MAGMA MIXING IN PLUTONIC ENVIRONMENT - GEOCHEMICAL AND ISOTOPIC EVIDENCE FROM THE CLOSEPET-BATHOLITH, SOUTHERN INDIA

The Closepet Batholith in southern India contains two groups of magmat ic intrusions: (i) mantle-derived quartz monzonites and monzogranites (SiO2-poor clinopyroxene bearing and porphyritic facies) display a nar row range of initial Sr-87/Sr-86 ratios (0.7017-0.7029 at 2.5 Ga) and epsilon Nd (-0.9 to -4.7 at 2.5 Ga). (ii) crustal-derived granites (Si O2-rich equigranular grey and pink granites) show wide range of initia l Sr-87/Sr-86 ratios (0.7028-0.7366 at 2.5 Ga) and E Nd (-2.7 to -8.91 at 2.5 Ga). Field data and single zircon Pb-207/Pb-206 ages demonstra te that the two groups are broadly contemporanous and mechanically mix ed. This observation is supported by geochemical and isotopic data tha t show well-defined mixing trends in both Harker binary diagrams and I -Sr vs epsilon Nd plots. The continuous chemical variation in the two magmatic bodies is interpreted in terms of interaction and mixing of t wo unrelated end-members derived from different source regions (enrich ed mantle vs Peninsular gneisses). The proposed model involves intrusi on of mantle-derived magmas into anatectic zone in the mid-continental crust: where they supply additional heat and fluids and promote large -scale melting of surrounding crust. During this event occurred mixing between mantle derived magma and anatectic melts.