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 typ ical crustal granite emplaced 2.5 Ga ago and derived through partial m elting 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 gro ups 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 late r SiO,-rich group (equigranular grey and pink granites) is located alo ng the interface between the SiO2-poor group and the Peninsular Gneiss es. 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 epsilo n(Nd) values from -2.7 to -8.3, at 2.52 Ga). Field and geochronologica l evidence shows that the two groups are broadly contemporaneous (2.51 8-2.513 Ga) and mechanically mixed. This observation is supported by t he chemical data that display well defined mixing trends in the epsilo n(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 di fferent source regions (enriched peridotitic mantle and Peninsular Gne isses). It is proposed that the intrusion of mantle-derived magmas int o mid-crustal levels occurred along a transcurrent shear zone; these m agmas supplied additional heat and fluids that initiated anatexis of t he surrounding crust. During this event, large-scale mixing occurred b etween mantle and crustal melts, thus generating the composite Closepe t batholith. The mantle-derived magmatism is clearly associated with g ranulite facies metamorphism 2.51 +/- 0.01 Ga ago. Both are interprete d as resulting from a major crustal accretion event, possibly related to mantle plume activity.