• R SRINIVASAN

      Articles written in Journal of Earth System Science

    • Archaean crustal thickness of greenstone granite belts of South India

      C S Pichamuthu R Srinivasan Fareeduddin Appaji Amrit Bhasker

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      Archaean crustal thickness for the Dharwar craton is estimated using potash index and Rb−Sr crustal thickness grid. The volcanics of the Dharwar greenstone belts appear to have evolved in a less than 20 km thick crust. Whereas the tonalite-trondhjemite pebbles of the Dharwar conglomerates (3250±150 m.y.) were derived from gneisses that evolved in a crust less than 20 km thick, the bulk of the peninsular gneisses and associated granitoids were emplaced in a crust 25 to 35 km thick. The 2000 m.y. old Closepet granite suite was emplaced in a crust thicker than 30 km. It is deduced that the continental crust in the region thickened from 15 to 35 km during a span of about 1000 m.y. between 3250±150 to 2000 m.y. ago. Calculations show that Archaean gecthermal gradients in Dharwar craton were three to four times steeper when compared to the present 10.5°C/km. The thin crust and the steep geothermal gradients are reflected by the emplacement of high magnesia basalts, layered igneous complexes and the strong iron enrichment trend shown by Dharwar metavolcanics.

    • The nature of the basement in the Archaean Dharwar craton of southern India and the age of the Peninsular Gneiss

      K Naha R Srinivasan K Gopalan GVC Pantulu M V Subba Rao A B Vrevsky Ye S Bogomolov

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      The Archaean Peninsular Gneiss of southern India is considered by a number of workers to be the basement upon which the Dharwar supracrustal rocks were deposited. However, the Peninsular Gneiss in its present state is a composite gneiss formed by synkinematic migmatization during successive episodes of folding (DhF1, DhF1a and DhF2) that affected the Dharwar supracrustal rocks. An even earlier phase of migmatization and deformation (DhF*) is evident from relict fabrics in small enclaves of gneissic tonalites and amphibolites within the Peninsular Gneiss. We consider these enclaves to represent the original basement for the Dharwar supracrustal rocks. Tonalitic pebbles in conglomerates of the Dharwar Supergroup confirm the inference that the supracrustal rocks were deposited on a gneissic basement.

      Whole rock Rb-Sr ages of gneisses showing only the DhF1 structures fall in the range of 3100–3200 Ma. Where the later deformation (DhF2) has been associated with considerable recrystallization, the Rb-Sr ages are between 2500 Ma and 2700 Ma. Significantly, a new Rb-Sr analysis of tonalitic gneiss pebbles in the Kaldurga conglomerate of the Dharwar sequence is consistent with an age of ∼2500 Ma and not that of 3300 Ma reported earlier by Venkatasubramanian and Narayanaswamy (1974). Pb-Pb ages based on direct evaporation of detrital zircon grains from the metasedimentary rocks of the Dharwar sequence fall into two groups, 3300–3100 Ma, and 2800–3000 Ma. Stratigraphic, structural, textural and geochronologic data, therefore, indicate that the Peninsular Gneiss of the Dharwar craton evolved over a protracted period of time ranging from > 3300 Ma to 2500 Ma.

    • Middle to late Archaean geology of the eastern Baltic shield, with a note on its similarity and contrast with the Archaean of southern India

      R Srinivasan K Naha Y J Bhaskar Rao A B Vrevsky S I Rybakov A I Golubev M Efimov

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      The middle to late Archaean rocks of Kola and Karelia in the eastern Baltic shield consist of the Infracomplex overlain by the Saamian complex, and the Lopian greenstone belts. The Infracomplex which forms the basement is a polymigmatite, parts of which are at least 3100 Ma old. The Saamian in the central Belomorian region comprises granite gneiss, amphibolite, garnet-kyanite gneiss and high alumina gneisses which belong to the Keret, Hetolombina and Chupa suites. The Lopian greenstone belts ranging in age from 3000 to 2700 Ma are composed of peridotitic, pyroxenitic and basaltic komatiites, tholeiitic basalts, andesites, dacites and rhyolites, together with tuffs, graywackes and iron formations. Whereas there is a dominance of volcanic over sedimentary rocks in the greenstone belts of the Baltic shield, a significant proportion of detrital and chemogenic sedimentary rocks characterizes the Dharwar succession of approximately the same time span in the southern Indian shield. Association of mature and immature detrital sedimentary rocks with bimodal volcanic assemblages points to a back-arc setting for the Dharwar belts. This contrasts with the association of immature sediments with calc-alkaline volcanic rocks in the greenstone belts of the eastern Baltic shield, suggesting an island arc environment there.

    • Nature of the Moyar and Bhavani shear zones, with a note on its implication on the tectonics of the southern Indian Precambrian shield

      K Naha R Srinivasan

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      The Moyar, Bhavani and Palghat-Cauvery shear zones have figured prominently in tectonic and metamorphic syntheses in the southern Indian Precambrian shield. Implicit in these studies is the assumption that in the Moyar shear zone in particular, transport has a large strike-slip component, with a dextral displacement of as high as 70 km. Detailed structural investigations in several key sectors and reconnaissance over a large terrane cast doubt on several aspects of the accepted tectonic model. Both the Moyar and the Bhavani shear zones are steeply-dipping thrusts, with the Moyar shear in particular characterized by a predominantly dip-slip transport. Such a movement on a subvertical plane striking EW could in no way rotate the northerly ‘trends’ to EW ones as believed by many worker. Further, the Moyar and Bhavani shear zones are neither as extensive nor as pervasive as envisaged. Veering of the ‘trends’ in southern Karnataka, northern Tamil Nadu and northern Kerala is an inherent feature of the superposed fold systems here. Isoclinal folds with axial planes of diverse attitudes, overprinted by upright folds of varying tightness, have resulted in this change in ‘trend’.

    • Zircon SHRIMP U–Pb geochronology, geochemical and Nd isotope systematics of Neoarchean granitoids, Gadag Greenstone Belt, Dharwar Craton, southern India: Petrogenesis and tectonic significance

      MONIKA SINGH S SARANGI R SRINIVASAN S BALAKRISHNAN V S HEGDE

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      Coupling of the geological processes in the arc-magmatic and back-arc provinces of accretionary orogens in Neoarchean plate tectonic setting is a subject of current research all over the world. The Dharwar Craton of southern India is an example of such an accretionary orogen, with an arc-magmatic province in the east and a back-arc province in the west, referred as the Eastern Dharwar Craton and the Western Dharwar Craton, respectively. The boundary between the two provinces is considered to be marked by a 400-km long shear zone along the eastern margin of the Gadag–Chitradurga–Karighatta greenstone belt which is called as the Chitradurga Boundary Shear Zone. Potassic, metaluminous, I type, calc-alkalic to alkali-calcic, arc-magmatic granitoids are widespread in the EDC. But they are also found to occur along the western margin of the Chitradurga–Gadag greenstone belt. SHRIMP U–Pb zircon ages of the granitoids in the western back-arc province in the Gadag region occurring near Srimant Gudda, Mulgund and Chabbi have been determined as 2565  $\pm$   24 to 2591  $\pm$   64 Ma old. Within errors, the ages of these granitoids are the same as the Lakundi and Turchihal granitoids occurring to the east of the Gadag Greenstone Belt in the arc-magmatic province. Nd isotope systematics of the granitoids suggest that they were formed from magmatic melts that were produced by remelting of 3200–3500 Ma old heterogenous continental crust. Rare inherited zircons support this antiquity of the protoliths. Occurrence of granitoids of similar age and origin, in the western back-arc province and eastern arc-magmatic province in the Gadag area was attributed to thrust duplex structure in the Gadag region. However, elsewhere, along the western margin of the Chitradurga greenstone belt near Harpanhalli, Hosdurga–Nagamangala–Pandavapura sector, or away from it in the Arsikere–Banavara, where repetition by thrusting is not obvious, late potash arc-magmatic type granitoids of similar age as the Gadag arc-magmatic granitoids are observed. The arc-magmatic type granitoids appear to have overstepped the boundary shear into the back-arc province at several places. Arc-magmatic and back-arc boundary may be diffuse rather than sharp, as also suggested by some earlier workers.

      $\bf{Highlights}$

      $\bullet$ The ages and petrogenesis of the granitoids around GGB of both the sides of EDC (i.e., eastern magmatic-arc province) and WDC (i.e., western back-arc provinces) are very much similar.

      $\bullet$ The geochemistry and isotope systematics (i.e., Nd TDM2 ages and εNdT values at 2.5–2.6 Ga) of the granitoids exposed one both the sides are also similar.

      $\bullet$ This shows not only a similar antiquity of both the provinces, but also gives evidence for a possible diffusive nature of boundary between the EDC and the WDC around GGB.

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