Contrasting petrogenesis of charnockites across the Kasargod–Mercara Shear Zone, southern India

This study presents an integrated petrographic and geochemical investigation of charnockites from the southern part of the Western Dharwar Craton (WDC), Coorg Block (CB), and the Kasargod–Mercara Shear Zone (KMSZ) separating the two crustal domains, with the aim of constraining their petrogenesis and tectonic setting. The charnockites from the three different domains display diverse textural and compositional characteristics reflecting their contrasting tectonomagmatic histories. Petrographically, WDC charnockites are foliated, medium-grained, and exhibit hornblende-biotite retrogression, while CB charnockites are coarse-grained and massive with limited retrograde alteration. KMSZ charnockites are strongly deformed, showing dynamic recrystallization textures. Geochemically, WDC charnockites are metaluminous, ferroan, and high-K with tholeiitic affinity, suggesting derivation from an evolved, crustally contaminated melt in a post-collisional setting. In contrast, CB charnockites are low-K, calcic, and magnesian with metaluminous compositions, suggestive of mantle-derived magmas generated in a subduction-related arc environment. The KMSZ charnockites are peraluminous to metaluminous, calc-alkaline, and magnesian, pointing to a mixed crustal-mantle source influenced by shear zone-related deformation and crustal assimilation in a convergent margin or syn-collisional regime. These findings reveal complex magma generation processes and tectonic evolution along the accretion window between the WDC and CB.