- Email: [email protected]
Recognition of Pre-Grenvillian and Grenvillian Tectonothermal Events in the Central Indian Tectonic Zones: Implications on Rodinian Crustal Assembly
755
RODINIA, GONDWANA AND ASIA
the fracture zones (e.g., the Tasman FZ, Balleny FZ), is still continuing. They strike NW-SE towards the continent. Normally, fracture zones do not affect continental lithosphere. But during the GANOVEX VIII expedition right-lateral strike-slip faults and km-wide fault zones in approximately the same strike direction were observed in the Rennick Glacier, the Matusevich Glacier and the Mertz Glacier areas. We can conclude that these rightlateral strike-slip movements were the dominating tectonic features and are still active today, as indicated by earthquakes in the lower Matusevich (1952) and Rennick (1972,1999) areas. The Rennick earthquakes had epicentres at depths of 33 km and 23.5 km. The 1999 earthquake was registered during the GANOVEX VIII expedition (Cattaneo et al., 2001). The
observations speak for an onshore continuation of fracture zones or related tectonic features, a topic, which has to be discussed further.
References Cattaneo, M., Chiappini, M. and De Gori, I? (2001) Seismological experiment in the BACKTAM project. In: Bozzo, E. and Damaske, D. (Eds.), Initial Reports - GANOVEXVIII - ItaliAntartide XV Antarctic Expedition 1999-2000. Terra Antarctica Rep., No. 5, pp. 29-44. Roland, N.W. and Worner, G. (1996) Kirkpatrick flows and associated pyroclastics: new occurrences, definition, and aspects of a Jurassic Transantarctic Rift. In: Roland, N.W. (Ed.), Polar Issue No. 6, GANOVEX: From Oates Coast to Marie Byrd Land. Geol. Jb., B 89, pp. 97-121.
Recognition of Pre-Grenvillian and Grenvillian Tectonothermal Events in the Central Indian Tectonic Zones: Implications on Rodinian Crustal Assembly Abhinaba Roy, M. Hanuma Prasad and S.K. Bhowmik Geological Survey of India,Seminary Hills, Nagpur - 440 006, Indza, E mail:[email protected];gsipgrs~na~ur.dot.net.in ~-1
I
The Precambrian crust of central India comprises two Archaean cratonic domains (ca. 3.5-2.5 Ga), namely the Bundelkhand Craton in the north and Bastar Craton in the south. They were accreted along a broad, ENE-WSW trending Proterozoic (ca. 2.5-1 .O Ga) Central Indian Tectonic Zone (CITZ) (Fig. 1). The CITZ, as defined by Radhakrishna (1989) and Acharyya and Roy (2000), contains low- to medium-grade supracrustal belts set in largely undifferentiated gneisses and granitoids. A few tracts of granulites are present as enclaves with in the gneisses. Amongst the supracrustal belts, the northern Mahakoshal belt (ca. 2.4-1.8 Ga), central Betul and southern Sausar belt (ca. 1.5-1.0 Ga) are prominent, of which Mahakoshal and Sausar belts are well studied. Further, the CITZ is traversed by number of crustal scale ductile shear zones, which mark either the boundaries of the supracrustal belts or discrete terrains. Sonh armada shear system, Tan shear and Central Indian Shear (CIS) (Jain et al., 1991) are conspicuous shear zones in CITZ. The earliest event in the CITZ is manifested by the deposition of Mahakoshal volcano-sedimentary rocks in the linear ENEWSW trending fault-bound rift basin. The basin was initiated in the form of a pericratonic basin along the southern margin of the ca. 3.5-2.5 Ga old Bundelkhand Granite Complex, in which shallow marine quartzite-carbonate-chert-BIFwere deposited (Roy and Devarajan, 2001). This was followed by rifting, magmatic underplating and consequent tholeiitic volcanism in the basin. Upliftment of the rifted margins led to the deposition of the debris flow deposits in the hanging wall basin, culminating with the deposition of the argillite-BIF during the thermal relaxation stage. Subsequently, the basin was closed by N to NNW-ly directed bulk subhorizontal compression, during which the supracrustal rocks were folded into upright to slightly overturned folds on ENE-WSW striking and steep southerly dipping axial planes. A high strain or shear zone, defined by a Gondwana Research, K 4, No. 4, 2001
~-
1111
1 1
- -- - _--
mylonite belt, developed along the southern margin of the belt, which served as the avenue for the emplacement of several linear, granitoid plutons. Detailed structural studies show that the granites and gneisses were thrusted over the supracrustal rocks along this high strain zone. Concomitant melting of the lower crust and advective heat transfer by syn-kinematic granitoids resulted in low-pressure and medium temperature regional metamorphism of the supracrustal rocks (Roy et al., 2001). These syn-kinematic granitoids are ca. 1.8 Ga in age and were generated by the recycling of the ca. 2.5 Ga old crust (Sarkar et al., 1998). Thus, the ca. 1.8 Ga events in the CITZ are marked by collisional tectonics and recycling of the continental crust. The second event in the CITZ is represented by the development of a gneissic complex from a range of granitic protoliths. This gneissic terrain occupies major portion between northern Mahakoshal and southern Sausar supracrustal belts. Available Rb-Sr geochronological data from Tirodi gneisses and Dudhi gneisses indicate this event to be around 1.5 Ga (Sarkar et al., 1986) and they are comparable to Chotanagpur gneisses further east. In addition, these gneisses contain enclaves of granulites. Three linear tracts of granulites were found in the CITZ, one along the southern margin of the Mahakoshal belt and other two along the southern and northern margins of the Sausar belt. The tract north of the Sausar belt is made up of mafic, felsic and cordierite-bearing granulites. P-T-t studies indicate that the granulites record peak P and T of -10.5 kb and 775°C respectively, followed by post-peak ITD at -5.5 kb and 750% (Bhowmik et al., 1999). This clock-wise P-T loop suggests initial collision related thickening followed by extensional collapse. Generation of ca. 1.5 Ga gneisses, viz., Tirodi Gneiss, is inferred to be linked with the granulite facies metamorphism, thus the granulite event is considered to be ca. 1.5 Ga in age. Granulitcs south of the Sausar belt are composed
756
RODINIA, GONDWANA AND ASIA
200 km I
p i Bijawar Group
1
1* * 1
1-1 Fig. 1.
Granulites
I
I
Mahakoshal Group
__
26"
Youngercover
Sausar Group
F]Sakoli Group
Gneisses and Granites
Dongargarh Supergroup
Geological map showing different lithotectonic units of the Central Indian Tectonic Zone.
of two pyroxene granulites, meta iron formation, enderbites and charnockites. They record peak P-T of 7 kbar and 900°C respectively and follow a post-peak IBC path (Ramachandra and Roy, 2001). Magmatic accretion was synchronous with the peak P-T, which took place in an extensional tectonic regime. No
reliable radiometric dates are available for this event. Both northern and southern granulites are superimposed by amphibolite foliation of younger Sausar orogeny (ca. 1.0 Ga), thus both the granulite tracts are pre-Sausar (Grenvillian) in age. In summary the 1.5 Ga event in the CITZ marks the ~~
Gondwana Research, V. 4, No. 4,2001
RODINIA, GONDWANA AND ASIA
formation of granulites and gneisses in a predominantly collisional tectonic environment. These gneisses and granulites formed the basement for the Sausar Group of rocks, which were deposited in an intracratonic basin. The basement-cover relationships are obliterated by the tectonic imbrication. The supracrustal rocks of the Sausar Group are represented by shallow water quartzite-carbonate-pelite-Mn formation. Volcanic rocks are completely absent. The Sausar Group of rocks have undergone three phases of deformation. The first phase is manifested by the ductile shear zones, coverbasement tectonism, thrusting and tectonic thickening. The second phase produced large and small-scale upright folds on E and ENE striking subvertical axial planes. The last phase has given rise to large-scale asymmetric (sinistral) folds on NW and NNW striking axial planes. P-T-t studies indicate early loading at 7 kbar and 675°C followed by decompressional heating at 4 kbar and 550-675°C. This clockwise P-T loop, coupled with thrust tectonics, suggests a collisional tectonic regime during Sausar orogeny. A series of syn- to post-kinematic linear and circular granitic plutons were emplaced in to the Sausar belt and the post-kinematic granite yielded a Rb-Sr age of ca. 950 Ma. A similar age by Ar-Ar method was obtained from cryptomelane in the Mn-formation (Lippolt and Hautmann, 1994). Thus, the Sausar Orogeny is inferred to be of ca. 1.0 Ga old and correlatable with Grenvillian Orogeny. Thus, the CITZ represents a Proterozoic mobile belt, which experienced polyphase tectonothermal events. Available geochronological data indicates at least three events, viz., ca. 1.8 Ga, ca. 1.5 Ga and ca. 1.0 Ga. Each event is characterized by collisional tectonism and crustal recycling. Our studies indicate that ca. 1.5 Ga event may have stitched northern Bundelkhand and the southern Bastar Cratons. Evolutionary trends in CITZ show certain similarities with those of AlbanyFraser Mobile belt of Australia. Detailed geochronological data is awaited for making global correlations.
757
References Acharyya, S.K. and Roy, A. (2000) Tectonothermal history of the Central Indian Tectonic Zone and reactivation of major faults/shear zones. J. Geol. SOC.India, v. 55, pp. 239-256. Bhowmik, S.K., Pal, T., Roy, A. and Pant, N.C. (1999) Evidence for PreGrenvillian high-pressure granulite metamorphism from the northern margin of the Sausar mobile belt in Central India. J. Geol. SOC. India, v. 53, pp. 385-399. Jain, S.C., Yedekar, D.B. and Nair, K.K.K. (1991) Central Indian Shear Zone: a major Precambrian crustal boundary. J. Geol. SOC.India, v. 37, pp. 521-548. Lippolt, H.J. and Hautmann, S. (1994) AP/A139 ages of Precambrian manganese ore minerals from Sweden, India and Morocco. Mineralium Deposita, v. 18, pp. 195-215. Radhakrishna, B.P. (1989) Suspect tectonostratigraphic terrane elements in the Indian subcontinent. J. Geol SOC.India, v. 34, pp. 1-24. Ramachandra, H.M. and Roy, A. (2001) Evolution of the BhandaraBaiaghat granulite belt along the southern margin of the Sausar mobile belt of Central India. In: Special Volume in tribute to Prof. S. Sen. Roy, A. and Devarajan, M.K. (2001) A reappraisal of the stratigraphy and tectonics of the Proterozoic Mahakoshal belt, Central India. In: S.K. Acharyya and M. Yoshida (Eds.), Precambrian crust of Eastern and Central India. IGCP-368, Seminar Volume, Geol. Sum. India (In Press). Roy, A., Devarajan, M.K. and Hanuma Prasad, M. (2001) Ductile shearing and syntectonic granite emplacement along the southern margin of the Palaeoproterozoic Mahakoshal supracrustal belt: evidences from Singrauli area, Madhya Pradesh. Geol. SOC.India, (Submitted). Sarkar, A., Bodas, M.S.,Kundu, H.K., Mamgain, V.V. and Ravishankar (1998) Geochronology and geochemistry of Mesoproterozoic intrusive plutonites from the eastern segment of the Mahakoshal greenstone belt, Central India. In: IGCP-368 Seminar on Precambrian Crust in Eastern and central India, Bhubaneswar (Abst.), pp. 82-85. Sarkar, S.N., Trivedi, J.R. and Gopalan, K. (1986) Rb-Sr whole rock and mineral isochron age of the Tirodi gneiss, Sausar Group, Bhandara district, Maharastra. J. Geol. SOC.India, v. 27, pp. 30-37.
Phanerozoic Reconstitution of Indian Shield as Aftermath of Gondwana Break-up A.B. Roy Department of Geology, Mohanlal Sukhadia University, Udaipur - 323 002, India, E-mail: [email protected] The crust constituting the Indian Shield is traditionally considered as a completely cratonised continental lithosphere since Precambrian. However, the fact that this cratonic block remained tectono-thermally active during the entire Phanerozoic provides proofs of reconstitution of the Indian lithospheric crust. The tectonic processes include the break-up of the Indian Shield from the Gondwana Supercontinent, its northward movement, and ultimate collision with northern continental blocks leading to evolution of the mighty Himalaya in the process. Two important factors, which brought a b o u t considerable reconstitution of t h e Indian lithospheric crust, are the development of several d e e p fractures a n d extensive Gondwana Research, K 4, No. 4,2001
underplating events at subcrustal level or at the crust-mantle boundary during India’s passage over a number of mantle plumes. The first major Phanerozoic tectono-thermal event, which 50 Ma (Le., close to affected the Indian crust during 500 Pc-C boundary), is manifested in the form of several anorogenic intrusions of per-alkaline granitoids all along the periphery of the pre-Himalayan Indian Shield. In spite of the fact that the event is virtually contemporaneous with the closing of the PanAfrican orogenic/thermal cycle, there is hardly any reason to describe the event as Pan-African in the Indian context. On the other hand, the 500 k 50 Ma per-alkaline granitoid events are
*
RODINIA, GONDWANA AND ASIA
the fracture zones (e.g., the Tasman FZ, Balleny FZ), is still continuing. They strike NW-SE towards the continent. Normally, fracture zones do not affect continental lithosphere. But during the GANOVEX VIII expedition right-lateral strike-slip faults and km-wide fault zones in approximately the same strike direction were observed in the Rennick Glacier, the Matusevich Glacier and the Mertz Glacier areas. We can conclude that these rightlateral strike-slip movements were the dominating tectonic features and are still active today, as indicated by earthquakes in the lower Matusevich (1952) and Rennick (1972,1999) areas. The Rennick earthquakes had epicentres at depths of 33 km and 23.5 km. The 1999 earthquake was registered during the GANOVEX VIII expedition (Cattaneo et al., 2001). The
observations speak for an onshore continuation of fracture zones or related tectonic features, a topic, which has to be discussed further.
References Cattaneo, M., Chiappini, M. and De Gori, I? (2001) Seismological experiment in the BACKTAM project. In: Bozzo, E. and Damaske, D. (Eds.), Initial Reports - GANOVEXVIII - ItaliAntartide XV Antarctic Expedition 1999-2000. Terra Antarctica Rep., No. 5, pp. 29-44. Roland, N.W. and Worner, G. (1996) Kirkpatrick flows and associated pyroclastics: new occurrences, definition, and aspects of a Jurassic Transantarctic Rift. In: Roland, N.W. (Ed.), Polar Issue No. 6, GANOVEX: From Oates Coast to Marie Byrd Land. Geol. Jb., B 89, pp. 97-121.
Recognition of Pre-Grenvillian and Grenvillian Tectonothermal Events in the Central Indian Tectonic Zones: Implications on Rodinian Crustal Assembly Abhinaba Roy, M. Hanuma Prasad and S.K. Bhowmik Geological Survey of India,Seminary Hills, Nagpur - 440 006, Indza, E mail:[email protected];gsipgrs~na~ur.dot.net.in ~-1
I
The Precambrian crust of central India comprises two Archaean cratonic domains (ca. 3.5-2.5 Ga), namely the Bundelkhand Craton in the north and Bastar Craton in the south. They were accreted along a broad, ENE-WSW trending Proterozoic (ca. 2.5-1 .O Ga) Central Indian Tectonic Zone (CITZ) (Fig. 1). The CITZ, as defined by Radhakrishna (1989) and Acharyya and Roy (2000), contains low- to medium-grade supracrustal belts set in largely undifferentiated gneisses and granitoids. A few tracts of granulites are present as enclaves with in the gneisses. Amongst the supracrustal belts, the northern Mahakoshal belt (ca. 2.4-1.8 Ga), central Betul and southern Sausar belt (ca. 1.5-1.0 Ga) are prominent, of which Mahakoshal and Sausar belts are well studied. Further, the CITZ is traversed by number of crustal scale ductile shear zones, which mark either the boundaries of the supracrustal belts or discrete terrains. Sonh armada shear system, Tan shear and Central Indian Shear (CIS) (Jain et al., 1991) are conspicuous shear zones in CITZ. The earliest event in the CITZ is manifested by the deposition of Mahakoshal volcano-sedimentary rocks in the linear ENEWSW trending fault-bound rift basin. The basin was initiated in the form of a pericratonic basin along the southern margin of the ca. 3.5-2.5 Ga old Bundelkhand Granite Complex, in which shallow marine quartzite-carbonate-chert-BIFwere deposited (Roy and Devarajan, 2001). This was followed by rifting, magmatic underplating and consequent tholeiitic volcanism in the basin. Upliftment of the rifted margins led to the deposition of the debris flow deposits in the hanging wall basin, culminating with the deposition of the argillite-BIF during the thermal relaxation stage. Subsequently, the basin was closed by N to NNW-ly directed bulk subhorizontal compression, during which the supracrustal rocks were folded into upright to slightly overturned folds on ENE-WSW striking and steep southerly dipping axial planes. A high strain or shear zone, defined by a Gondwana Research, K 4, No. 4, 2001
~-
1111
1 1
- -- - _--
mylonite belt, developed along the southern margin of the belt, which served as the avenue for the emplacement of several linear, granitoid plutons. Detailed structural studies show that the granites and gneisses were thrusted over the supracrustal rocks along this high strain zone. Concomitant melting of the lower crust and advective heat transfer by syn-kinematic granitoids resulted in low-pressure and medium temperature regional metamorphism of the supracrustal rocks (Roy et al., 2001). These syn-kinematic granitoids are ca. 1.8 Ga in age and were generated by the recycling of the ca. 2.5 Ga old crust (Sarkar et al., 1998). Thus, the ca. 1.8 Ga events in the CITZ are marked by collisional tectonics and recycling of the continental crust. The second event in the CITZ is represented by the development of a gneissic complex from a range of granitic protoliths. This gneissic terrain occupies major portion between northern Mahakoshal and southern Sausar supracrustal belts. Available Rb-Sr geochronological data from Tirodi gneisses and Dudhi gneisses indicate this event to be around 1.5 Ga (Sarkar et al., 1986) and they are comparable to Chotanagpur gneisses further east. In addition, these gneisses contain enclaves of granulites. Three linear tracts of granulites were found in the CITZ, one along the southern margin of the Mahakoshal belt and other two along the southern and northern margins of the Sausar belt. The tract north of the Sausar belt is made up of mafic, felsic and cordierite-bearing granulites. P-T-t studies indicate that the granulites record peak P and T of -10.5 kb and 775°C respectively, followed by post-peak ITD at -5.5 kb and 750% (Bhowmik et al., 1999). This clock-wise P-T loop suggests initial collision related thickening followed by extensional collapse. Generation of ca. 1.5 Ga gneisses, viz., Tirodi Gneiss, is inferred to be linked with the granulite facies metamorphism, thus the granulite event is considered to be ca. 1.5 Ga in age. Granulitcs south of the Sausar belt are composed
756
RODINIA, GONDWANA AND ASIA
200 km I
p i Bijawar Group
1
1* * 1
1-1 Fig. 1.
Granulites
I
I
Mahakoshal Group
__
26"
Youngercover
Sausar Group
F]Sakoli Group
Gneisses and Granites
Dongargarh Supergroup
Geological map showing different lithotectonic units of the Central Indian Tectonic Zone.
of two pyroxene granulites, meta iron formation, enderbites and charnockites. They record peak P-T of 7 kbar and 900°C respectively and follow a post-peak IBC path (Ramachandra and Roy, 2001). Magmatic accretion was synchronous with the peak P-T, which took place in an extensional tectonic regime. No
reliable radiometric dates are available for this event. Both northern and southern granulites are superimposed by amphibolite foliation of younger Sausar orogeny (ca. 1.0 Ga), thus both the granulite tracts are pre-Sausar (Grenvillian) in age. In summary the 1.5 Ga event in the CITZ marks the ~~
Gondwana Research, V. 4, No. 4,2001
RODINIA, GONDWANA AND ASIA
formation of granulites and gneisses in a predominantly collisional tectonic environment. These gneisses and granulites formed the basement for the Sausar Group of rocks, which were deposited in an intracratonic basin. The basement-cover relationships are obliterated by the tectonic imbrication. The supracrustal rocks of the Sausar Group are represented by shallow water quartzite-carbonate-pelite-Mn formation. Volcanic rocks are completely absent. The Sausar Group of rocks have undergone three phases of deformation. The first phase is manifested by the ductile shear zones, coverbasement tectonism, thrusting and tectonic thickening. The second phase produced large and small-scale upright folds on E and ENE striking subvertical axial planes. The last phase has given rise to large-scale asymmetric (sinistral) folds on NW and NNW striking axial planes. P-T-t studies indicate early loading at 7 kbar and 675°C followed by decompressional heating at 4 kbar and 550-675°C. This clockwise P-T loop, coupled with thrust tectonics, suggests a collisional tectonic regime during Sausar orogeny. A series of syn- to post-kinematic linear and circular granitic plutons were emplaced in to the Sausar belt and the post-kinematic granite yielded a Rb-Sr age of ca. 950 Ma. A similar age by Ar-Ar method was obtained from cryptomelane in the Mn-formation (Lippolt and Hautmann, 1994). Thus, the Sausar Orogeny is inferred to be of ca. 1.0 Ga old and correlatable with Grenvillian Orogeny. Thus, the CITZ represents a Proterozoic mobile belt, which experienced polyphase tectonothermal events. Available geochronological data indicates at least three events, viz., ca. 1.8 Ga, ca. 1.5 Ga and ca. 1.0 Ga. Each event is characterized by collisional tectonism and crustal recycling. Our studies indicate that ca. 1.5 Ga event may have stitched northern Bundelkhand and the southern Bastar Cratons. Evolutionary trends in CITZ show certain similarities with those of AlbanyFraser Mobile belt of Australia. Detailed geochronological data is awaited for making global correlations.
757
References Acharyya, S.K. and Roy, A. (2000) Tectonothermal history of the Central Indian Tectonic Zone and reactivation of major faults/shear zones. J. Geol. SOC.India, v. 55, pp. 239-256. Bhowmik, S.K., Pal, T., Roy, A. and Pant, N.C. (1999) Evidence for PreGrenvillian high-pressure granulite metamorphism from the northern margin of the Sausar mobile belt in Central India. J. Geol. SOC. India, v. 53, pp. 385-399. Jain, S.C., Yedekar, D.B. and Nair, K.K.K. (1991) Central Indian Shear Zone: a major Precambrian crustal boundary. J. Geol. SOC.India, v. 37, pp. 521-548. Lippolt, H.J. and Hautmann, S. (1994) AP/A139 ages of Precambrian manganese ore minerals from Sweden, India and Morocco. Mineralium Deposita, v. 18, pp. 195-215. Radhakrishna, B.P. (1989) Suspect tectonostratigraphic terrane elements in the Indian subcontinent. J. Geol SOC.India, v. 34, pp. 1-24. Ramachandra, H.M. and Roy, A. (2001) Evolution of the BhandaraBaiaghat granulite belt along the southern margin of the Sausar mobile belt of Central India. In: Special Volume in tribute to Prof. S. Sen. Roy, A. and Devarajan, M.K. (2001) A reappraisal of the stratigraphy and tectonics of the Proterozoic Mahakoshal belt, Central India. In: S.K. Acharyya and M. Yoshida (Eds.), Precambrian crust of Eastern and Central India. IGCP-368, Seminar Volume, Geol. Sum. India (In Press). Roy, A., Devarajan, M.K. and Hanuma Prasad, M. (2001) Ductile shearing and syntectonic granite emplacement along the southern margin of the Palaeoproterozoic Mahakoshal supracrustal belt: evidences from Singrauli area, Madhya Pradesh. Geol. SOC.India, (Submitted). Sarkar, A., Bodas, M.S.,Kundu, H.K., Mamgain, V.V. and Ravishankar (1998) Geochronology and geochemistry of Mesoproterozoic intrusive plutonites from the eastern segment of the Mahakoshal greenstone belt, Central India. In: IGCP-368 Seminar on Precambrian Crust in Eastern and central India, Bhubaneswar (Abst.), pp. 82-85. Sarkar, S.N., Trivedi, J.R. and Gopalan, K. (1986) Rb-Sr whole rock and mineral isochron age of the Tirodi gneiss, Sausar Group, Bhandara district, Maharastra. J. Geol. SOC.India, v. 27, pp. 30-37.
Phanerozoic Reconstitution of Indian Shield as Aftermath of Gondwana Break-up A.B. Roy Department of Geology, Mohanlal Sukhadia University, Udaipur - 323 002, India, E-mail: [email protected] The crust constituting the Indian Shield is traditionally considered as a completely cratonised continental lithosphere since Precambrian. However, the fact that this cratonic block remained tectono-thermally active during the entire Phanerozoic provides proofs of reconstitution of the Indian lithospheric crust. The tectonic processes include the break-up of the Indian Shield from the Gondwana Supercontinent, its northward movement, and ultimate collision with northern continental blocks leading to evolution of the mighty Himalaya in the process. Two important factors, which brought a b o u t considerable reconstitution of t h e Indian lithospheric crust, are the development of several d e e p fractures a n d extensive Gondwana Research, K 4, No. 4,2001
underplating events at subcrustal level or at the crust-mantle boundary during India’s passage over a number of mantle plumes. The first major Phanerozoic tectono-thermal event, which 50 Ma (Le., close to affected the Indian crust during 500 Pc-C boundary), is manifested in the form of several anorogenic intrusions of per-alkaline granitoids all along the periphery of the pre-Himalayan Indian Shield. In spite of the fact that the event is virtually contemporaneous with the closing of the PanAfrican orogenic/thermal cycle, there is hardly any reason to describe the event as Pan-African in the Indian context. On the other hand, the 500 k 50 Ma per-alkaline granitoid events are
*