Late Archaean (2550-2520 Ma) juvenile magmatism in the Eastern Dharwar craton, southern India: constraints from geochronology, Nd-Sr isotopes and whole rock geochemistry

The results of held, geochronologic, geochemical and isotopic studies are p resented for the granitoids that occur east of the Closepet batholith up to the Kolar schist belt (KSB). Field data, such as common foliation, strong shear deformation occasionally leading to mylonitization, together with pet rographic data, including reduction in grain size with corroded borders, sh ow characteristics of the syn-kinematic emplacement of the granitoids. Sing le zircon evaporation ages define a minimum age of 3127 Ma for the tonaliti c-trondhjemitic-granodioritic (TTG) basement and 2552-2534 Ma plateau ages for the emplacement of the granitoids, which slightly predate (20-30 Ma) th e emplacement of the 2518 Ma Closepet batholith. Major and trace element da ta, together with isotopic data, suggest at least four magmatic suites from Closepet batholith to the east, which have independent magmatic evolution histories. The observed data are compatible with magma mixing for the Close pet batholith, melting of TTG and assimilation-fractional crystallization p rocesses for Bangalore granites, either melting of heterogeneous source or different degree of melting of the same source for the granitoids of Hoskot e-Kolar and fractional crystallization for the western margin of the KSB. I sotopic (Nd-Sr) and geochemical data (LREE and LIL elements) suggest highly enriched mantle and ancient TTG crust for the Closepet batholith, enriched mantle and TTG crust for the Bangalore granites, c.a. chondritic mantle so urce for the granitoids of Hoskote-Kolar and the quartz monzonites of the w estern margin of the KSB and slightly depleted mantle for granodiorites of the eastern margin of the KSB. We interpret all these geochronologic, geoch emical and isotopic characteristics of granitoids from the Closepet batholi th to the east up to the KSB in terms of a plume model. The centre of the p lume would be an enriched 'hot spot' in the mantle that lies below the pres ent exposure level of the Closepet batholith. Melting of such an enriched m antle hot spot produces high temperature magmas (Closepet) that penetrate o verlying ancient crust, where they strongly interact and induce partial mel ting of the surrounding crust. These magmas cool very slowly, as the hot sp ot maintains high temperatures for a longs time; thus they appear younger ( 2518 Ma). On the contrary, to the east the plume induces melting of c.a. ch ondritic or slightly depleted mantle that produces relatively colder and le ss enriched magmas, which show less or no interactions with the surrounding s crust and cool rapidly and appear slightly older (2552-2534 Ma). This plu me model can also account for late Archaean geodynamic evolution, including juvenile magmatism, heat source for reworking, inverse diapirism and granu lite metamorphism in the Dharwar craton. (C) 2000 Elsevier Science B.V. All rights reserved.