LATE ARCHEAN CRUST-MANTLE INTERACTIONS - GEOCHEMISTRY OF LREE-ENRICHED MANTLE DERIVED MAGMAS - EXAMPLE OF THE CLOSEPET BATHOLITH, SOUTHERN INDIA

The Closepet batholith in South India is generally considered as a typical crustal granite emplaced 2.5 Ga ago and derived through partial melting of the surrounding Peninsular Gneisses (3.3 to 3.0 Ga). In the field, it appears as a composite batholith made up of at least two groups of intrusions. (a) An early SiO2-poor group (clinopyroxene quartz-monzonite and porphyritic monzogranite) is located in the central part of the batholith. These rocks display a narrow range in both initial Sr-87/Sr-86 (0.7017-0.7035) and epsilon(Nd) (-0.9 to -4.1). (b) A later SiO,-rich group (equigranular grey and pink granites) is located along the interface between the SiO2-poor group and the Peninsular Gneisses. They progressively grade into migmatised Peninsular Gneisses, thus indicating their anatectic derivation. Their isotopic characteristics vary over a wide range (Sr-87/Sr-86 ratios = 0.7028-0.7336 and epsilon(Nd) values from -2.7 to -8.3, at 2.52 Ga). Field and geochronological evidence shows that the two groups are broadly contemporaneous (2.518-2.513 Ga) and mechanically mixed. This observation is supported by the chemical data that display well defined mixing trends in the epsilon(Sr) VS epsilon(Nd) and elemental variation diagrams. The continuous chemical variation of the two magmatic bodies is interpreted in terms of interaction and mixing of two unrelated end-members derived from different source regions (enriched peridotitic mantle and Peninsular Gneisses). It is proposed that the intrusion of mantle-derived magmas into mid-crustal levels occurred along a transcurrent shear zone; these magmas supplied additional heat and fluids that initiated anatexis of the surrounding crust. During this event, large-scale mixing occurred between mantle and crustal melts, thus generating the composite Closepet batholith. The mantle-derived magmatism is clearly associated with granulite facies metamorphism 2.51 +/- 0.01 Ga ago. Both are interpreted as resulting from a major crustal accretion event, possibly related to mantle plume activity.