Magma mixing in plutonic environment: Geochemical and isotopic evidence from the Closepet batholith, southern India

Abstract

The Closepet Batholith in southern India contains two groups of magnetic intrusions: (i) mantle derived quartz monzonites and monzgranites (SiO₂-poor clinopyroxene bearing and porphyritic facies) display a narrow range of initial ⁸⁷Sr/⁸⁶Sr ratios (0.7017-0.7029 at 2.5 Ga) and ε Nd (-0.9 to -4.7 at 2.5 Ga). (ii) crustal-derived granite (SiO₂-rich equigranular grey and pink granites) show wide range of initial ⁸⁷Sr/⁸⁶Sr ratios (0.7028-0.7366 at 2.5 Ga) and ε Nd (-2.7 to -8.91 at 2.5 Ga). Field data and single zircon ²⁰⁷Pb/²⁰⁶Pb ages demonstrate that the two groups are broadly contemporaneous and mechanically mixed. This observation is supported by geochemical and isotopic data that show well-defined mixing trends in both Harker binary diagrams and I_Sr vs ε Nd plots. The continuous chemical variation in the two magmatic bodies is interpreted in terms of interaction and mixing of two unrelated end-members derived from different source regions (enriched mantle vs Peninsular gneisses). The proposed model involves intrusion 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.