity between 656 and 649 Ma ago produced S-type granitoids, whereas a second event at 583-552 Ma marks voluminous melting of upper crustal material. Nd-Sr isotopic data suggest the granitoid rocks of both magmatic phases to be of upper crustal origin. Monazites in a 656±8 Ma old meta-granite yielded an age of 558±4 Ma which is interpreted to reflect a high-T metamorphic event. Field observations suggest that this high-grade metamorphism caused anatexis within Pan-African metasediments and culminated in widespread anatexis coinciding with the intrusion of crustal melts. A monazite age of 539±7 Ma determined for a 1510±10 Ma old basement orthogneiss from the central part of the Kaoko belt indicates that the pre-Pan-African basement is also affected by this high-grade event. Field relations further suggest that the ~650 Ma magmatic activity is also related to a high-grade thermal event as shown by an age of 645.0±3.5 Ma for metamorphic zircons from a granulite-facies metasediment from the same area (Franz et al., in press).
The Ribeira and northern Dom Feliciano belts of SE Brazil are considered to represent the western continuation of the Kaoko belt. A high-grade tectono-thermal event culminating in partial melting and granitoid intrusions at 590-565 Ma is reported from the central Ribeira belt (Machado et al., 1996). This event corresponds to the peak of the second magmatic and metamorphic event in the Kaoko belt, thus indicating a synchronous evolution and supporting the model that both the Namibian and Brazilian belts belonged to the same orogen in late Neoproterozoic times.
Franz L, Romer RL & Dingeldey DP, Europ.J.Mineral., (in press)
Machado N, Valladares C, Heilbron M & Valeriano C, Precambrian Res., 79, 347-361, (1996).
The overwhelming part of the central zone of the Pan-African Damara Orogen, Namibia, consists mainly of medium- to high grade grt-crd-sil-Kf-bearing metasediments, migmatites and numerous crd-grt-bearing leucogranite intrusions. The central zone is a high temperature area with estimated peak metamorphic conditions of ca.700°C/4 kbar in the north-east to ca. 750°C/5 kbar along the coast in the south-west.U-Pb monazite ages obtained on non-migmatized metapelites indicate that the peak of regional metamorphism occurred between 520 and 510 Ma. Anatectic garnets are anhedral and show flat elemental profiles and abundant inclusions. Such garnets gave Sm-Nd gnt-whole rock ages between 522 Ma and 508 Ma, close to the inferred thermal peak of regional metamorphism.Sm-Nd gnt-whole rock ages from magmatic garnets (euhedral grains with major element zoning and rare inclusions) indicate intrusion of felsic melts between 470 and 509 Ma, relatively late in the metamorphic history. The observation that clearly magmatic garnets still retain their magmatic major element zonation suggest that the final thermal peaks during the metamorphic evolution were not long enough to homogenize the garnet by volume diffusion. The flat profiles of the anatectic garnets might be explained by a longer period of static annealing between 522-508 Ma or by the growth of garnet in the presence of a melt or fluid during migmatization which probably allows a more rapid diffusion of the elements. The combined monazite and garnet ages indicate that in-situ migmatization is confined to the culmination of regional metamorphism. U-Pb monazite ages from intrusion-related migmatites give distinct ages of 540 Ma, 520 Ma, 485 Ma, 475 Ma and 465 Ma indicating that local migmatization occurred also prior to the major thermal peak. The Sm-Nd garnet ages and the U-Pb monazite ages obtained on the migmatites and metasediments give evidence for a polymetamorphic history associated with discrete thermal pulses due to intrusion of felsic melts. The intrusion of grt-bearing felsic melts is confined to the post-collisional phase of the orogeny probably related to the rapid exhumation of the belt.
Cambrian arc belts in SE-Australia, Tasmania, New Zealand and Antarctica are part of a coherent convergent margin, active over 20-30 Ma from the latest Early Cambrian to the Late Cambrian. Two key sequences are exposed in Western Tasmania and the northern South Island of New Zealand. Throughout the Mid Cambrian, magmatism in these two fragments and in SE-Australia/Antarctica is documented by intra-oceanic arc and back arc sequences. In the Late Mid Cambrian, the collision of theses arc segments with the Gondwana continent is evident from obducted ophiolites and tectonic overprint in Tasmania and SE Australia (Adelaide Fold Belt). Post-collisional magmatism of Latest Mid to Early Late Cambrian age (e.g. Mt. Read Volcanics of Tasmania) terminates convergent tectonics in SE Australia and Tasmania. In contrast, no such volcanism is known from Antarctica and New Zealand, where volcanism ends in the Latest Mid Cambrian (Glasgow Volcanics, Antarctica) or continues into the Late Cambrian in an intra-oceanic arc setting (Devil River Volcanics, New Zealand), respectively. Common to most Cambrian fragments in SE-Gondwana, however, is the tectonic overprint by the Ross Delamerian orogeny in the Late Cambrian/Early Ordovician which paleogeographically links all the above fragments by the end of the Cambrian. The overall synchroneity of distinct tectonomagmatic events in the different SE-Gondwana fragments therefore suggests that they were all part of one arc system. A change in collision style from continental against oceanic crust (Australia/Tasmania) to oceanic against oceanic crust (New Zealand/Antarctica) explains the differences between the Australian/Tasmanian and New Zealand/Antarctic arc segments. Limited Pb-Nd isotope and U-Pb detrital zircon data obtained for sediments and volcanic rocks in SE Gondwana suggest that the Cambrian arc segments all originated along the Australian-Antarctic margin. No exotic fragments which possibly originated from Laurentia or South America, both located nearby in Cambrian time, could be identified that far. Separation of Laurentia from Gondwana must have therefore occurred prior to the late Early Cambrian.
Geological mapping and petrological work carried out within the frame ofthe international 1995/1996 GeoMaud Expedition, in the Orvin and Wohlthatranges, indicate that Neo-Proterozoic/Early Palaeozoic basement complex ofcentral Dronning Maud Land (cDML), consists of several rock units showingdistinctive lithological assemblage and partly contrasting metamorphichistories. The oldest units comprise: 1) a layered sequence of banded hornblende-bearing gneiss, plagiogneiss and amphibolite, minor ultramafic and mafic granulite lenses and rare metagabbro; and 2) a metasedimentary unit including biotite-sillimanite-garnet, biotite-hornblende or orthopyroxene gneiss, Ca-silicate rocks, diopside+/-forsterite marble, quartzite and garnet-bearing amphibolite. Both units were intruded by voluminous granitoids, comprising an early granitic phase and a latertonalitic phase, both extensively metamorphosed to Garnet-bearing migmatitic augen gneiss (3) and to hornblende-bearing augen gneiss (4), respectively. Subsequently a network of garnet-bearing leucogranites (5) intensively deformed and migmatitized, was emplaced. Mafic boudins from units 1 and 2 from Conrad Range and Dalmann Ranges retain relics of a pristine clinopyroxene-garnet-rutile+/-quartz or olivine assemblage, indicative of an early metamorphic event (M1) under high P granulite facies/HT eclogite (?) facies conditions. Delicate kelyphitic coronas of plagioclase-orthopyroxene-clinopyroxene ±spinel around garnet and symplectitic aggregates of plagioclase-clinopyroxene point to a decompressional path and partial reequilibration under lower P granulite facies conditions (M2). M1/M2 parageneses were subsequently variably affected by upper amphibolite to granulite facies transformations (M3), as well as by minor lower amphibolite to low grade retrogressions (M4). Coarse-grained orthopyroxene-bearing leucosomes commonly occurring within the necks of mafic boudins formed late- to post-tectonically during M3. These later metamorphic stages are more pervasively developed in the country felsic rocks, and particularly M3 lead to the most significant regional re-equilibration of most rock type at the regional scale. Preliminary PT estimates for M3 stage closely matches those estimated for mafic boudins of the Shirmacher region (Rao et al., 1997) and in marble and calcsilicate rocks from cDML (Markl & Piazolo, 1998), whereas inferred M1 conditions, as well as the decompressional path, are similar to those reported by Groenewald & Hunter (1991) for mafic granulites from western Dronning Maud Land (H.U.Sverdrupfjella). On the basis of the most recent U-Pb SHRIMP zircon ages available in cDML (Jacobs et al, 1998), the M1 to M2 metamorphic evolution may be tentatively assigned to an earlier Grenville-age tectonic cycle, and the M3-M4 stages to the pan-African (Ross-age) thermo-tectonic reactivation.
Groenewald PB, Hunter DR, Thomson et al., Geological evolution of Antarctica, Cambridge University Press, 61-66, (1991).
Jacobs J, Fanning CM, Henjes-Kunst F, Olesh M, Paech H-J, J. Geology, 106, 385-406, (1998).
Markl G, Piazolo S, Contrib. Mineral. Petrol, 132, 246-268, (1998).
Rao DR, Sharma R, Gururajan NS, Transactions of the Royal Society of Edinburgh, 88, 1-17, (1997).
The evolution of the arch-shaped, symmetrical Araçuaí-West-Congo Orogen (AWCO) was confined to the São Francisco-Congo cratonic embayment during the Brasiliano-Pan-African Cycle (cf. Trompette, 1994). The 21° S parallel marks the AWCO's southern limit in Brazil. In fact, the AWCO constitutes the northern branch of the complex orogenic system that includes the Ribeira, Kaoko, Dom Feliciano, Damara, and Gariep belts. Since 1992, the major part of the Brazilian territory of the AWCO has been mapped in the 1:100000 scale or greater. We base our interpretations in this up-to-date field knowledge of the AWCO Brazilian region, and on new geochemical and geochronological data. The AWCO internal orogenic domain shows remnants of a Neoproterozoic oceanic lithosphere recorded by metamorphosed MORB-type basalts, deep-sea sediments and tectonic slabs of ultramafic rocks (Pedrosa-Soares et al., 1998a). These oceanic slivers are evidence of a suture located along the 42° W meridian, between the 17°S and 19° S parallels. This suture zone is also marked by thrusts, flower-like structures, and regional magnetic and gravimetric anomalies. Five granitoid suites of Neoproterozoic to Cambrian age occur along the AWCO internal domain (Pedrosa-Soares et al., 1998b). Two are I-type, calc-alkaline granitoids, and depict a west-to-east zoning indicated by decreasing ages and increasing LILE contents. The tonalitic to granodioritic, calc-alkaline plutons of the Galiléia Suite (ca. 600-575 Ma) have consistent pre-collisional, volcanic arc geochemical signatures (Nalini, 1997; Noce and Pedrosa-Soares, unpublished data). These plutons display gneissic foliation following the Brasiliano structural trend, and compose N-S elongated batholiths parallel to the suture zone. This extensive occurrence of calc-alkaline granitoids indicates that a reasonably large amount of oceanic crust was consumed via an east-dipping subduction zone. Nonetheless, Rb-Sr and Sm-Nd isotopic data suggests involvement of continental crust in the pre-collisional calc-alkaline granitogenesis (Nalini, 1997). The post-collisional, high-K calc-alkaline Aimorés Suite (ca. 570-520 Ma) occurs eastward of the Galiléia Suite (Pedrosa-Soares et al., 1998b). In fact, the cratonic connection that once linked the São Francisco and Congo cratons might have imposed mechanical constraints for the AWCO development (cf. Trompette, 1994). However, the coeval evolution of the Paramirim (Brazil) and Sangha (Africa) aulacogens, as well as other extensional structures in the cratonic region, could have accommodated the required ocean spreading (Pedrosa-Soares et al., 1992, 1998a; D'Agrella-Filho et al., 1998). In conclusion, we propose that the AWCO accretionary stage was related to a B-subduction-controlled, continental magmatic arc.
D'Agrella-Filho MS, Trindade RIF, Siqueira R, Ponte-Neto CF & Pacca IG, Int. Geol. Rev., 40, 171-188, (1998).
Nalini, HA, Dr Thesis, École des Mines, Saint-Etienne, France, 237 p., (1997).
Pedrosa-Soares AC, Noce CM, Vidal Ph, Monteiro RLBP & Leonardos OH, J. South Amer. Earth Sci., 6, 33-47, (1992).
Pedrosa-Soares AC, Vidal Ph, Leonardos OH & Brito-Neves BB, Geology, 26, 519-522, (1998a).
Pedrosa-Soares AC, Wiedemann C, Fernandes MLS, Faria LF & Ferreira JCH, Revista Brasileira de Geociências, in press, (1998b).
Trompette R, Geology of Western Gondwana, Balkema Publ, 350 p, (1994).
The Palaeozoic rocks of the W Pontides tectonic belt of northern Turkey comprise a transgressive sedimentary sequence, known as 'Palaeozoic of Istanbul". In a few areas, the basement of the Palaeozoic sequence is exposed, the largest of which is the Bolu Massif, located in the middle of the west Pontides. The lowermost unit of the "Palaeozoic of Istanbul" in the Bolu area is the Isigandere Formation, made up of fluvial red conglomerates and sandstones of Lower Ordovician age. Three different units are exposed unconformably beneath these continental clastics, forming the Bolu Massif. These are, from the structural base to the top: i) a high grade metamorphic unit (the Sunnice Group); ii) granitoid intrusions (the Bolu Granitoid Complex-BGC); and iii) a greenschist meta-volcanic sequence (the Çasurtepe Formation). The structurally lowest Sünnice Group is a SW-NE trending belt of migmatitic basement, consisting of amphibolites and paragneisses, cut by small (< 10 m) metagranitic intrusions. The Sunnice Group is tectonically overlain by the BGC and the Çasurtepe Formation along the Karadere Fault. In the study area the BGC is represented by two distinct, NNE-SSW trending én echelon intrusions, the Tüllükiris and Kapikaya Plutons. The granitoids are mainly tonalitic and granodioritic in composition, cut by lamprophyre and aplite dykes and intruded into the Çasurtepe Formation. The Çasurtepe Formation is composed of mainly andesitic and minor rhyolitic lavas, and a meta-ignimbrite sequence.The lavas have geochemical characteristics indicative of eruption in a subduction-related tectonic setting. The geochemistry of the intrusions also suggests emplacement in an arc-type setting. Initial Nd isotope data for the Çasurtepe Formation indicate derivation from a depleted mantle source, whereas those for the granitoids are consistent with greater degrees of crusal contamination. Structural data indicate that the calc-alkaline granitic intrusions were emplaced at high crustal levels in a strike-slip setting associated with an oblique subduction. Extension then led to the exhumation of the high grade metamorphic rocks (the Sunnice Group) along a north dipping, top to the north detachment zone, later reactivated and became a thrust during the Eocene compression. The extension created irregular topography with structural highs and lows, into which the sediments of the Isigandere Formation were deposited in a fluvial setting. Further extension then resulted in construction of a Palaeozoic passive margin sequence, known as the Palaeozoic Istanbul.
Aeromagnetic data reveal a pattern of anomalies that can be interpreted in terms of the structure and evolution of the Avalonian basement beneath southern Britain, south of the Iapetus Suture. These data are of particular importance in view of the small number of isolated Precambrian outcrops and borehole provings in the region and the paucity of deep seismic evidence. Magnetic anomalies are associated with late Precambrian magmatic rocks in the English Midlands (Charnian), the Welsh Borderlands, and SW and NW Wales. The anomalies relating to the Charnian arc-magmatic rocks form a NNW-trending belt which appears to cross-cut an anomaly due to a deeper magnetic source beneath SE England. The latter is interpreted as a block of older Precambrian magnetic basement; the western edge of this block forms a N-S linear feature parallel to, but to the east of, the Malvern Lineament. There is a hiatus in the magnetic anomaly pattern at the Variscan Front, although it is possible to identify a possible continuation of the Charnian feature within the basement beneath the Variscides. NW-trending magnetic anomalies in eastern England are probably due in part to Ordovician plutonic rocks associated with the closure of the Tornquist Ocean, but the magnetic evidence suggests that these were intruded into a pre-existing crystalline basement and that structures within this basement controlled their emplacement. Cross-cutting magnetic anomaly relationships can be used to infer that a NNE-trending magnetic basement boundary beneath the Craven Basin in northern England dates from the late Precambrian - Cambrian assembly of eastern Avalonia; this feature lies on the projection of the Welsh Borderland Fault System. A further, NNE-trending magnetic feature beneath the Irish Sea is interpreted as the probable north-western limit (at least in the upper-middle crust) of the Avalonian crystalline basement. This boundary can be traced from the northern margin of such rocks in SE Ireland to the Isle of Man; it is interpreted to be an Avalonian basement structure which was reactivated as a major extensional fault during early Palaeozoic times. ENE-trending structures were clearly active during Iapetus closure and subsequent Acadian deformation, but there is evidence to suggest that this was a pre-existing trend; this is demonstrable in the case of the Menai Strait Fault Zone, and may also apply with the (subsequently reactivated) structures with a similar trend in Northern England. Analysis of the regional magnetic anomaly pattern therefore suggests that the structural fabric established during the assembly of Eastern Avalonia has had an important influence on the subsequent evolution of southern Britain.
1. A recent paper by V.E.Khain and S.G. Rudakov (1995) shows the global range, in the plate-tectonic context, of the Baikalian era of tectogenesis. This contribution sets out to highlight this important stage in the Earth's geologic history (1000-500 Ma) using the tectonotype of the Siberian craton and its southern folded surroundings. 2. The onset of the Baikalian era of tectogenesis, following the Grenville one, is marked by the breakup of the Rodinia supercontinent and inception of the large Paleo-Asian Ocean, to separate the Siberian craton, Baltia, North China, and Tarim-Kazakhstan cratons, between 1000 and 900 Ma. This is evidenced by the thick sedimentary sequences at the Siberian craton's passive margins formed in the Yenisei Range, the Baikal-Patom region, and in marginal parts of the Tarim-Kazakhstan craton, as well as by the ca. 1 Ga dating for the protoceanic crust in the Baikal-Muya belt. 3. The 850/830 Ma boundary (the Yenisey phase) is marked by (a) crustal contraction, orogeny, granitoid magmatism, and regional metamorphism reported for the Siberian craton's margins facing the Yenisey Range and (b) the inception of primary suprasubduction-related island-arc systems of Paleo-Asian Ocean (East Sayan, the Baikal-Muya belt). 4. The pre-Vendian (Khantaishirian) epoch of tectogenesis (740-690 Ma) is manifest by the rifting phase reported in marginal parts of the Kuvay, Olokit, and Dzabkhan cratons. A new extended system of volcanic arcs and the associated East Sayan, Central Mongolian, Polar Urals and Taimyr basins arose there. 5. The Kadomian tectonic epoch (630-510 Ma) shows a more complicated structure of Paleoasian Ocean, possibly due to changes in the plates movement kinematics. It is the time of extinction of the old island-arc systems, massive appearance of marginal accretionary prisms, and the ophiolite obduction, eventually leading to the erosion of ophiolites and related rocks. Foreland basins tend to form at the cratonic margins. In the environment of general compression, new spreading and subduction zones, as well as new basinal and volcanic-arc systems, arise. 6. The Salair epoch of tectogenesis (500-460 Ma) is marked in the tectonotype under study by massive accretion of perioceanic structures and microcontinents onto the Siberian craton, as well as by abundant granitoid magmatism and high-T metamorphism, along with generation of molasses. The orogenic processes are greater in scale than those known in earlier epochs of the Baikalian era of tectogenesis. In V.E.Khain's opinion, this epoch is an important turning point in the Earth's Early Paleozoic history, corresponding to the transition from Baikalian to Caledonian era of tectogenesis.
The Palaeo-Asian Ocean, whose fragments are found in the Urals-Mongolia foldbelt, existed throughout the Neoproterozoic, early, and middle Palaeozoic. The first events related to the formation of this ocean occured 950-1050 Ma ago. This period characterizes the development of passive continental margins on the Siberian and Tarim cratons and on the central Kazakhstan microcontinents.
Evidence for active margins first appeared at about 900-950 Ma. The Baikal-Muya zone (Baikal region) contains arc fragments (825-880 Ma supra-subduction zone gabbros) and coeval back-arc basins. The Dunzhugur complex (East Sayan, U-Pb and Sm-Nd ages ca. 900 Ma), and Yenisei Range (825-850 Ma) contain late Riphean ophiolites. Associated volcanics have supra-subduction zone geochemical fingerprints.
Between 850 and 650 Ma, a new, larger system of mature ensimatic arcs and related basins evolved at the paleocean's margin. Fragments thereof are the Polar Urals and Taimyr ophiolites (670 and 733 Ma, respectively) and the East Sayan and North Mongolian Sarkhoi-Darkhat volcanics (720 Ma). Coeval arc volcanics occur in the Daribi-Khantaishir-Ulanshandin volcanic belt of central and western Mongolia. In the Daribi Range, these volcanics overrode spreading-related ophiolites with a Sm-Nd age of 695±25 Ma.
During the late Riphean, the Kazakhstan-Tarim paleoceanic margin remained passive. Crustal heating and melting during the initial rifting produced bimodal volcanic and plutonic rocks (rhyolite and basalt-rhyolite sequences, subalkali granites, granosyenites, alkali syenites). Further spreading in the late Riphean created a vast ocean. At first, its eastern and northern margins rimming the Siberian craton evolved as passive, and later, as active ones. Simultaneously, the Kazakhstan-Tarim complexes arose along passive margins of this ocean. The ocean was bounded, on the west and south (present reference frame), by the Baltic, Tarim-Kazakhstan, and North China cratons and, on the east and north, by the Siberian craton and a system of microcontinents, likely to be fragments of Rodinia.
During the Vendian, the marginal structures of the Paleo-Asian Ocean became more complex. Along the Siberian margin, a major system of volcanic arcs and related basins became extinct. Pre-existing volcanic arcs converged with continents and microcontinents, whose margins show evidence of accretionary prisms and obduction of the oldest ophiolites. Although the general environment was compressional, new spreading occurred, and subduction zones and volcanic arcs evolved. These supra-subduction zone structures are found in the North Baikal region, the Djida zone of North Mongolia, the Olkhon region, and the Bayankhongor zone in central Mongolia. The Kazakhstan-Tarim margin remained passive during this time. Thus, during the Baikalian stage of tectogenesis, the marginal structures of the paleocean became more complex, most likely through changes in global plate kinematics.
Structural data and 40Ar/39Ar cooling ages of the metamorphic complex of Beloretzk (MCB) and the provenance signature of Riphean and Vendian siliciclastic rocks in the western fold-and-thrust belt of the southern Urals give clear evidence for a Neoproterozoic orogenic event at the eastern margin of Baltica. The MCB is composed of low to high grade (locally migmatitic) metamorphic rocks which are intruded by granites. Towards the West the prominent Zuratkul fault (ZF) separates the metamorphic rocks of the MCB from low to medium grade metamorphic rocks of the western and central Bashkirian Megaanticlinorium (BMA). As described in detail by Bauer et al. (1999) three pre-Ordovician deformation phases can be identified in the MCB, a first S-vergent, isoclinal folding phase synchronous to the peak metamorphism which is followed by later extensional event and a third major E-vergent folding and thrusting phase at retrograde metamorphic conditions. The MCB is unconformably overlain by Early Paleozoic sediments which have experienced only a week Late Paleozoic Uralian deformation and incipient metamorphism. The Late Vendian clastic deposits of the western part of the BMA are distinctly different from the Riphean clastics but similar to the Ordovician clastics. Both indicate derivation from a recycled orogen. The reminiscence of the break up of Rodinia as revealed by an 40Ar/39Ar amphibole age of 718±5 Ma and the occurrence of mafic intrusions suggest the rifting period followed by the drift of the ´Beloretzk terrane´. 40Ar/39Ar cooling ages of muscovite (543±4 to 597±4 Ma) and the distinctive pre-Ordovician deformation history indicate that the Neoproterozoic orogeny was initiated by the accretion of this terrane with the E-margin of Baltica. The recognition of the exotic ´Beloretzk terrane´ at the E-margin of Baltica provides further constraints for Late Proterozoic paleogeographic plate reconstruction as well as important implications for the structural evolution of the SW Uralian fold belt.
Bauer, W, Glasmacher, U, Giese, U, Ladage, St, Alekseyev, AAand Puchkov, VN, J. Conf. Abs., 4, (1999).
Central Asia is composed of a collage of continental blocks, ancient island arc terranes, subduction complexes and fragments of ocean crust that amalgamated during the Palaeozoic. Sengör et al. (1993) suggested a continental growth mechanism for Central Asia of subduction accretion and arc migration within one long-lived subduction zone along which the so-called Altaids were accreted. Conversely, Hsü et al. (1991) and Coleman (1989) identified distinct ophiolite belts in Northern China and proposed a punctuated accretion mechanism of collision and closure of multiple ocean basins. One approach to reconcile these opposing models is to determine whether ophiolitic rocks represent true suture zones marking positions of former oceans, or simply offscraped crustal fragments incorporated within an accretionary complex.
Mongolia has some of the best preserved ophiolites in Central Asia. The Bayankhongor ophiolite, a NW-SE striking linear belt 300 km long and up to 20 km maximum wide, is the largest ophiolite occurrence in Mongolia and possibly in all of Central Asia. We present initial results of three detailed cross-strike transects of the ophiolite and its bounding zones. The transects can be divided into five zones from south to north; the Baydrag and Bömbögör complexes, Bayangol, Bayankhongor and Dzag zones. The Archaean Baydrag complex is composed of tonalitic gneiss and granulite. The Bömbögör complex consists of granitic orthogneiss and amphibolite, as well as minor marble and quartzite. The Bayangol zone comprises a tectonic mélange of sedimentary and igneous lenses with abundant quartz veining. Metamorphic zonation occurs with increasing grade towards the north. The Bayankhongor zone contains a full ophiolite stratigraphy, dated at 569 Ma (Sm-Nd whole rock on gabbro; Kepezhinskas et al., 1991) which was dismembered during obduction and collision to form a mélange of blocks within a serpentinite matrix. Within this zone, to the north and south of the ophiolite are imbricated sedimentary and volcanic rocks of Precambrian to Carboniferous age. The Dzag zone consists of asymmetrically folded chlorite-mica schists. In less deformed areas these resemble meta-turbidites. The overall structure is dominated by steeply dipping, NE-vergent thrusts and folds. Near-horizontal lineations in the Bayankhongor and Dzag zones, also indicate a left lateral strike slip component accompanied deformation.
Initial conclusions suggest closure of an ocean separating two microcontinents; the Baydrag-Bömbögör complex to the south and a northern continent below the Hangay region. The Dzag zone is interpreted to represent a passive margin to the Hangay continent and the Bayangol zone is interpreted as an accretionary complex attached to the southern continent. Subduction was to the SW, based on the polarity of thrusting, with oblique ophiolite obduction to the NE. Geochemical work in progress will further define the original tectonic environment for the Bayanhongor ophiolite and adjacent lithotectonic domains.
Sengör AMC, B. A. Natal'in BA & Burtman VS, Nature, 364, 299-307, (1993).
Hsü KJ, Qingcheg W, Liang L & Jie H, Eclogae Geologicae Helvetiae, 84, 1-31, (1991).
Coleman RG, Tectonics, 8, 621-635, (1989).
Kepezhinskas PK, Kepizhinskas KB & Pukhtel IS, Geophysical Research Letters, 18, 1301-1304, (1991).
The Tuva-Mongolia terrane (TMT) lies in the SW fold fringing of the Siberian Craton. It is the product of the Baikalian orogeny occurring at the Late Riphean/Vendian boundary and is manifested by accretion of Late Riphean island arcs and ancient sialic blocks. The fold-nappe orogen generation was associated with accumulation of Lower Vendian continental molasses. During the Vendian-Cambrian time, the TMT appeared as a microcontinent with a plarformal carbonate sedimentation (Kuz'michev, 1996, 1997). Later, the Riphean and Vendian-Cambrian strata suffered folding and granitoid intrusions, which made the role of Baikalian orogeny less evident. The Paleozoic deformations have mainly resulted in numerous en-echelon normal faults, dipping northwards and accompanied by folding and cleavage. The mapping has revealed the occurrence of dextral faults and the related Z-shape folds. The latter resulted from superposition of sublatitudinal Paleozoic dextral deformations upon the submeridional Upper Riphean linear folds. The related magmatism is presented by calc-alkaline and alkaline granites and syenites. Geochemically and geologically, the bulk of granitoids may be considered anorogenic. These form discordant plutons, whose intrusion directly followed the main stage of Paleozoic faulting, simultaneously with dextral deformations. The synshift intrusion is revealed by the morphology of the Ikhe-Khaigass pluton. The latter is an oval-sygmoidal intrusion associated with dikes formed in the pressure shadows. The age of granites is determined with the Rb-Sr method by a mineral isochrone (5 points): T = 460,4±2,5, I = 0,70429±3, MSR = 0,85. The obtained value also date the dextral deformations. The Mid-Ordovician events are related with the Kaledonian orogeny - the closure of the ocean areas fringing the TM microcontinent. Presence of superposed deformations and anorogenic granitoids do not prevent to attribute the terrane to the Baikalides, since the Paleozoic cratonization has not yield the continental crust generation, though it increased the crustal sialic properties. The work is supported by RBRF, grant 98-05-64876.
Kuz'michev AB, Geology and prospecting. Izvestiya VUZOV, 3, 11-25, (1996).
Baikalian-age complexes are exposed in the cores of anticlines along the western front of the Uralide orogen. These Precambrian rocks were accreted to the margin of the East European Craton in the Vendian, prior to deposition of Baltica's passive margin platform successions in the Early Ordovician (locally, in the late Cambrian). In the southern and central Urals, the Baikalian complexes strike more or less longitudinally. In the north, the Neoproterozoic fold belt strikes northwesterly, from the northern Urals as far west as the Timan range, continuing out into the south Barents Sea and the Varanger Peninsula of northern Norway. Beneath the Pechora Basin, northeast of the Timan Range, widespread Vendian magmatism (c. 560 Ma) was related to Baikalian deformation. Further east in the front of the Polar Urals, fragmented Neoproterozoic ophiolites (c. 670 Ma) are present within the pre-Ordovician complexes, e.g. the Engenape anticline. Even further east, in the Uralian thrust sheets, new ion-microprobe U-Pb zircon data from the high-pressure, ecologite-bearing, Marun-Keu complex provide clear evidence of latest Proterozoic (and Cambrian?) protolith ages (577 ± 7 Ma & 545 ± 10 Ma). Local metamorphic overprinting occurred as a result of thermal highs associated with igneous intrusion (~540 Ma) or Devonian deformation (~380 Ma). An inherited component of ~630 Ma is also well defined, along with single grains as old as 3.3 Ga.
To better assess the character of the Baikalian-age basement beneath the Ordovician shelf sandstones and carbonates of Baltica, a program has been started to identify provenance ages in the strata directly overlying the early Paleozoic unconformity. In the Polar Urals, detrital zircons in Ordovician quartzites from the Engenape anticline and an anticlinal window beneath the Marun-Keu nappe, yield a dominant c. 575 Ma signature, with a significant contribution (up to 50%) from Precambrian sources (1.0 to 2.3 Ga). In addition, an Ordovician (450 to 475 Ma) component of unknown origin has been identified and represents a maximum age for these sediments. It can be concluded that the entire northeastern continental margin of Baltica, from the Timan Range into the Polar Urals, was dominated by Baikalian-age complexes and that older (Early and Middle Proterozoic) crustal components were also involved.
There are many examples demonstrating how new geochronological data result in the revision of traditional geological models. This situation is particularly characteristic for Russian geology because reliable and precise geochronological data were only obtained during the last 10 years or so.
In previous models for the evolution of Central Asia the Tuvino-Mongolian Massif (TMM) was mostly considered as an ancient microcontinent like one of the larger Precambrian blocks within the Central Asian mobile belt separating the ancient cratons of northern Eurasia. In these models, "pre-Riphean basement rocks" experienced two episodes of regional metamorphism, and Lower to Middle Riphean cover rocks were reworked together with the basement during high-grade metamorphism. These high-grade rocks have been correlated with the Precambrian basement of the Siberia Craton.
Our new precise U-Pb zircon data demonstrate that the earliest metamorphic event in the "basement" (536±6 Ma) occurred after deposition of the supracrustal cover. These data, combined with Sm-Nd mean crustal residence ages, permit to bracket the depositional age of the basement rocks between 536 and 1840 Ma. Sm-Nd data for granitoids emplaced in the time interval 536±6 Ma to 465±6 Ma indicate the involvement of long-lived early Proterozoic and juvenile Vendian-Cambrian crustal sources in their formation.
On the basis of these data and detailed fieldwork, we propose the following new model for the evolution of the TMM: Metasedimentary sequences were derived from Palaeoproterozoic crustal rocks of the Siberian Craton (Eastern Gondwanaland?) and were the main source for granitoids formation some 800-900 Ma ago in an Andean-type active continental margin setting. The granitoids generated during this event have subsequently been the source for Neoproterozoic supracrustal rocks of the TMM. The first metamorphic event and related magmatic processes affected this rock association in latest Neoproterozoic times along an active continental margin during accretion processed that occurred close to the southern margin of the Siberian Craton. The second metamorphic event occurred at 520-500 Ma and was related to collision processes following closure of the Khantaishir ocean and amalgamation of microcontinents, island arc and ocean basin complexes (oceanic plateaux?). Structures resembling those in modern accretionary belts were found in the metasedimentary units of the Tuvino-Mongolian Massif.
Vendian - Cambrian rocks have been topic of active paleomagnetic research since the late 1950's (Khramov, 1958). But the majority of paleomagnetic poles were obtained inside folded zones around Siberian platform. Main purpose of previous years studies were magnetostratigraphic correlation of sediments at long distances in different folded zones. Using of these data for paleoreconstuctions of Siberian platform is not quite correct and may lead to many problems. Scatter of paleomagnetic poles of this age is about 360° of great circle. As a result, position of Siberian platform for Vendian - Early Cambrian varies greatly from one author to the other (Kravchinsky, 1979; Kirschvink et al., 1997; Pisarevsky et al., 1997; Kravchinsky and Konstantinov, 1997, Smethurst et. al., 1998). That is why obtaining of paleomagnetic pole directly for Siberia was a most actual task.We carried out paleomagnetic studies of the Vendian - Early Cambrian aged rocks (520 - 650 Ma) in South Siberia in order to reconstruct paleoposition of main lithospheric blocks of Siberian platform (Aldan and Angarian blocks), Barguzin and Tuva-Mongolian blocks and Baikal-Patom folded zone. During this study we:- obtained the Vendian - Early Cambrian paleomagnetic poles of different lithospheric blocks composing Siberian continent and revised last version of the Siberian APWP for 520 - 650 Ma.- reconstructed primary features of Sayan-Baikal folded belts and relations between Angarian and Tuva-Mongolian blocks.- evaluated the character and extent of post-Cambrian intercontinental deformation.
Khramov AN, Paleomagnetic correlation of sediment formations, Gostechizdat, Leningrad, 218, (1958).
Kravchinsky AYa, Paleomagnetism and paleogeographic evolution ofcontinents, Novosibirsk, Nauka, 264, (1979).
Kirschvink JL, Ripperdan RL & Evans DA, Science, 277, 541-545, (1997).
Pisarevsky SA, Gurevich EL & Khramov AN, Geophys. J. Int, 130, 746-756, (1997).
Kravchinsky VA & Konstantinov KM, To carry paleomagnetic investigations aimed at obtaining number of paleomagnetic poles for the basic geological formations of Vendian and Palaeozoic age of Eastern Trans-Baikal region (for the Geological survey of scale 1:200,000 and 1:1,000,000), Report, 193, (1997).
Smethurst MA, Khramov AN & Torsvik TH, Earth-Science Reviews, 43, 1-24, (1998).
In South India, a major system of ductile shear zones - the Palghat-Cauvery shear system - separates the Archaean Dharwar craton in the north from the early Proterozoic Madurai Unit and the adjacent Trivandrum Block in the south. The E-W trending Moyar shear zone (MSZ) as well as the NE-SW trending Bhavani shear zone (BSZ), which bound the late Archaean Nilgiri Hills terrane, both form part of this suture. Within these shear zones retrogression and bleaching led to the development of amphibolite facies gneisses with mylonitic fabrics. Relict granulite mineral assemblages and transitions to undeformed enderbitic and mafic granulites occur.
To unravel the presumably polyphase tectonic and metamorphic history of the suture belt Rb-Sr and Sm-Nd analyses on minerals, small slabs and whole rock samples were carried out in both shear zones and the neighbouring crustal blocks.
In the MSZ, garnets of an undeformed enderbite yielded an age of 2355 ± 8 Ma (all quoted errors are 95% conf.) which is in agreement with Sm-Nd-garnet ages from the Nilgiri Hills (Köhler and Srikantappa, 1996). The data indicate garnet crystallisation during postmetamorphic cooling after a climax of high grade metamorphism at approximately 2.5 Ga (Buhl, 1987). In the same quarry, cataclastic garnets of an enderbite affected by retrogression and shearing give a significantly younger age of 613 ± 120 Ma. This reveals a thermal event as well as tectonic reactivation in the MSZ during Pan-African times. A concordant value results from a Rb-Sr small slab investigation (624 ±74 Ma, 9 slabs). Rb-Sr mineral ages of syn- and postkinematically grown biotite about 565 Ma reflect the subsequent postmetamorphic cooling stage.
In the BSZ the Pan-African event is also obvious: coronitic garnets in a metadolerite yielded an age of 552 ± 8 Ma. Rb-Sr biotite ages from enderbites scatter in a narrow range around 508 Ma. Muscovites taken from a deformed pegmatite indicate an age of 591 ± 15 Ma, while two samples of a post-tectonic pegmatite generation (muscovite 499 ± 12 Ma; biotite 492 ± 12 and 472 ± 12 Ma) give a minimum value for ductile tectonics. Thus, our data clearly reflect the Pan-African imprint which has also been recorded in the proterozoic Madurai and Trivandrum blocks. The results are highly discordant with mineral ages from the Nilgiri Hills and the Dharwar craton.
Other results may give evidence for an older thermal event and reflect the complexity of the shear zones: in the BSZ, a 954 ± 110 Ma Sm-Nd hornblende age has been obtained for a bi-hbl-gneiss. Near the intersection of the BSZ to the MSZ, garnets from mylonitic kyanite-bearing quarzites revealed an age of 1212 ± 4 Ma.
Buhl D, PhD Thesis, Univ. Münster, Germany, (1987).
Köhler H, Srikantappa C, J. Conf. Abs, 1(1), 320, (1996).
The Egersund basaltic dyke swarm is made up of 11 subvolcanic dykes trending ESE-WNW (d1 to d11, from N to S). They intruded the Sveconorwegian gneiss basement of SW Norway. 616 ± 3 Ma (U-Pb baddeleyite age; Bingen et al, 1998).
Two types of dyke occur: (1) porphyritic dykes (d3, d8-d11) with plagioclase phenocrysts (up to An 81) in a subophitic matrix of pl, aug (up to 6.1% Al2O3), ol (up to Fo 82.5) and minor ilm, mag and bt, and (2) aphyric dykes (d1, d4-d6) with subophitic texture of pl, aug, ± ol, and minor ilm, mag, bt and ap. Limited magmatic differentiation occurred in the dykes during shallow level intrusion as compositional parameters are constant along strike. Porphyritic dykes and aphyric dyke 6 have subalkaline to mildly alkaline compositions whereas aphyric dykes 1, 4 and 5 have alkaline compositions. The most primitive samples of the swarm (approaching magmatic liquids) occur in the porphyritic dykes; they have Mg# of 56 - 61, TiO2 of 2.0 - 2.1%, with enriched incompatible trace element contents, and initial isotopic compositions, Sri and <epsilon>(Nd)i, of 0.7034 - 0.7039 and +2.0 - +3.1. The most evolved liquids in alkaline dykes 4-5 are SiO2-poor (45 - 46%) ferrobasalts (Mg# = 38 - 40), very enriched in TiO2 (3.2 - 3.4%), P2O5 (2.0 - 2.5%) and incompatible trace elements. They have Sri of 0.7058 - 0.7060 and <epsilon>(Nd)i of +1.0.
The composition of the most primitive magmas and the core compositions of the most primitive minerals suggest that porphyritic dykes taped a magma chamber situated at the crust-mantle boundary, at ca. 10 kbar. It is shown that dyke 6 is possibly related to the porphyritic dyke magma by a fractional crystallization process accompanied by limited crustal assimilation at this pressure. The alkaline dykes represent a distinct suite with a distinct mantle source.
The emplacement of the Egersund swarm is discussed in relation to the late-Neoproterozoic rifting leading to opening of Iapetus ocean. At the Baltoscandian passive margin of Baltica, there is a first order oceanwards decrease of alkalinity and incompatible trace element content of basaltic dyke swarms, from the Egersund swarm in the continental basement (SW) to the sheeted dyke complexes of subalkaline affinity at the continent-ocean transition (Seve superterrane, to the NE). This geochemical trend is interpreted as a change from high-pressure partial melting of garnet peridotite with residual garnet in a mildly depleted mantle to lower-pressure partial melting of a depleted mantle.
Bingen B, Demaiffe D & van Breemen O, J Geol, 106, 565-574, (1998).
Average U-Pb (SHRIMP) zircon ages of 581± 20 Ma were obtained for the orthogneisses of the Vila Mendo region (northern central Portugal). At the presently exposed level, these gneisses occur as thin, intrusive, sheet-like bodies interlayered in a high-grade metasedimentary complex of pre-Ordovician age. Both the gneisses and the metasediments show tectonic evidence of having been affected by the three main Variscan deformation phases (D1+D2+D3). Orthogneisses with this age are rare in the Hercynian autochthonous of the Iberian Fold Belt and reveal an old episode of acid magmatism probably related with the Cadomian orogeny. Previous studies show that the Pan-African Cadomian orogenic activity is recorded in the Iberian Massif by the presence of 700-550 Ma detrital zircons in metasediments with more radiogenic Nd isotope compositions and the occurrence of meta-igneous rocks with 600±50 Ma.
The metasedimentary units of the Vila Mendo region are part of a monotonous megasequence of metapelites and metagreywackes, known as "Complexo Xisto Grauváquico" (CXG). This complex occupies large domains of the Iberian Massif and has been interpreted as a thick, syn-orogenic, flysch sequence deposited at an active continental margin during the Pan-African / Cadomian orogeny. A late Precambrian to Cambrian age has been generally assumed for the CXG. However, no precise dating of these lithologies is established. As such, the ages obtained for the orthogneisses also provide constraints on the age of the CXG.
The Pan-African orogen in the Eastern Desert of Egypt is characterized by the occurrence of metamorphic core complexes arranged parallel to the strike of the orogen. Core complexes are bordered by distinct ductile subvertical sinistral NW-trending strike-slip zones. Low angle normal faults (LANF) form the NW and SE limits of these core complexes. The age of this deformation is considered to be late Pan-African.
This study compares both, quartz-textures in shear-zones and normal faults to get information about differences in deformation geometry, temperature during deformation and other deformation parameters. Quartzitic mylonites have been measured with X-ray-texture goniometry and partly with Neutron-goniometry.
Quartz-c-axis data show the following distribution in the kinematic XZ-section:
Meatic core complex: The SW shear-zone is characterized by asymmetric sinistral crossgirdles which are typical for low grade ductile shear-zones. The SE directed LANF indicates symmetric crossgirdles due to mainly coaxial deformation.
Sibai core complex: Sinistral asymmetric crossgirdles to oblique single girdles dominate the NE shear-zone. The more complicated SW shear-zone shows a variation from asymmetric crossgirdles to maxima in Y. Asymmetric crossgirdles occur along the El Shush shear-zone which occupies the position of a synthetic shear to the SW shear-zone. Along the SE directed LANFs in the SE of the core complex exclusively maxima in Y with slight tendency to oblique single girdles were observed.
Hafafit core complex: The NW directed LANFs in the N of Hafafit show maxima in Y, too. The NE shear-zone is characterized by asymmetric sinistral single girdles.
A general trend in the change of quartz-c-axis patterns from N to S can be observed. In the N crossgirdle to asymmetric crossgirdle distributions dominate whereas towards the S oblique single girdles and maxima in Y have been observed. This change starts further N for the LANFs than for the sinistral shear-zones.
The appearance of crossgirdles implies the dominance of basis <a> gliding with less pronounced rhomboeder <a> and prism <a><c> gliding which is common for deformation under low grade metamorphic conditions. Quartz- c-axis patterns showing maxima in Y could be explained by the dominance of prism <a> gliding and are typical for medium to high grade metamorphic conditions.
The variation of quartz-c-axis patterns from N toward the S is explained in term of changing of metamorphic conditions during deformation from the S (medium to high grade) towards the N (low grade). This is related to the progradation of transpressional tectonics from the S (c. 640 Ma) towards the N (c. 570 Ma) under decreasing metamorphic conditions (Loizenbauer et al., 1999). Differences in quartz-c-axis patterns between LANFs and steep dipping shear-zones are explained in terms of higher strain-rates within the LANFs.
Loizenbauer J, Wallbrecher E & Fritz H, J. Conf. Abs., 4, (1999).
The boundary between Wilson Terrane (WT) and Bowers Terrane (BT) is one of the major tectonic discontinuities of northern Victoria Land (Antarctica) and is referred to as Lanterman Fault by most authors (Dow & Neall, 1974). Fieldwork in this area during three ItaliAntartide expeditions (1993-94, 94-95 and 96-97) shows that the structural evolution of this boundary is polyphase. Four main tectonic phases can be recognized: I) one phase in which thrusting movement from the west over the east predominated. It was syntectonic with amphibolite and amphibolite-greenschist facies transition metamorphism and is related to the Ross orogeny (i.e. around 500 Ma); II) a second phase with a prevailing strike-slip sinistral shearing, syntectonic with greenschist facies metamorphism. This event may be a late event of the Ross orogeny or may represent an evidence of the Borchgrevink orogeny (i.e. around 360 Ma); III) a third phase characterized by large wavelenght folding of late-Ross or Borchgrevink age; IV) a Cenozoic brittle tectonics, expressed by small- to km-scale structures, with dextral strike-slip displacement.
The Ross aged tectonic transport is directed towards the northeast, so that generally the higher grade units are thrust onto the lower grade units; the higher temperatures are in the highest structural position and the metamorphic isogrades result inverted. A thermal increase in the BT confined to the proximity of the boundary with the WT and not related to a regional metamorphic zoning, could result from a combined action of cover-effect of the hot WT onto the colder BT and possibly of shear-heating. This is in agreement with the occurrence of the highest grade (greenschist-amphibolite facies transition) in the position closest to the WT. The metamorphic telescoping between the high-grade rocks of the WT and the low grade rocks of the BT, as well as the occurrence of the magmatic arc granites close to the boundary, requires a major loss of crust during the accretionary process that can be explained with the amphibolite facies thrusting and the greenschist facies strike-slip reworking.
We want to stress that the tectonic history of the docking between WT and BT has been complex and polyphase: it was the site of contrasting tectonic regimes in different times and at different structural levels therefore its evolving character shows the impossibility to define a unique tectonic role for the Lanterman Fault. Evidences of the polyphase evolution are not confined along a single tectonic lineament, but occurr all over the area close to the boundary between WT and BT. Hence we prefer to use the term Lanterman Fault with the meaning of the zone around the WT-BT boundary rather than to indicate a single tectonic line.
Dow JAS. & Neall VE, NZ J. Geol. Geophys, 17, 659-714, (1974).
A significant gap of middle Paleozoic apparent polar wander (APW) path still vacillates polar definition of early Paleozoic paleopoles for North China, and leave the hemisphere for the North China block unconstrained. This problem can be resolved by the inter-continental correlation of the magnetic polarity pattern across a small time interval. A magnetic stratigraphic study was therefore carried out on the Cambrian/Ordovician boundary and lowest Ordovician, around the Zhaogezhuang section, North China. After stepwise thermal or thermal and alternating field demagnetizations a characteristic component, with normal and antipode reversed direction, was identified from 49 out of 139 samples, yielding a paleopole at long 294.6E, lat 32.9N (dp=3.0, dm=5.3). We therefore extend the available reliable Cambrian apparent polar wander path (APWP) to the Cambrian/Ordovician boundary and lowest Ordovician. Magnetic stratigraphic results sugggest that a dominant reversal polarity for the Arenig and upper Tremadoc epoch. The Cambrian/Ordovician boundary, however, is of dominantly normal polarity which is consistent with these obtained from the Black Mountain of Australia, the Kulumbe river section of northwestern Siberia and Dayangcha section of Northeastern China. This coherent magnetic polarity pattern around the Cambrian/Ordovician boundary and lowest Ordovician obtained from different continents favors the southern hemispheric origin (~17) for the North China block. By comparing Cambrian-Ordovician APW paths between North China and Gondwana, we suggest that the NCB was very probably a part of the Gondwanaland during the Cambrian and lowest Ordovician, which is not conflict with available paleobiogeographic evidence during Cambrian time. However, the close affinity of Early Ordovician tropical bathyurid trilobites and conodont fauna found from the NCB, Laurentia and Siberia, would suggest the NCB started break away from Gondwana in the lower Ordovician.
The Metamorphic Complex of Beloretzk (MCB), situated in the southern Urals, is part of the Central Uralian Zone (Puchkov, 1997). The MCB is separated from Riphean sedimentary rocks in the west by the prominent Zuratkul fault. In the east the complex is unconformably overlain by Palaeozoic rocks of the Zilair synform but also separated by faults from the Ural-Tau zone. The MCB mainly comprises metamorphic rocks of supracrustal origin, i.e. mica schists, quartzites, marbles, and volcanosedimentary rocks. This supracrustal sequence was intruded by mafic dykes and a small granite body prior to the main metamorphic event. Mineral parageneses of the metasedimentary rocks are indicative of uppermost greenschist-facies and lowermost amphibolite-facies whereas Alekseyev (1984) reports relics of eclogite-facies parageneses from mafic dykes in the south of the MCB, therefore the complex has been overprinted by a pervasive retrograde event.
Regarding recently published (Glasmacher et al., in press) metamorphic age data of 600 to 540 Ma, the MCB is a Cadomian-aged basement which was affected by temperatures less than 200°C during the Uralian orogeny. Therefore MCB allows a look at the Cadomian-aged structures at the easternmost edge of the East European Craton.
Four tectonic events have been distinguished for the MCB. The oldest deformation D1 resulted in south-vergent, tight to isoclinal folds and a continuous schistosity. Relics of a later extensional event (D2) are preserved as discrete quartzite mylonites, showing quartz c-axis patterns typical for a simple shear regime indicating a top-down to NW kinematic. The third deformation D3 is documented by close, east-vergent folds. A widely spaced schistosity which dipping steeply to the west is developed parallel to the F3 axial planes. In the eastern part of the MCB wide mylonite zones with top-to-east shear sense indicators mark D3 nappe thrusts. These D3 structures probably result from the collision of the MCB terrane with the East European Craton. The final Uralian (D4) tectonothermal overprint was very weak and led only to a gentle folding of the already consolidated crystalline basement.
Alekseyev AA, Riphean and Vendian Magmatism in the Southern Urals. - 136 pp, Nauka; Moskov [in russian], (1984).
Glasmacher U, Reynolds P, Alekseyev AA, Puchkov VN, Taylor K, Gorozhanin V & Walter R, Geologische Rundschau, (in press).
Puchkov VN, Geological Society Special Publication, 121, 201-236, (1997).
The Timans and the Ynisey Ridge are two geomorphologically comparable superorder structures which extend for about 1000 km in the marginal parts of ancient platforms. The Late Precambrian complex which makes the basement of the Ynisey Ridge, is exposed on the major part of its territory and is relatively well studied. Timanides (Preuralides) of the European North-East locally crop out in the Timans and north of the Urals. The structure and geological history of the Timans and Yenisey Ridge have both common features and essential differences. The former are: 1. The Timans and the Yenisey Ridge are boundary structures of younder plates: the Pechorian and West-Siberian ones with the ancient East-European and Siberian platforms, respectively. 2. In Late Precambrian, these structures belonged to the regions of pericratonic subsidence, found along the platform's margins: north-eastern and eastern region of the East-European platform, and western and southern - of the Siberian one. 3. In the Late Precambrian paleotectonic plan the Yenisey Ridge and European North - East both have similar patterns of the successive of the zone of pericratonic subsidence; wich included the shelf, the continental slope and foot, the internal zone of the mobile belt consisting of island ares, «windous» of the oceanic crust, and microplates with the continental crust. 4. The pericraton - mobile belt boundary is made by deep faults: Prepechorian in the north-east of the European platform and Tatarian on the Yenisey Ridge. Distinctions in the Late Precambrian history of the Timans and the Yenisey Ridge are as follows: 1) The orogeny and magmatism on the Yenisey Ridge were far stronger and are observed in a wider time range compared to the north-east of the European platform. Granitic batholith formation is absent in the Timans. Its possible analogues are disclosed by drilling only in the zone of Prepechorian deep fault, but they are developed on a far smaller scale. 2) Development of the structures was completed by different phases of the baikalian cycle of tectonogenesis. In the Timans, the stage dates back to 680-550 mln. years, on the Yenisey ridge, two stages of granitoid intrusion and metamorphism are distinguished: 950 and 650 mln.years (Postelnikov, 1980; Volobuev et al., 1976; Vernikovsky, 1995). 3) Successive change of Late Precambrian paleotectonic environment (shelf - continental slope - island arc) in contemporary tectonic plan takes 150 km on the Yenisey Ridge while in the north-east of the European platform it stretches to 350-400 km. The Late Precambrian structure of the Timans and the Yenisey ridge was formed during accretion of massifs with the continental crusts (terranes) to the pericratonic regions of ancient platforms; the area of the Yenisey Ridge, however, underwent a longer and more complex development.
Results of isotope investigations by Sm-Nd and U-Pb methods on zircon, including «single grain» technique, show that late Neoproterozoic complexes of Baikalian Folding Area (BFA) were formed during two stages, which correspond early- and late Panafrican events. During early stage (1.0-0.8 Ga) there were formed carbonate-terrigenous complexes of sedimentary basins and continental rifts with platobasalts, ferropicrites and low-titan basaltoides. These basaltoides are characterized by low negative <epsilon>Nd values, which indicate the old continental lithosphere mantle source. The rift opening in inner part of BFA was accompanied by formation of N-MORB type oceanic crust (1.0 Ga) with tholeiitic basalts (<epsilon>Nd = +6.9). The evolution of rifting processes was breaked by deformations, metamorphism and formation of crust gneiss-granite massifs at 0.8 Ga.
The beginning of late stage (0.8-0.55 Ga) marked large layered plutons and continental basins. Main events of this stage connected with Vend evolution of supra-subduction magmatic front in rear part of opening paleo-basin of active continental margin of Paleoasian ocean with thick sedimentary-volcanogenic layers; tonalite, peridotite- pyroxenite-gabbro plutons and high-baric metamorphites. All these rocks are characterizes by high positive <epsilon>Nd values and short crust prehistory, which point out their connection with depleted mantle source. This stage ended by regional zonal metamorphism at 0.55 Ga.
The isotope data allow to conclude, that the most of late Precambrian formations of BFA are products of destruction and reworking of old early Proterozoic continental crust of Siberian paleocontinent. Juvenile crust formation at 1.0 and 0.66-0.62 Ga and accretion of late Precambrian continental crust at the end of Vend have in BFA a limited scale.
This work was supported by Russian Foundation of Basic Researches, grants 95-05-15104 and 98-05-65593.
The Taimyr Upper Precambrian ophiolites are located within the Central Taimyr Zone, representing a Neoproterozoic fold-thrust accretionary complex. This complex is thrusted south-eastwardly onto a former passive margin of the Siberian continent, the present-day South Taimyr zone. The ophiolites are exposed as two narrow strips, each stretching for more than 70 km. One is in the Chelyuskin Peninsula and another, more southwestern, trends from the Stanovaya River to the Zimovochnaya Bay.The Taimyr ophiolites are composed of metaperidotites transformed into serpentinites, massive or layered metagabbroids, tholeiite basalts, and, in minor quantities, of turbidites and carbonate rocks, also metamorphosed. In places the basalts are metamorphosed into garnet amphibolites. In the Chelyuskin belt, plagiogranites (trondhjemites) of the oceanic type were also found. The submarine and subaerial island-arc calc-alkaline volcanics (basalts, andesites, felsic tuffs) are widespread in the same zone and spatially associated with ophiolites, and occuring in tectonic contact with them. Petrological-geochemical studies suggest that the formation of ophiolites relates to marginal-sea basin geodynamic environment. For the Chelyuskin belt this environment is characterised by the eruption of nearly MORB that is followed by tholeiitic volcanism (basalt to rhyolites), and formation of ensimatic arc. Stanovoy ophiolites seem to have more complex history. They are assumed to originate in the spreading zone and to undegro metamorphism in the subduction zone where basalts are transformed into garnet amphibolites at the high pressures of amphibolite facies. Zircon U-Pb dating (740 Ma) and Sm-Nd isotopic study (model age 850-785 Ma) of the plagiogranite from the Chelyuskin ophiolitic belt suggest the Neoproterozoic age for the upper and lower age boundaries for the formation of the ophiolites. These age data very good correlate with the same U-Pb data on island-arc volcanics and plagiogranites, dikes and sills of diabases, traced from Canadian Cordillera through Yukon and Franklin belt of North America, through Alaska-Chukotka to Taimyr. It may be indicative of the time of opening of the Northern Paleo-Pacific at 850-800 Ma. Study of Sm-Nd, Rb-Sr, Ar-Ar and K-Ar isotopic systems in the garnet amphibolites of the Stanovoy ophiolite belt allowed us to establish the age of their metamorphism (626-575 Ma), which probably corresponds to the obduction of ophiolites and the whole accretion block onto the margin of the Siberian craton.
Three metamorphic phases are recognized in the Wadi Kid area, SE Sinai, Egypt. These phases are related to the Pan-African orogeny. The oldest M1 metamorphic phase, dated at approximately 720-65OMa, is of greenschist grade. M1 is characterized by the growth of chlorite in schistose rocks. The schistosity was formed axial planar to F1 folds. M1 is only found in the upper crustal rocks of the southern Wadi Kid area. Metamorphic conditions of M1 are estimated at 300°C and 3-5 kbar. The F1-folds are related to arc-accretion.
The M2 metamorphic phase is best preserved in the southern Wadi Kid area and occasionally in the central and northern Wadi Kid area. This phase is represented by steeply dipping biotite-cordierite schists in the southern area and sub-horizontal biotite-garnet-staurolite schists in the central and northern areas. Metamorphic conditions are estimated at 400-500°C and 5-7 kbar. The M2 upper-greenschist/lower-amphibolite-grade phase is dated at approximately 650-600 Ma. M2 is associated with the earlier stages of a D2 deformation phase. The schists are interpreted as mylonites (Blasband et al., 1997).
Relicts of the M3 metamorphic phase are found in the central and northern Wadi Kid area. Biotite schists contain homblende-, andalusite-, sillimanite- and garnet porphyroblasts. These schists were interpreted as metapelites, metasandstones and metavolcanics. The M3 metamorphic phase overprints and locally obscures the older M1 and M2 phases. It is related to the later stages of D2 at approximately 600-530 Ma. Garnet-biotite geothermometry in metapelites of the central Wadi Kid area reveal temperatures of 540-620°C. Plagioclase-biotite-garnet-muscovite geobarometry indicates pressures of 3-4.5 kbar. Peak temperatures are represented by garnet rims. This indicates progressive metamorphism. Hornblende-plagioclase geothermobarometry in metavolcanics of the northern Wadi Kid area indicate temperatures of 549-678°C and pressures of 4 kbar.
The M1-phase is related to a compressional arc-accretion phase. The M2 resulted from shearheating in crustal-scale shearzones. These shearzones are related to NW-SE extension (Blasband et al., 1997). The HT-LP metamorphism of M3 is related to the intrusion of granites in a continuously extending crust during D2. The higher temperatures in the Northern Wadi Kid are related to the intrusion of a gabbroic body which originated in the lowest parts of the crust in the Northern Wadi Kid area, dated at 59OMa (Moghazi et al., 1998). The P-T results together with structural data from Blasband et al. (1997) support a core complex model for the Wadi Kid area. During the Pan-African the transition from the arc-accretion and crustal thickening phase of M1 to the extensional regime present at M2 and M3 is interpreted to be caused by extensional collapse.
This project was financed by the Dr. Schunnann Foundation grant no.1994/06 and 1997/04.
Blasband, B, P Brooijmans, P Dirks, W Visser, S White, A Pan-African core complex in the Sinai, Egypt, Geologie en Mijnbouw, 76, 247-266, (1997).
Moghazi, AM, T Andersen, GA Oweiss, AMEl Bouseily, Geochemical and Sr-Nd-Pb isotopic data hearing on the origin of Pan-African granitoids in the Kid area, southeast Sinai, Egypt, J. Geo. Soc. London, 155, 697-710, (1998).
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