Neoarchaean Felsic Volcanic Rocks in Tracing Evolution of Arcs: An Insight from Geochemical Data of the Gadag Schist Belt, Western Dharwar Craton

Abstract

Quartz porphyry of rhyolitic composition though a volumetrically minor is an important component of metavolcanic successions in the Gadag schist belt in the western Dharwar craton. The quartz porphyries are calc-alkaline, enriched in light rare earth elements (Lan/Ybn ~ 10–20) and with a strong negative Eu anomaly (Eu/Eu*N=0.45–0.58). These features, coupled with high K2O/Na2O (0.8 to 1.46), low Nb/Th (<1), and Lan/Smn ratios of 4.2 to 6.0, suggest that rhyolitic magma formed from melt generated by partial melting of basaltic crust, subsequently modified by crustal assimilation and crystal fractionation. The geological setting of the quartz porphyries and the geochemical relationships suggest magmatism at a convergent plate tectonic setting. The zircon U-Pb age of 2588.0 ± 2.4 Ma for the quartz porphyry, together with an age of 2732 Ma for similar volcanic rocks in the Chitradurga schist belt further south, and 2614 Ma for felsic volcanics from the Shimoga schist belt, point to an extended period with episodic arc magmatism in the western Dharwar craton of at least ~144myr.