The petrochemistry and geologic implications of conglomerates from Archaean geosynclinal piles of southern India

Abstract

Conglomerates are abundant in the 2.6 Ga old geosynclinal piles of southern Peninsular India and some also occur in the 3.4–3.2 Ga old greenstone belts. The conglomerates in the greenstone belts are autoclastic or pyroclastic, in contrast to the conglomerates in the geosynclinal piles, which generally contain graywackes indicative of sedimentation by turbidity currents.Analyses of 104 pebbles and 28 matrices from the 2.6 Ga-old conglomerates and petrological data show that their source areas were made up of basic–ultrabasic volcanics, trondhjemitic–tonalitic acid plutonics, and phyllite–amphibolite–quartzite–chert–ironstone metasediments. The variations in the composition of the trondhjemite–tonalite pebbles from the Shimoga, Bababudan, Chitradurga, and Gadag geosynclinal piles indicate the significant overall low abundance and differences in the availability of free plutonic quartz for sedimentation. The abundance of Na₂O in the matrices and associated graywackes may be due to the high Na₂O content of detrital plagioclase fragments derived from trondhjemites and tonalites. The Na₂O contents of Archaean graywackes do not appear to be a result of post-depositional albitization of plagioclase. The scarcity of K-rich pebbles, and the K-depleted nature of most of the matrices, indicate that the earlier acid plutonic rocks associated with the true greenstone belts were mainly trondhjemitic–tonalitic in character, and K-rich acid plutonism was largely a post-geosynclinal development. The plutonic source terranes of the geosynclinal piles are similar in some aspects to those of much younger geosynclines throughout the world.