FORMATION AND TECTONIC EVOLUTION OF GRANULITES FROM THE BILIGIRI RANGAN AND NILIGIRI HILLS, S-INDIA - GEOCHEMICAL AND ISOTOPIC CONSTRAINTS

The granulites of the Biligiri Rangan (BR) and Niligiri Hill ranges ar e characterised by distinct field, geochemical and isotopic signatures . The B.R. Hill granulites contain a significant metasedimentary compo nent (pelite-BIF-Mn-horizons), whilst metasediments are rare in the Ni ligiri granulite terrain. Lithologic and structural continuity between the two terrains cannot be demonstrated as they are separated by E-W running Moyar shear zone. Isotopic data suggest that B.R. Hills and Ni ligiri granulites have had different pre-crustal histories. Sm-Nd mode l TDM ages and U-Pb zircon concordia upper intercept age suggest that the protoliths of B.R. Hill quartzo-feldspathic charnockites accreted at 3.4 Ga. On the other hand, Sm-Nd model TDM ages indicate that the m agmatic protoliths of Niligiri granulites differentiated from mantle a t 2.6 Ga. Geochemical data from the B.R. Hills charnockites, particula rly low-Mg numbers (0.2-0.3), low content of transition and HFS elemen ts and strongly fractionated REE patterns with HREE depletion, suggest that their magmatic protoliths were derived by parti al melting of yo ung, hydrated oceanic crust, in which garnet and amphibole were import ant residual phases. The high content of transition elements together with low total LREE and less-or unfractionated REE patterns of the B.R . Hills mafic granulites imply the derivation of their protoliths from long term depleted mantle sources. On the other hand the hi gh Mg num bers (0.5-0.6), high content of transition and HFS elements, moderatel y fractionated REE patterns with HREE enrichment suggest that the prot oliths of the Niligiri granulites are interpreted to have formed by me lting of metasomatized mantle wedge with olivine and spinel as residua l phases. The Niligiri terrain was accreted onto the Dharwar Craton at C.2.6 Ga and the two terrains witnessed a common major granulite even t at 2.51 Ga.