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Archaean-Neoproterozoic Terrane Boundary in the South Indian Granulite Belt in East Gondwana
RODINIA, GONDWANA AND ASIA ~
711
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Archaean-Neoproterozoic Terrane Boundary in the South Indian Granulite Belt in East Gondwana Dhruba Mukhopadhyayl, R. Srinivasan2,P. Senthilkumar2,Tapas Bhattacharya' and Pr abir Sengupta I
Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Calcutta - 700 019, India National Geophysical Research Institute, Hyderabad - 500 007, India
The Palghat-Cauvery shear zone (PCSZ) in the South Indian Granulite terrane is perceived to mark the boundary between the Neoproterozoic Madurai granulite belt to the south and the late Archaean granulite belt to the north (Harris et al., 1994, Jayananda and Peucat, 1996) and is generally regarded as a dextral crustal scale shear zone. We present here the results of a detailed structural study in the eastern segment of the PCSZ near Namakkal. The region south of Namakkal is characterised by the presence of several mappable mafic granulite bands in a terrane of garnetiferous enderbitic and garnetiferous quartzofeldspathic gneiss. Thin bands of BIF are present within the enderbitic gneiss, the contact between them being razor sharp in many outcrops and parallel to the common gneissosity in both the rock types. The BIF-mafic rock-granitic gneiss association is reminiscent of similar association in the Dharwar schist belts and it is hypothesised that Dharwar craton rocks are exposed in the Namakkal region in a metamorphic dress. U-Pb zircon ages from enderbites, granitic gneisses and mafic granulites indicate an age of ca. 2.5 Ga (Ghosh, 1998), similar to the age of metamorphism-magmatism in the low grade Dharwar craton. The granulites are followed to the south by another terrane occupied by biotite-hornblende bearing amphibolite facies gneiss, granite plutons, aplites and pegmatites. There are enclaves of marble and calc-silicate gneiss, a few metres to hundreds of metres long. Xenoliths of mafic granulite showing various stages of retrogressive alteration occur within the gneisses. Zircon ages from the granites and gneisses range from 0.55 to 0.63 Ga (Ghosh, 1998). Within the granulite terrane the compositional banding in BIF, mafic granulites and enderbites are involved in two principal phases of folding. The early folds are outcrop scale rootless isoclinal folds. In the BIF a fine streaky gneissic banding is axial planar to these folds. In the enderbites and mafic granulites the original layering is transposed to parallelism with the main penetrative gneissosity. The second deformation has produced a series of upright mappable folds with E-W axial planes; these are defined by the formational contacts and variation in attitude of gneissic banding. The D2 fold axes show extreme variability of plunge from gentle easterly through vertical to gentle westerly; outcrop scale sheath folds are observed at places. An axial planar foliation is defined by parallel alignment of quartzose lenses and mafic streaks. Myriad shear zones are present in the granulite terrane. At many localities a mylonitic foliation parallel to gneissosity is developed in thin felsic bands in mafic granulites. S-C fabric is often associated with these zones and the sense of shear is Gondwana Research, V. 4, No. 4, 2001
reversed on the two limbs of the D2 folds. These layer parallel shear zones are related to the D2 folding episode. In addition, zones of high ductile strain are developed on the limbs of both outcrop scale and mappable D2 folds; such zones are characterised by the presence of straight planar banding parallel to the D2 axial planes. Rarely brittle dislocations have also developed parallel to the D2 axial planes. Other shear zones of diverse orientation cut across the gneissosity; some of them show dextral sense of movement and others sinistral. Steep plunging slickenlines on the mylonitic foliation parallel to the gneissosity may be related to movement during exhumation of the granulite belt. Enderbites have the typical opx-plagioclase-garnet assemblage and the mafic granulites have cpx-garnet-plagioclase and opx-cpx-garnet-plagioclaseassemblages. Garnet crystallised after opx and cpx. Hornblende occurring as an accompanying phase is of two generations; an earlier prograde variety preceded the crystallization of pyroxenes and the later one is retrogressive. D2 shearing parallel to the gneissosity is later than the crystallization of pyroxene and garnet, which show crystal plastic deformation pointing to prevalence of fairly high temperature (> 600° C) during D2 deformation. Hornblende-plagioclase symplectite is common in the southern part of the granulite terrane in the transition zone to the amphibolite facies terrane. At places garnet is completely replaced by the hornblendeplagioclase symplectitic aggregates and the granulites are totally downgraded to amphibolites. The textural evidence conclusively proves that this retrogressive change postdates the D2 shearing. The retrogression can be tied up with fluid activity related to migmatization and granite emplacement in the southern terrane. A narrow transition zone separates the late Archaean granulites from the Neoproterozoic amphibolite facies gneisses to the south. There is extensive development of migmatites in the southern terrane and there are several small plutons of aplogranite. Structurally these gneisses underlie the northern granulites. Though there is an overall parallelism of the gneissosity in the two terranes, particularly near the boundary region, the presence of xenoliths of variously retrograded granulite with internal structure discordant to the host gneissosity clearly indicates that the granulites were deformed earlier than the Neoproterozoic granite emplacement. The present study brings out the difference in the structural set-up in the two terranes. The folds in the hornblende-biotite gneisses are often disharmonic and show great variation in form, wavelength, amplitude and axial planar orientation, which is typical of many migmatites. Open folds on gneissosity with N-S
712
RODINIA, GONDWANA AND ASIA
to NE-SW trending axial planes are common and segregation of partial melts parallel to these axial traces is conspicuous. Folds of this orientation are absent in the granulites. PCSZ has been interpreted by many workers as a dextral shear zone (Drury et al., 1984), which has dragged the north-easterly EasternGhaVDharwar trend to an easterly orientation in the Namakkal-Salem region. Though small scale shears are ubiquitous in the study area, the preservation of granulite facies assemblages and overall granoblastic textures argues against any crustal scale shear zone passing through the area. Layer parallel shearing observed in the granulites is related to D2 folding. Further, the fold pattern in the Namakkal region does not fit with the interpretation of dextral shear, because such a shear sense would have caused layer parallel elongation and not folding of initially N-S or NE-SW trending layering. The fold pattern is suggestive of shortening across the zone rather than simple transcurrent motion. Though the granulites and amphibolite facies gneisses represent two disparate terranes, in the area of study no mylonite belt separates the two terranes.
The observed structural and textural features do not support the interpretation that the two terranes are brought into juxtaposition by a major transcurrent motion. The boundary, as observed in the present area, is more of the nature of a Neoproterozoic migmatization front, possibly related to a collisional process.
References Drury, S.A., Harris, N.B.W., Holt, R.W., Reeves-Smith, G.J. and Wightman, R.T. (1984) Precambrian tectonics and crustal evolution in South India. J. Geol., v. 92, pp. 3-20. Ghosh, J.G. (1998) U-Pb geochronology and structural geology across major shear zones of the Southern Granulite Terrain of India and 6l3COrg stratigraphy of the Gondwana coal basins of India. Ph. D. Thesis at University of Cape Town, 300p. Harris, N.B.W., Santosh, M.and Taylor, PN. (1994) Crustal evolution in South India: constraints from Nd isotopes. J. Geol., v. 102, pp. 139-150. Jayananda, M. and Peucat, J.J. (1996) Geochronological framework of Southern India. In: Santosh, M. and Yoshida, M. (Eds.), The Archaean and Proterozoic terrains in southern India within East Gondwana. Gondwana Res. Group Mem. No. 3, pp. 53-70.
The East-Carpathian Metamorphic Terranes (Romania): Evolution from Gondwana to the East European Platform Marian Munteanu' and Mihai Tatd
' National Agency for Mineral Resources, Mendeleev Str. 36-38, 70169 Bucharest, Romania Geological Institute of Romania, Caransebe Str. 1, 78344 Bucharest, Romania, E-mail: [email protected] The East-Carpathian Alpine orogen includes pre-Alpine terranes amalgamated during the Paleozoic. Medium-grade Bretila Group (augen-gneisses, pelitic gneisses, micaschists, granites and amphibolites), showing East European Platform (EEP) affinities, is thrust upon the Rebra-Tulghe composite terrane, formerly a single pile that has been sheared by two main pre-Alpine thrusts (Sandulescu, 1984). This tectonic setting resulted from the collision of Rebra-Tulghe terrane with the EEP. Re b r a -Tulgh e t e r r a n e , exhibiting Go n d w an a n fe a t ur e s (Munteanu and Tatu, ZOOO), is built up by Upper Proterozoic carbonate platform (Rebra Group, built up by pelitic gneisses, dolomites, limestones, micaschists, amphibolites, quartzites) overlain by a Late Proterozoic volcano-sedimentary formation (Pietrosu Bistriei porphyroids) and by the Lower Paleozoic Tulghe Group (sericite-chlorite schists, metarhyolites, quartzites, metagreywackes, limestones and greenschists). Remnants of the same terrane could be found in the Supragetic Nappe of the South Carpathians a n d in Apuseni Mountains, where metamorphic sequences similar to Rebra Group, Tulghe Group and Pietrosu Bistriei porphyroids are known. Rebra Group and Pietrosu Bistriei porphyroids evolved in Proterozoic as parts of Gondwana. Tulghe Group is made up of four formations that represent parts of an island arc complex. Tgl and Tg2formations were sediments deposited in a back-arc basin; Tg, formation
comprises a volcanic arc with the related epiclastic products and an intra-arc basin; Tg4 formation gather the fore-arc sequences. They have accummulated partly predrift and partly after the rifting from Gondwana. This is the reason why the age of Tulghe Group is critical for the timing of the rifting and drift of Rebra-Tulghe terrane. On the basis of the Silurian age of Tulghe Group metamorphism (Zincenco, 1994), a n d of the lithostratigraphic similarities with the Avalonian-Cadomian terranes, Rebra-Tulghe terrane is assumed an Early Ordovician rifting from Gondwana and a Silurian docking to the EEP. This would mean that the Variscan event in the Eastern Carpathians is much less important than previously thought and the pre Alpine nappes, previously considered Variscan, are Silurian in age.
References Munteanu, M. and Tatu, M. (2000) The East-Carpathian CrystallineMesozoic Zone: a n exhumed TESZ segment. Joint Meeting of EUROPROBE (TESZ) and PACE Projects. Zakopane/Holy Cross Mountains, Poland, Sept. 16-23, 2000. Abst. Vol., pp. 63-64. Siindulescu, M. (1984) Geotectonics of Romania (in Romanian). 336p. Editura tiinific, Bucharest. Zincenco, D. (1994) Chronostratigraphic scale of the pre-Permian metamorphites and granitoids from Romanian Carpathians. Carpathian-Balkan Geological Association XVth Congress, v. 4, pp. 647 - 652.
Gondwana Research, K 4, No. 4,2001
711
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Archaean-Neoproterozoic Terrane Boundary in the South Indian Granulite Belt in East Gondwana Dhruba Mukhopadhyayl, R. Srinivasan2,P. Senthilkumar2,Tapas Bhattacharya' and Pr abir Sengupta I
Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Calcutta - 700 019, India National Geophysical Research Institute, Hyderabad - 500 007, India
The Palghat-Cauvery shear zone (PCSZ) in the South Indian Granulite terrane is perceived to mark the boundary between the Neoproterozoic Madurai granulite belt to the south and the late Archaean granulite belt to the north (Harris et al., 1994, Jayananda and Peucat, 1996) and is generally regarded as a dextral crustal scale shear zone. We present here the results of a detailed structural study in the eastern segment of the PCSZ near Namakkal. The region south of Namakkal is characterised by the presence of several mappable mafic granulite bands in a terrane of garnetiferous enderbitic and garnetiferous quartzofeldspathic gneiss. Thin bands of BIF are present within the enderbitic gneiss, the contact between them being razor sharp in many outcrops and parallel to the common gneissosity in both the rock types. The BIF-mafic rock-granitic gneiss association is reminiscent of similar association in the Dharwar schist belts and it is hypothesised that Dharwar craton rocks are exposed in the Namakkal region in a metamorphic dress. U-Pb zircon ages from enderbites, granitic gneisses and mafic granulites indicate an age of ca. 2.5 Ga (Ghosh, 1998), similar to the age of metamorphism-magmatism in the low grade Dharwar craton. The granulites are followed to the south by another terrane occupied by biotite-hornblende bearing amphibolite facies gneiss, granite plutons, aplites and pegmatites. There are enclaves of marble and calc-silicate gneiss, a few metres to hundreds of metres long. Xenoliths of mafic granulite showing various stages of retrogressive alteration occur within the gneisses. Zircon ages from the granites and gneisses range from 0.55 to 0.63 Ga (Ghosh, 1998). Within the granulite terrane the compositional banding in BIF, mafic granulites and enderbites are involved in two principal phases of folding. The early folds are outcrop scale rootless isoclinal folds. In the BIF a fine streaky gneissic banding is axial planar to these folds. In the enderbites and mafic granulites the original layering is transposed to parallelism with the main penetrative gneissosity. The second deformation has produced a series of upright mappable folds with E-W axial planes; these are defined by the formational contacts and variation in attitude of gneissic banding. The D2 fold axes show extreme variability of plunge from gentle easterly through vertical to gentle westerly; outcrop scale sheath folds are observed at places. An axial planar foliation is defined by parallel alignment of quartzose lenses and mafic streaks. Myriad shear zones are present in the granulite terrane. At many localities a mylonitic foliation parallel to gneissosity is developed in thin felsic bands in mafic granulites. S-C fabric is often associated with these zones and the sense of shear is Gondwana Research, V. 4, No. 4, 2001
reversed on the two limbs of the D2 folds. These layer parallel shear zones are related to the D2 folding episode. In addition, zones of high ductile strain are developed on the limbs of both outcrop scale and mappable D2 folds; such zones are characterised by the presence of straight planar banding parallel to the D2 axial planes. Rarely brittle dislocations have also developed parallel to the D2 axial planes. Other shear zones of diverse orientation cut across the gneissosity; some of them show dextral sense of movement and others sinistral. Steep plunging slickenlines on the mylonitic foliation parallel to the gneissosity may be related to movement during exhumation of the granulite belt. Enderbites have the typical opx-plagioclase-garnet assemblage and the mafic granulites have cpx-garnet-plagioclase and opx-cpx-garnet-plagioclaseassemblages. Garnet crystallised after opx and cpx. Hornblende occurring as an accompanying phase is of two generations; an earlier prograde variety preceded the crystallization of pyroxenes and the later one is retrogressive. D2 shearing parallel to the gneissosity is later than the crystallization of pyroxene and garnet, which show crystal plastic deformation pointing to prevalence of fairly high temperature (> 600° C) during D2 deformation. Hornblende-plagioclase symplectite is common in the southern part of the granulite terrane in the transition zone to the amphibolite facies terrane. At places garnet is completely replaced by the hornblendeplagioclase symplectitic aggregates and the granulites are totally downgraded to amphibolites. The textural evidence conclusively proves that this retrogressive change postdates the D2 shearing. The retrogression can be tied up with fluid activity related to migmatization and granite emplacement in the southern terrane. A narrow transition zone separates the late Archaean granulites from the Neoproterozoic amphibolite facies gneisses to the south. There is extensive development of migmatites in the southern terrane and there are several small plutons of aplogranite. Structurally these gneisses underlie the northern granulites. Though there is an overall parallelism of the gneissosity in the two terranes, particularly near the boundary region, the presence of xenoliths of variously retrograded granulite with internal structure discordant to the host gneissosity clearly indicates that the granulites were deformed earlier than the Neoproterozoic granite emplacement. The present study brings out the difference in the structural set-up in the two terranes. The folds in the hornblende-biotite gneisses are often disharmonic and show great variation in form, wavelength, amplitude and axial planar orientation, which is typical of many migmatites. Open folds on gneissosity with N-S
712
RODINIA, GONDWANA AND ASIA
to NE-SW trending axial planes are common and segregation of partial melts parallel to these axial traces is conspicuous. Folds of this orientation are absent in the granulites. PCSZ has been interpreted by many workers as a dextral shear zone (Drury et al., 1984), which has dragged the north-easterly EasternGhaVDharwar trend to an easterly orientation in the Namakkal-Salem region. Though small scale shears are ubiquitous in the study area, the preservation of granulite facies assemblages and overall granoblastic textures argues against any crustal scale shear zone passing through the area. Layer parallel shearing observed in the granulites is related to D2 folding. Further, the fold pattern in the Namakkal region does not fit with the interpretation of dextral shear, because such a shear sense would have caused layer parallel elongation and not folding of initially N-S or NE-SW trending layering. The fold pattern is suggestive of shortening across the zone rather than simple transcurrent motion. Though the granulites and amphibolite facies gneisses represent two disparate terranes, in the area of study no mylonite belt separates the two terranes.
The observed structural and textural features do not support the interpretation that the two terranes are brought into juxtaposition by a major transcurrent motion. The boundary, as observed in the present area, is more of the nature of a Neoproterozoic migmatization front, possibly related to a collisional process.
References Drury, S.A., Harris, N.B.W., Holt, R.W., Reeves-Smith, G.J. and Wightman, R.T. (1984) Precambrian tectonics and crustal evolution in South India. J. Geol., v. 92, pp. 3-20. Ghosh, J.G. (1998) U-Pb geochronology and structural geology across major shear zones of the Southern Granulite Terrain of India and 6l3COrg stratigraphy of the Gondwana coal basins of India. Ph. D. Thesis at University of Cape Town, 300p. Harris, N.B.W., Santosh, M.and Taylor, PN. (1994) Crustal evolution in South India: constraints from Nd isotopes. J. Geol., v. 102, pp. 139-150. Jayananda, M. and Peucat, J.J. (1996) Geochronological framework of Southern India. In: Santosh, M. and Yoshida, M. (Eds.), The Archaean and Proterozoic terrains in southern India within East Gondwana. Gondwana Res. Group Mem. No. 3, pp. 53-70.
The East-Carpathian Metamorphic Terranes (Romania): Evolution from Gondwana to the East European Platform Marian Munteanu' and Mihai Tatd
' National Agency for Mineral Resources, Mendeleev Str. 36-38, 70169 Bucharest, Romania Geological Institute of Romania, Caransebe Str. 1, 78344 Bucharest, Romania, E-mail: [email protected] The East-Carpathian Alpine orogen includes pre-Alpine terranes amalgamated during the Paleozoic. Medium-grade Bretila Group (augen-gneisses, pelitic gneisses, micaschists, granites and amphibolites), showing East European Platform (EEP) affinities, is thrust upon the Rebra-Tulghe composite terrane, formerly a single pile that has been sheared by two main pre-Alpine thrusts (Sandulescu, 1984). This tectonic setting resulted from the collision of Rebra-Tulghe terrane with the EEP. Re b r a -Tulgh e t e r r a n e , exhibiting Go n d w an a n fe a t ur e s (Munteanu and Tatu, ZOOO), is built up by Upper Proterozoic carbonate platform (Rebra Group, built up by pelitic gneisses, dolomites, limestones, micaschists, amphibolites, quartzites) overlain by a Late Proterozoic volcano-sedimentary formation (Pietrosu Bistriei porphyroids) and by the Lower Paleozoic Tulghe Group (sericite-chlorite schists, metarhyolites, quartzites, metagreywackes, limestones and greenschists). Remnants of the same terrane could be found in the Supragetic Nappe of the South Carpathians a n d in Apuseni Mountains, where metamorphic sequences similar to Rebra Group, Tulghe Group and Pietrosu Bistriei porphyroids are known. Rebra Group and Pietrosu Bistriei porphyroids evolved in Proterozoic as parts of Gondwana. Tulghe Group is made up of four formations that represent parts of an island arc complex. Tgl and Tg2formations were sediments deposited in a back-arc basin; Tg, formation
comprises a volcanic arc with the related epiclastic products and an intra-arc basin; Tg4 formation gather the fore-arc sequences. They have accummulated partly predrift and partly after the rifting from Gondwana. This is the reason why the age of Tulghe Group is critical for the timing of the rifting and drift of Rebra-Tulghe terrane. On the basis of the Silurian age of Tulghe Group metamorphism (Zincenco, 1994), a n d of the lithostratigraphic similarities with the Avalonian-Cadomian terranes, Rebra-Tulghe terrane is assumed an Early Ordovician rifting from Gondwana and a Silurian docking to the EEP. This would mean that the Variscan event in the Eastern Carpathians is much less important than previously thought and the pre Alpine nappes, previously considered Variscan, are Silurian in age.
References Munteanu, M. and Tatu, M. (2000) The East-Carpathian CrystallineMesozoic Zone: a n exhumed TESZ segment. Joint Meeting of EUROPROBE (TESZ) and PACE Projects. Zakopane/Holy Cross Mountains, Poland, Sept. 16-23, 2000. Abst. Vol., pp. 63-64. Siindulescu, M. (1984) Geotectonics of Romania (in Romanian). 336p. Editura tiinific, Bucharest. Zincenco, D. (1994) Chronostratigraphic scale of the pre-Permian metamorphites and granitoids from Romanian Carpathians. Carpathian-Balkan Geological Association XVth Congress, v. 4, pp. 647 - 652.
Gondwana Research, K 4, No. 4,2001