Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

B. Mahabaleswar ¹, J. J. Peucat ²

Affiliations
1 Department of Geology, Bangalore University, Bangalore-560056, IN
2 Centre Armoricain d'Etudes Structurales des Socles, CNRS, Laboratoire de geochronologie et geochimie isotopique, 35042 Rennes, FR

Abstract

Preliminary Rh-Sr isotopic studies of the granulites occurring SW of Kabbaldurga were undertaken to verify the possibility of an earlier granulite facies event. The Rb-Sr data obtained gives an imperfect isochron age of 2950 ± 234 m.y. with an initial ratio of 0.7032 ± 22 (MSWD = 52). The age obtained may correspond to subconcomitant protolith-metamorphism age or metamorphism alone. The high error may be due to non-cogenetic nature of the rocks analysed or a small disturbance of the Rb-Sr system during the 2500 m.y. high-grade event. The present study indicates probability of the occurrence of an early granulite facies event prior to 2500 m.y.

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B. M. Reddy ¹, A. S. Janardhanan ¹, J. J. Peucat ²

Affiliations
1 Department of Studies and Research in Geology, University of Mysore, Manasagangothri, Mysore - 570 006, IN
2 Institute of Geology, University of Rennes, 35042 Rennes Cedex, FR

Abstract

Alkaline rocks spatially associated with magnesite bearing dunites occur in the vicinity of Salem, Tamil Nadu. This association occurs with in the gneiss-charnockite terrain and is confined to the intersection of two deep seated lineaments. Based on mineralogy the alkaline rocks are recognised as missourite, shonkinite and syenite.

The alkaline rocks have high Mg numbers and are enriched both in compatible and incompatible trace elements. Majority of the samples show ultrapotassic character and are enriched in REE (30-200 ppm) with LREE enrichment and no Euanarnoly. The Rb-Sr isochron gives an age of 808 ± 18 Ma and shows fairly high Sr initial ratio of 0.7051 ± 1. The unusual chemistry of these rocks suggests that they are derived, from an enriched mantle source, with the dunite representing the mantle residua.

Keywords

Geochemistry, Geochronology, Alkaline Rocks, Ultramafic Rocks, Salem, Tamil Nadu.

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B. Mahabaleswar ¹, M. Jayanand ¹, J. J. Peucat ², N. Shadakshasra Swamy ¹

Affiliations
1 Department of Geology, Bangalore University, Bangalore - 560 056, IN
2 Laboratoire de Geochimie Isotopique et Geochronologie, Geosciences Rennes-CNRS, 35042 Rennes Cedex, FR

Abstract

The high-grade gneiss complex of the Satnur-Halagur-Sivasamudram area in southern Karnataka forms a part of amphibolite-granulite facies transition zone of southern India. The major lithologies are metasediments (quartzites-pelites-BIF-Mn-horizons), amphibolite facies gneisses, foliated charnockites, mafic granulites, granite sheets (Closepet-type). All these lithologies show a prominent N-S fabric which appears to have been produced during late Archaean shear deformation. Rb-Srwhole rock isochron and U-Pb zircon and monazite ages suggest that much of the crust accreted during 2.96 Ga magmatic event followed by the 2.5 Ga granulite facies metamorphism; the occurrence of a previous high-grade metamorphism close to 2.9 Ga remains debatable.

Both gneisses and foliated charnockites show similar chemical characteristics except in LIL elements, which appear to have been disturbed during granulite metamorphism. Low Mg# (0.18-0.27), low content of HFS elements (Ti, Nb, Ta, Hf and Y) and strongly fractionated REE patterns with HREE depletion suggest that magmatic history of the gneisses and charnockites are similar to the classical Archaean trondhjemite, tonalite, granodiorite suites (TTG).

The geodynamic evolution of the Satnur-Halagur-Sivasamudram area implies a change in the thermal structure and melting conditions of lithosphere from 3.0 to 2.5 Ga. Following the model of Martin (1987), the magmatic protoliths of gneisses and charnockites could have been generated in an accretion zone (subduction-mantle plume ?)2.9 Ga ago, where young oceanic crust melted before dehydration. Granulite facies metamorphism occurred during latest Archaean: synchronous mantle derived melts tapped into mid-crustal levels along a major shear-zone where they induced migmatization (Closepet-type batholith; Jayananda et al. 1994) immediately followed by granulite metamorphism, related to a possible 2.5 Ga old mega-plume (Peucat et al. 1993b).

Keywords

Petrology, Granulites, Archaean, Southern Karnataka.

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