Geology and geochemistry of metabasalts of Shimoga schist belt, Dharwar Craton: implications for the late Archean basin development

Abstract

The Late Archaean Shimoga schist belt in the Western Dharwar Craton, with its huge dimensions and varied lithological associations of different age groups, is an ideal terrane to study Archean crustal evolution. The rock types in this belt are divided into Bababudhan Group and Chitradurga Group. The Bababudhan Group is dominated by mafic volcanic rocks followed by shallow marine sedimentary rocks while the Chitradurga Group is dominated by greywackes, pillowed basalts and deep marine sedimentary rocks with occasional felsic volcanics. The Nb/Th and Nb/La ratios of the studied metabasalts of the Bababudhan Group indicate crustal contamination. They were extruded onto the vast Peninsular Gneisses through the rifting of the basement gneiss.The Nb/Yb ratios of high magnesium basalts and tholeiitic basalts of Chitradurga Group suggest the enrichment of their source magma. Based on the flat primitive mantle normalized multi element plot with negative Nb anomalies and Th/Ta-La/Yb ratios, the high magnesium basalts and tholeiitic basalts are considered to have erupted in an oceanic-plateau setting with minor crustal contamination. The high magnesium basalts and tholeiitic basalts formed two different pulses of same magma type, in which first pulse of magma gave rise to high magnesium basalts which were derived from deep mantle sources and underwent minor crustal contamination enroute to the surface. While the second pulse of magma which gave rise to tholeiitic basalts formed at similar depths to that of high magnesium basalts and escaped crustal contamination. The associated lithological units found with the studied metavolcanic rock types of Bababudan and Chitradurga Group of Dharwar Supergroup of rocks in Shimoga schist belt of Western Dharwar Craton confirms the mixed-mode basin development with a transition from shallow marine to deep marine settings.