Petrology of Gneisses and the Associated Granites from Devanahalli Area, Karnataka, Southern India

Abstract

Granites and gneisses are the principal lithological components of the Precambrian crystalline terrain in Devanahalli Taluk, Bangalore Rural District, southern India, within the eastern segment of the Dharwar Craton. This study synthesizes field observations, petrographic analysis, and whole-rock geochemical data to delineate the characteristics, genesis, and tectonic relevance of these rocks. Field investigations reveal that the gneisses typically manifest as well-banded to streaky units exhibiting pronounced foliation, and are locally intruded by younger granitic bodies, pegmatites, and quartz veins. Petrographic examination identifies mineral assemblages primarily composed of quartz, plagioclase, K-feldspar, biotite, and hornblende, accompanied by accessory phases such as zircon, apatite, and opaque minerals. Major element geochemistry indicates that the granites are silica-enriched and range from peraluminous to weakly metaluminous, while the gneisses show tonalitic to granodioritic compositions. Trace and rare earth elements (REEs) highlight REE enrichment and negative Eu anomalies, consistent with plagioclase fractionation and crustal involvement. Integrated petrographic and geochemical evidence suggests that these granites and gneisses resulted from Archaean crustal evolution processes, including partial melting of older TTG-type crust followed by deformation and metamorphism. The record preserved in these rocks is indicative of significant crust-forming events associated with the stabilization of the Dharwar Craton.