Structural and Metamorphic Relatioms Between Sargur and Dharwar Supracrustal Rocks and Peninsular Gneiss in Central Karnataka

Abstract

The Dharwar Supergroup and its basement of Peninsular Gneiss and Sargur supracrustal rocks in the areas of Ghatti Hosahalli and southeast Bababudan display certain textural, structural and unconformable relations which have important implications for the Archaean chronology of the Karnataka craton. In the first. instance these relations show that certain tonalitic-granitic parental rocks of the Peninsular Gneiss basement to the Dharwar supracrustal rocks were formed as a series beginning with polyphase gneisses and ending with discordant plutons such as the Chikmagalur granite s.l. The Sargur rocks were deformed and metamorphosed to medium-high grade during intrusion of the polyphase gneisses. After cooling, uplift and erosion of the Peninsular Gneiss and the tracts and enclaves of Sargur rocks, the Dharwar supracrustal association was deposited unconformably on the medium-high grade basement. The pre-Dharwar metamorphic minerals in the Sargur rocks were partly retrogressed and then overprinted by a second major metamorphism, mainly low grade, whose climax was attained after the main deformation of the belts and basins of the Dharwar supracrustal rocks. This major low grade metamorphism in central Karnataka is correlated with the later Archaean high grade terrane (ca. 2600 Ma) in southern Karnataka and elsewhere in Peninsular India.