EARLY ARCHEAN CRUST IN BASTAR CRATON, CENTRAL INDIA - A GEOCHEMICAL AND ISOTOPIC STUDY

High-Al2O3 trondhjemitic gneisses occur in enclaves within granites at Markampara in the Bastar Craton, Central India. These gneisses exhibi t a highly fractionated rare earth element (REE) pattern, [(La/Yb)n = 60-83 ] and may have been produced by partial melting of an amphibolit ic protolith. A zircon U-Pb age of 3 509 -7(+14) Ma dates primary crys tallization of the trondhjemites. A granite surrounding the gneissic e nclaves yields a zircon U-Pb age of 2480 +/- 3 Ma. Rb-Sr whole-rock da ta for the trondhjemitic gneisses are somewhat disturbed and indicate initial Sr-87/Sr-86 ratios of approximately 0.7016. Pb isotopic data a re poorly correlated and suggest a three-stage evolution with an early period of high U-Pb growth (mu almost-equal-to 9.5 - 10.5) after empl acement at 3.5 Ga, followed by isotopic disturbance and U depletion at 2.5 Ga during widespread magmatism and metamorphism. Loss of Rb and o ther mobile elements ma also have occurred at this time.