Petrogenesis of Neoarchean granitoids beneath the Koyna-Warna region, Deccan Volcanic Province, India

Abstract

• A new find of ca. 2.5 Ga granitoids in the western Dharwar craton. • These I-type clac-alkaline granitoids are bimodal in composition. • Trace element patterns are identical to Archean TTGs. • Precursor magmatic sources evolved in a Neoarchean subduction zone. The continental crust that is concealed beneath the voluminous flood basalt provinces of the Deccan Traps (India) is inaccessible except by few deep bore-holes. The well-core extracted from the deep borehole KBH-8 (> 1000 m) penetrating the massive flood basalt cover provides an exclusive opportunity to examine the Precambrian continental crust in this part of the Indian shield. Here, we present detailed petrology, geochemistry and zircon U-Pb geochronology of the KBH-8 granitoids, which are granodiorite and monzogranites by their normative composition showing a magnesian calc-alkaline metaluminous I-type geochemical affinity. The granitoids are categorically identical to low-and high-HREE TTGs. Their fractionated REE patterns exhibit positive Eu anomaly. Besides the negative Nb and Ti anomalies, they exhibit positive Sr and Zr anomalies. Such attributes are atypical of the granitoids formed from a reworked older (> 3.0 Ga) TTG crust. The geochemistry of these granitoids cannot be reconciled with assimilation and fractional crystallization of the felsic crust in the Dharwar craton. Alternatively, the Ba+Sr enrichments in these granitoids are ascribed to melting of a slab fluid/melt metasomatized mantle wedge. Further, the Sr-Y and La-Yb systematics are consistent with melting of a low-Mg amphibolite source with garnet in the restite. In addition to the above, the variable Gd N /Yb N =1-2.3 ratios and Nb-Zr systematics suggest that the melts were produced from variable depths involving both depleted and undepleted segments of the sub-arc mantle. Consequently, we propose that the KBH-8 granitoids were sourced from a heterogeneous thickened arc-like crust in a Neoarchean subduction zone.