Sanidinite facies metamorphism at Nagarahal Village of Bilgi taluk, Bagalkot district, Karnataka, India

Abstract

Nagarahal village in the Bilgi taluk of Bagalkot district in Karnataka state shows the contact between the younger Bilgi granites and the Deccan basalts. The thermal impact that has happened on the pre-existing granites by the basalts has been studied by identifying the field occurrence of the contact zone of granites and basalts. The unaffected portion of the granites and the basalts that are far away from the contact are studied as well. The petrographic studies of the contact rocks between the granites and basalts show the polygonal recrystallized quartz with the triple point contact with other grains indicating the decussate texture indicating the high-temperature conditions, crushed quartz grains having contact with each grain appear as sugary crystals from the granoblastic texture, the irregular oriented mafic and felsic crystals show the hornfelsic texture and micrographic, and intergrowth textures showing the effect of thermal metamorphism near to the contact between the granite and basalt. The post-hydrothermal solution activity is at the peripheral region of the contact between granite and basalt which is seen by the formation of tourmaline. The mineral assemblages that are identified by the petrographic studies are muscovite, quartz, orthoclase, sanidine, sillimanite, chlorite, and Fe-Ti oxide minerals. The mineral chemistry data shows the high potash K-feldspar that consists of Or from 84% to 97.97%, Ab2%- 4.26%, and An0.09%-1.34% was identified as sanidine variety of K-feldspar, the plagioclase feldspar shows the dominant composition of Ab ranging from 84.92% to 97.10%, An content in plagioclase was found to be ranging from 0.87% to 14.02%, and the Or content is ranging from 1.06%-7.41%. indicating the presence of the albite variety of plagioclase, the Fe2+ dominating chlorite, and the presence of tourmaline indicates the hydrothermal effects at the periphery of the contact. The two feldspar thermometry indicates the formation temperature was 750°C at the contact and the chlorite formation temperature calculated is found to be 313°C which shows a decrease in the temperature away from the contact of the basalt and the granitic rocks. Whereas the granite and the basalts that are far away from the contact do not show any changes. The mineral assemblages, mineral chemistry data, and thermometric results indicate the high temperature and low-pressure sanidine facies of contact metamorphism in the studied area.