- Email: [email protected]
Geology of the Achankovil Shear Zone, Southern India
744
RODINIA, GONDWANA AND ASIA
References Chetty, T.R.K. (1995) A correlation of Proterozoic shear zones between Eastern Ghats, India and Enderby Land, East Antarctica, based on LANDSAT imagery. Mem. Geol. SOC.India, No. 34, pp. 205-220. Dobmeier, C., Simmat, R. and Raith, M.M. (2001) New constraints on the Proterozoic crustal evolution of the Eastern Ghats Belt, India. This volume. Hofmann, J. (1996) Fragmente intragondwanischer Rifte als Werkzeug der Gondwana-Rekonstruktion-das Beispiel des LambertMahanadi-Riftes (Ostantarktis-Peninsula Indien). N. Jb. Miner. Mh., V. 199, pp. 33-48. Kovach, VP., Berezhnaya, N.G., Salnikova, E.B., Narayana, B.L., Divakara Rao, V. and Yoshida, M. (1998) U-Pb zircon age and Nd isotope systematics of megacrystic charnockites in Eastern Ghats granulite belt, India, and their implication for East Gondwana reconstruction. J. African Earth Sci., Special abstract issue 'Gondwana-10: Event stratigraphy of Gondwana', v. 27, pp. 125-127. Kovach, V., Simmat, R., Rickers, K., Berezhnaya, N.G., Salnikova, E.B., Dobmeier, C. and Raith, M.M. (2001) The Western charnockite zone
of the Eastern Ghats Belt (India)-an independent Palaeoproterozoic (1.7-1.6 Ga) crustal province. This volume. Paul, D.K., Ray Barman, T., McNaughton, N.J., Fletcher, I.R., Potts, R.J., Ramakrishnan, M. and Augustine, PE (1990) Archean-Proterozoic evolution of Indian charnockites: isotopic and geochemical evidence from granulites of the Eastern Ghats Belt. J. Geol., v. 98, pp. 253-263. Mezger K. and Cosca M.A. (1999) The thermotectonic history of the Eastern Ghats Belt (India), as revealed by U-Pb and 4nAr-39Ar dating of metamorphic and magmatic minerals: implications for the SWEAT correlation. Precamb. Res., v. 94, pp. 251-271. Ramakrishnan, M., Nanda, J.K. and Augustine, P.E (1998) Geological evolution of the Proterozoic Eastern Ghats Mobile Belt. Geol. Surv. India, Spl. Publ., v. 44, pp. 1-21. Rickers, K., Mezger, K. and Raith, M.M. (2001) Evolution of the continental crust in the Proterozoic Eastern Ghats Belt, India: implications from Sm-Nd, Rb-Sr and Pb-Fb isotopes. Precamb. Res., (in Press). Sengupta, P., Sen, J., Dasgupta, S., Raith, M.M., Bhui, U. and Ehl, J. (1999) Ultra-high temperature metamorphism of metapelitic granulites from Kondapalle, Eastern Ghats Belt: implications for the Indo-Antarctic correlation. J. Petrol., v. 40, pp. 1065-1087.
Geology of the Achankovil Shear Zone, Southern India V.J, Rajeshl, M. Arimal and M. Santosh2 I
Geological Institute, Yokohama National University, Yokohama, Japan Department of Natural and Environmental Science, Kochi University, Kochi, Japan
Proterozoic granulite facies rocks are extensively exposed in East Gondwanaland terrains particularly southern India, Sri Lanka, East Antarctica and Madagascar. Southern Peninsular India comprises an assembly of terrains of varying metamorphic grades and tectonic histories and dissected into a number of blocks by major Proterozoic transcontinental shear zones. In general the shear zones define terrain boundaries and zones of more intense deformation within the terrain. The Achankovil Shear Zone (ACSZ) appears as a prominent lineament in aeromagnetic and satellite imageries. This feature is identified as a shear zone based on the change in rock types to the north and south, and by the sharp change from NE trending structures north of the shear zone to NW trending structures within and south of the zone. Field mapping to date suggests a lateral extension of 120 km and in places a thickness of 10-20 km for ACSZ which passes through the states of Kerala and Tamil Nadu in a NW-SE direction separating the supracrustal sequences of Trivandrum block (also known as the Kerala Khondalite Belt, KKB) in south with the massif charnockites in the North. ACSZ preserves abundant fabrics that can be used to derive the apparent sense of tectonic transport which include asymmetric fold patterns (s and z shaped), sigmoidal augens and boudins, asymmetric porphyroblasts with or without tails, pinch and swell structures, rolling on structures, minor shear bands, stretching lineations, etc. Earlier studies based on mesoscopic shear sense indicators in smaller areas recorded both dextral and sinistral shear sense along the ACSZ. Our detailed structural studies on larger domains along the shear zone indicate that the opposite sense of movements were developed at different times along the shear zone.
The dominant rock types in ACSZ are charnockites, garnetbiotite gneisses, calc-silicates and mafic granulites. Aluminous bands within the gneisses carry sillimanite and spinel. Charnockites are mostly garnet bearing, but garnet-absent massif type is also seen. Patches and veins of incipient charnockite develop within gneisses in several localities. One of the characteristic lithological units within ACSZ is cordierite and orthopyroxene bearing charnockite showing various reaction textures indicating garnet breakdown and cordierite formation in a decompression setting. Calc-silicate rocks within ACSZ comprise both garnet-bearing and garnet-absent varieties. The granulite facies rocks are variously intruded by a number of syn- or post-tectonic granitic plutons. An ultramafic body (dunite) has also been mapped from the ACSZ. The various lithounits preserve evidence for polyphase deformation and metamorphism. Maximum foliation attitudes have been measured and they have been subjected to detailed geometrical analysis. The D1 folds developed on bedding planes are tight to isoclinal in nature, the foliations were developed on second phase of deformation (D2) and were subsequently superposed by the second generation of folds and finally the D 3 episode results in the development of third generation folds and significantly affected the attitudes of previously formed folds and the relicts of pre-existing folds in the form of rootless folds. Geometric analysis also indicates the varying attitudes for these subsequently formed superposed folds. The ACSZ plays a key role in Gondwana correlations and global tectonics. It is generally held that the final amalgamation of Gondwanaland took place during the Pan-African, and Madagascar is placed against India in the Gondwana assembly. Gondwana Research, V 4, No. 4,2002
RODINIA, GONDWANA AND ASIA
One of the important criteria for this is the correlation between Bongolova Ranotsara Shear Zone (BRSZ) in Madagascar with the ACSZ in southern India. Our ongoing detailed studies on
745
the lithology, structural characteristics, petrology a n d geochronology along ACSZ and BRSZ are expected to provide better constraints on Gondwana reconstruction.
Geochemistry and Evolution of Charnockite-Khondalite-Anorthosite Association of Eastern Ghat Mobile Belt - A Fragment of Gondwana D.C.L. Raju and V. Divakara Rao2 Geological Survey of India, A.M.S.E Wing,Seminary Hills, Nagpur - 440 006, India, E-mail: [email protected] 'National Geophysical Research Institute, Geochemistry Division, Hyderabad - 500 007, India, E-mail: [email protected] ""
1 1 1 "
,.
"""
..... . ..
The curvilinear Eastern Ghats Mobile Belt (EGMB) trending NE-SW along the east coast of India over 900 km with a maximum width of about 300 km forms a part of a continuous Precambrian metamorphic terrain in Gondwana (Yoshida et al., 1996). Based on the relative abundances of various lithotectonic units and their spatial distribution, the EGMB is divided into three longitudinal zones, viz., the western charnockiteanorthosite zone; the central khondalite zone and the eastern migmatite zone, separated by major ductile shear zones (Ramakrishnan et al., 1994). The charnockites occur mostly as huge massifs and exhibit intrusive relationship with the metasedimts, the khondalites. Mineralogical and geochemical characteristics coupled with their field observations indicate a magmatic origin for these charnockites. It is suggested that calc-alkaline fractionation of parent magma has given rise to the Ca-rich tonalitic and potashrich granodioritic-granitic types of charnockites. The source of alkali fluids, the evidence for which are preserved in the geochemical signature of these rocks, may be the mantle, and these fluids could have been brought up through deep-seated crustal shear zones. The presence of a number of alkaline and ultrabasic rocks in the EGMB along NE-SW trending deep seated shear zone which are of upper mantle origin also supports this and suggests large scale fluid movement from mantle to the upper crust. The khondalite suite comprising garnet-sillimanite gneisses, garnet-sillimanite quartzite, garnetiferous granite gneisses/ migmatitic gneisses, leptynite and sapphrine granulites exhibit typical metasedimentary characters in terms of lithology and geochemistry (Raju and Divakara Rao, 1994). The mineralogical and chemical characteristics observed in these rocks reflect heterogeneity in the metapelite composition and indicate more than one source for the parent sediments, predominantly basic in nature. Geochemical characteristics presented in this study suggest that the provenance for these rocks was a mixed source of basic rocks and tonalite/trondhjemite, probably the Achaean crust. Occurrence of extensive manganese, iron oxides, laterite and bauxite along with pockets of graphite in these rocks of EGMB support rapid changes both in the physico-chemical and environmental conditions, during and/or after the formation of the khondalites. The layered complex of anorthosites, gabbroic anorthosites, anorthositic gabbros, websterites, chromitites, pyroxenites and Gondwana Research, V. 4, No. 4,2001
dunites of intrusive nature (Raju et al., 1999) into charnockitic rocks occurs as discontinuous, conformable layers, lenses and bands. Detailed major- and trace-elemental compositions of these layered intrusive suggest that the chromitites, dunites, pyroxenite rocks of the suite are cumulates and the gabbro-anorthosite suite of rocks are the fractionates of the same source, a basic magma of tholeiite composition. The type of deformation, grade of metamorphism and the structural disposition of the intrusives suggest that the source-magma is a syn- to post-kinematic intrusive and intruded into a substantially thickened crust. The available structural, P-T-t and geochemical data are in accord with continent-continent collision model of the EGMB. The contrasting tectonothermal histories of EGMB (Mezger et al., 1996) in the north and south of Godavari rift suggest the possible correlation with the Napier-Rayner boundary in east Antarctica (Harley and Hansen, 1990). In other words, the EGMB along the east coast of India is juxtaposed against portions of East Antarctica (Dasgupta et al., 1999) and forms a part of the 950-1000 Ma SWEAT orogen (Yoshida, 1995). The correlation of shear zone geometry between the EGMB and the Enderby Land by LANDSAT imagery study (Chetty, 1995) also suggest the fit of these Gondwana crustal fragments during the Precambrian.
References Chetty, T.R.K. (1995) A correlation of Proterozoic shear zones between Eastern Ghats, India and Enderby Land, East Antarctica, based on LANDSAT Imagery. Geol. SOC.India, Mem., No. 34, pp. 205-220. Das Gupta, S., Sengupta, F!, Pal, S. and Fukuoka, M. (1999) Evidence of superposed metamorphism from Gokavaram area, Eastern Ghats Belt, and its relation with the Kemp land coast, East Antarctica. Gondwana Res., v. 2, pp. 227-236. Harley, S.L. and Hansen, B.J. (1990) Achaean and Proterozoic highgrade terrains of East Antarctica (40-80"E): a case study of diversity in granulite facies metamorphism. In: Ashworth, J.R. and Brown, M. (Eds.), high-temperature metamorphism and Crustal Anatexis. Unwin Hyman, pp. 320-370. Mezger, K., Cosca, M.A. and Raith, M. (1996) Thermal history of the Eastern Ghats Belt (India) deduced from U-Pb and Ar-Ar dating of metamorphic minerals. Abstracts Journal, V.M. Goldschmidt Conf., v. 1, p. 334. Raju, D.C.L. and Divakara Rao, V. (1994) Geochemistry and evolution of EGMB with special reference to Ibrahimpatnam-Kondapalle area, Krishna district, Andhra Pradesh. In workshop on Eastern Ghats Mobile Belt, Visakhapatnam, India 15-16 June 1994, Abstr. vol., p. 55.
RODINIA, GONDWANA AND ASIA
References Chetty, T.R.K. (1995) A correlation of Proterozoic shear zones between Eastern Ghats, India and Enderby Land, East Antarctica, based on LANDSAT imagery. Mem. Geol. SOC.India, No. 34, pp. 205-220. Dobmeier, C., Simmat, R. and Raith, M.M. (2001) New constraints on the Proterozoic crustal evolution of the Eastern Ghats Belt, India. This volume. Hofmann, J. (1996) Fragmente intragondwanischer Rifte als Werkzeug der Gondwana-Rekonstruktion-das Beispiel des LambertMahanadi-Riftes (Ostantarktis-Peninsula Indien). N. Jb. Miner. Mh., V. 199, pp. 33-48. Kovach, VP., Berezhnaya, N.G., Salnikova, E.B., Narayana, B.L., Divakara Rao, V. and Yoshida, M. (1998) U-Pb zircon age and Nd isotope systematics of megacrystic charnockites in Eastern Ghats granulite belt, India, and their implication for East Gondwana reconstruction. J. African Earth Sci., Special abstract issue 'Gondwana-10: Event stratigraphy of Gondwana', v. 27, pp. 125-127. Kovach, V., Simmat, R., Rickers, K., Berezhnaya, N.G., Salnikova, E.B., Dobmeier, C. and Raith, M.M. (2001) The Western charnockite zone
of the Eastern Ghats Belt (India)-an independent Palaeoproterozoic (1.7-1.6 Ga) crustal province. This volume. Paul, D.K., Ray Barman, T., McNaughton, N.J., Fletcher, I.R., Potts, R.J., Ramakrishnan, M. and Augustine, PE (1990) Archean-Proterozoic evolution of Indian charnockites: isotopic and geochemical evidence from granulites of the Eastern Ghats Belt. J. Geol., v. 98, pp. 253-263. Mezger K. and Cosca M.A. (1999) The thermotectonic history of the Eastern Ghats Belt (India), as revealed by U-Pb and 4nAr-39Ar dating of metamorphic and magmatic minerals: implications for the SWEAT correlation. Precamb. Res., v. 94, pp. 251-271. Ramakrishnan, M., Nanda, J.K. and Augustine, P.E (1998) Geological evolution of the Proterozoic Eastern Ghats Mobile Belt. Geol. Surv. India, Spl. Publ., v. 44, pp. 1-21. Rickers, K., Mezger, K. and Raith, M.M. (2001) Evolution of the continental crust in the Proterozoic Eastern Ghats Belt, India: implications from Sm-Nd, Rb-Sr and Pb-Fb isotopes. Precamb. Res., (in Press). Sengupta, P., Sen, J., Dasgupta, S., Raith, M.M., Bhui, U. and Ehl, J. (1999) Ultra-high temperature metamorphism of metapelitic granulites from Kondapalle, Eastern Ghats Belt: implications for the Indo-Antarctic correlation. J. Petrol., v. 40, pp. 1065-1087.
Geology of the Achankovil Shear Zone, Southern India V.J, Rajeshl, M. Arimal and M. Santosh2 I
Geological Institute, Yokohama National University, Yokohama, Japan Department of Natural and Environmental Science, Kochi University, Kochi, Japan
Proterozoic granulite facies rocks are extensively exposed in East Gondwanaland terrains particularly southern India, Sri Lanka, East Antarctica and Madagascar. Southern Peninsular India comprises an assembly of terrains of varying metamorphic grades and tectonic histories and dissected into a number of blocks by major Proterozoic transcontinental shear zones. In general the shear zones define terrain boundaries and zones of more intense deformation within the terrain. The Achankovil Shear Zone (ACSZ) appears as a prominent lineament in aeromagnetic and satellite imageries. This feature is identified as a shear zone based on the change in rock types to the north and south, and by the sharp change from NE trending structures north of the shear zone to NW trending structures within and south of the zone. Field mapping to date suggests a lateral extension of 120 km and in places a thickness of 10-20 km for ACSZ which passes through the states of Kerala and Tamil Nadu in a NW-SE direction separating the supracrustal sequences of Trivandrum block (also known as the Kerala Khondalite Belt, KKB) in south with the massif charnockites in the North. ACSZ preserves abundant fabrics that can be used to derive the apparent sense of tectonic transport which include asymmetric fold patterns (s and z shaped), sigmoidal augens and boudins, asymmetric porphyroblasts with or without tails, pinch and swell structures, rolling on structures, minor shear bands, stretching lineations, etc. Earlier studies based on mesoscopic shear sense indicators in smaller areas recorded both dextral and sinistral shear sense along the ACSZ. Our detailed structural studies on larger domains along the shear zone indicate that the opposite sense of movements were developed at different times along the shear zone.
The dominant rock types in ACSZ are charnockites, garnetbiotite gneisses, calc-silicates and mafic granulites. Aluminous bands within the gneisses carry sillimanite and spinel. Charnockites are mostly garnet bearing, but garnet-absent massif type is also seen. Patches and veins of incipient charnockite develop within gneisses in several localities. One of the characteristic lithological units within ACSZ is cordierite and orthopyroxene bearing charnockite showing various reaction textures indicating garnet breakdown and cordierite formation in a decompression setting. Calc-silicate rocks within ACSZ comprise both garnet-bearing and garnet-absent varieties. The granulite facies rocks are variously intruded by a number of syn- or post-tectonic granitic plutons. An ultramafic body (dunite) has also been mapped from the ACSZ. The various lithounits preserve evidence for polyphase deformation and metamorphism. Maximum foliation attitudes have been measured and they have been subjected to detailed geometrical analysis. The D1 folds developed on bedding planes are tight to isoclinal in nature, the foliations were developed on second phase of deformation (D2) and were subsequently superposed by the second generation of folds and finally the D 3 episode results in the development of third generation folds and significantly affected the attitudes of previously formed folds and the relicts of pre-existing folds in the form of rootless folds. Geometric analysis also indicates the varying attitudes for these subsequently formed superposed folds. The ACSZ plays a key role in Gondwana correlations and global tectonics. It is generally held that the final amalgamation of Gondwanaland took place during the Pan-African, and Madagascar is placed against India in the Gondwana assembly. Gondwana Research, V 4, No. 4,2002
RODINIA, GONDWANA AND ASIA
One of the important criteria for this is the correlation between Bongolova Ranotsara Shear Zone (BRSZ) in Madagascar with the ACSZ in southern India. Our ongoing detailed studies on
745
the lithology, structural characteristics, petrology a n d geochronology along ACSZ and BRSZ are expected to provide better constraints on Gondwana reconstruction.
Geochemistry and Evolution of Charnockite-Khondalite-Anorthosite Association of Eastern Ghat Mobile Belt - A Fragment of Gondwana D.C.L. Raju and V. Divakara Rao2 Geological Survey of India, A.M.S.E Wing,Seminary Hills, Nagpur - 440 006, India, E-mail: [email protected] 'National Geophysical Research Institute, Geochemistry Division, Hyderabad - 500 007, India, E-mail: [email protected] ""
1 1 1 "
,.
"""
..... . ..
The curvilinear Eastern Ghats Mobile Belt (EGMB) trending NE-SW along the east coast of India over 900 km with a maximum width of about 300 km forms a part of a continuous Precambrian metamorphic terrain in Gondwana (Yoshida et al., 1996). Based on the relative abundances of various lithotectonic units and their spatial distribution, the EGMB is divided into three longitudinal zones, viz., the western charnockiteanorthosite zone; the central khondalite zone and the eastern migmatite zone, separated by major ductile shear zones (Ramakrishnan et al., 1994). The charnockites occur mostly as huge massifs and exhibit intrusive relationship with the metasedimts, the khondalites. Mineralogical and geochemical characteristics coupled with their field observations indicate a magmatic origin for these charnockites. It is suggested that calc-alkaline fractionation of parent magma has given rise to the Ca-rich tonalitic and potashrich granodioritic-granitic types of charnockites. The source of alkali fluids, the evidence for which are preserved in the geochemical signature of these rocks, may be the mantle, and these fluids could have been brought up through deep-seated crustal shear zones. The presence of a number of alkaline and ultrabasic rocks in the EGMB along NE-SW trending deep seated shear zone which are of upper mantle origin also supports this and suggests large scale fluid movement from mantle to the upper crust. The khondalite suite comprising garnet-sillimanite gneisses, garnet-sillimanite quartzite, garnetiferous granite gneisses/ migmatitic gneisses, leptynite and sapphrine granulites exhibit typical metasedimentary characters in terms of lithology and geochemistry (Raju and Divakara Rao, 1994). The mineralogical and chemical characteristics observed in these rocks reflect heterogeneity in the metapelite composition and indicate more than one source for the parent sediments, predominantly basic in nature. Geochemical characteristics presented in this study suggest that the provenance for these rocks was a mixed source of basic rocks and tonalite/trondhjemite, probably the Achaean crust. Occurrence of extensive manganese, iron oxides, laterite and bauxite along with pockets of graphite in these rocks of EGMB support rapid changes both in the physico-chemical and environmental conditions, during and/or after the formation of the khondalites. The layered complex of anorthosites, gabbroic anorthosites, anorthositic gabbros, websterites, chromitites, pyroxenites and Gondwana Research, V. 4, No. 4,2001
dunites of intrusive nature (Raju et al., 1999) into charnockitic rocks occurs as discontinuous, conformable layers, lenses and bands. Detailed major- and trace-elemental compositions of these layered intrusive suggest that the chromitites, dunites, pyroxenite rocks of the suite are cumulates and the gabbro-anorthosite suite of rocks are the fractionates of the same source, a basic magma of tholeiite composition. The type of deformation, grade of metamorphism and the structural disposition of the intrusives suggest that the source-magma is a syn- to post-kinematic intrusive and intruded into a substantially thickened crust. The available structural, P-T-t and geochemical data are in accord with continent-continent collision model of the EGMB. The contrasting tectonothermal histories of EGMB (Mezger et al., 1996) in the north and south of Godavari rift suggest the possible correlation with the Napier-Rayner boundary in east Antarctica (Harley and Hansen, 1990). In other words, the EGMB along the east coast of India is juxtaposed against portions of East Antarctica (Dasgupta et al., 1999) and forms a part of the 950-1000 Ma SWEAT orogen (Yoshida, 1995). The correlation of shear zone geometry between the EGMB and the Enderby Land by LANDSAT imagery study (Chetty, 1995) also suggest the fit of these Gondwana crustal fragments during the Precambrian.
References Chetty, T.R.K. (1995) A correlation of Proterozoic shear zones between Eastern Ghats, India and Enderby Land, East Antarctica, based on LANDSAT Imagery. Geol. SOC.India, Mem., No. 34, pp. 205-220. Das Gupta, S., Sengupta, F!, Pal, S. and Fukuoka, M. (1999) Evidence of superposed metamorphism from Gokavaram area, Eastern Ghats Belt, and its relation with the Kemp land coast, East Antarctica. Gondwana Res., v. 2, pp. 227-236. Harley, S.L. and Hansen, B.J. (1990) Achaean and Proterozoic highgrade terrains of East Antarctica (40-80"E): a case study of diversity in granulite facies metamorphism. In: Ashworth, J.R. and Brown, M. (Eds.), high-temperature metamorphism and Crustal Anatexis. Unwin Hyman, pp. 320-370. Mezger, K., Cosca, M.A. and Raith, M. (1996) Thermal history of the Eastern Ghats Belt (India) deduced from U-Pb and Ar-Ar dating of metamorphic minerals. Abstracts Journal, V.M. Goldschmidt Conf., v. 1, p. 334. Raju, D.C.L. and Divakara Rao, V. (1994) Geochemistry and evolution of EGMB with special reference to Ibrahimpatnam-Kondapalle area, Krishna district, Andhra Pradesh. In workshop on Eastern Ghats Mobile Belt, Visakhapatnam, India 15-16 June 1994, Abstr. vol., p. 55.