Petrography, geochemistry and petrogenesis of calc-alkaline granitoids from the Mercara Shear Zone (MSZ), southwest of West Dharwar Craton, India: Indication of continental arc magmatism and deep-crustal melting

Abstract

The Mercara Shear Zone (MSZ) is an intensely deformed, curvilinear, mylonitised zone juxtaposed between the Western Dharwar Craton (WDC) and the Coorg Block (CB). In the vicinity of Madikeri town, the MSZ exposes many bands and enclaves of granulite-grade meta-supracrustals and mafic granulites hosted within charnockite or felsic orthogneiss (retrogressed charnockite). Subsequently, the area is intruded by a suite of granitoids along the mid-axis of the shear zone, showing no signature of metamorphism. Views on the origin and timing of the formation of the granulite-grade rocks of the MSZ are not streamlined, and the unmetamorphosed suite of granitoids has not been studied in detail in light of the evolution of the terrain. The field relationship, petrography and bulk rock geochemistry of the suite of unmetamorphosed granitoids from the MSZ were carried out to address this issue. Petrological and geochemical data indicate an I-type affinity of the granitoids. Trace and rare earth element (REE) patterns suggest a magmatic arc setting. The rocks are calc-alkaline, and the REE pattern is fractionated, enriched in Ba, Sr, and depleted in high field strength elements like Nb, Ta, and Ti with almost no significant negative Eu anomaly. These characteristics indicate the dominance of crustal involvement over the mantle in the generation of melt. Studied samples show ‘adakite-like’ geochemical characteristics (high Sr/Y, La/Yb) but are not the product of typical slab melting. The present study indicates that the granitoids were derived from a thick crustal source by partial melting in an oxidized condition (fO₂ between NNO and HM buffer) at 941–985°C and 10–12.5 kbar pressure (corresponding to 30–40 km depth).