Petrogenesis and Tectonomagmatic Development of Palaeoproterozoic Hornblende Granite Gneiss from Madurai Block, Southern India: Insights from Phase Petrology, Zircon Typology and U-Pb SHRIMP Dating

ABSTRACT

The Madurai Block is the most extensive continental block within the Southern Granulite Terrain (SGT) of the southern Indian peninsula. It witnessed multiple magmatic and tectonothermal events during its geological history. We provide zircon U-Pb SHRIMP dating, typology, and phase petrology data for a Hornblende Granite Gneiss (HGGn) from the northwestern region of the Madurai Block to investigate the timing, origin, and tectonomagmatic evolution of the Palaeoproterozoic basement granite gneiss. The HGGn contains an assemblage of hbl-pl-Kf-qz-ap-zrn±mag, and modally represents tonalite. Geochemically, the HGGn is classified as a metaluminous (molar Al2O3/CaO+Na2O+K2O = 0.83), calc-alkaline, ilmenite-series (reduced) granite, formed through the melting of mafic crust in a subduction environment. The amphibole compositions suggest their crystallisation at ca. 5-7 kbar and 800-900°C, and low oxygen fugacity ( f O2) in a calc-alkaline host magma (with 3-4 wt.% H2O) likely originated from a mantle-derived source rock. A relatively narrow range of plagioclase compositions (An24-An27; Oligoclase) demonstrates a high-silica nature of the host magma, which experienced subsolidus fluid-driven re-equilibration during later tectono-metamorphic events. The primary zircon subtypes (S25, S19, S15, P4, and P5) in the HGGn strongly align with zircon formed in a calc-alkaline felsic magma with metaluminous (I-type) character, originating from the melting of the lower crust. High-precision zircon U-Pb SHRIMP isotope analysis from the HGGn yields an upper intercept at 1781±17 Ma and 206Pb/207Pb weighted mean age of 1783±15 Ma (MSWD=0.072; n=32), indicating the timing of zircon crystallisation or HGGn magma emplacement during the Columbia Supercontinent formation. However, the lower intercept at 802±21 Ma, with 206Pb/207Pb weighted mean age of 790±30 Ma (MSWD=0.33; n=6), reflects a subsequent tectono-thermal event affecting the HGGn zircon rims, linked with the breakup of the Rodinia supercontinent.