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 is a volumetrically minor but 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 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 by 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.

Provenance of the conglomerate and siliciclastic rocks from the Gadag Greenstone Belt, Western Dharwar Craton, India: Implications for understanding Neoarchean basin margin sedimentation

“… Note : Biotite‐bearing Granite (BG): Moyen et al (2003), Tonalite‐trondhjemite‐granodiorite (TTG): Ram Mohan et al (2019), Ingaldhal Felsic volcanic (IFV): Hedge et al (2021), Bababudan Group mafic volcanic (BMV), Ingaldhal Formation mafic volcanic (IMV): Kamalakannan (2014), Sargur Quartzite (SQ): Bhushan and Sahoo (2010), Banded iron formation (BIF): from unpublished average clast composition. …”

“…The Gadag-CGB developed in the western continental foreland basin adjacent to an accretionary arc forming to its east (Chadwick et al, 2003(Chadwick et al, , 2007Jayananda et al, 2000;Krapež et al, 2020). The major lithotypes in the GGB include greywacke, polymictic conglomerate, interbedded sandstone-siltstone, argillites, BIF, mafic, intermediate, and felsic volcanic rocks with compositions akin to active continental margin to island arc settings (Hedge et al, 2021;Jayananda et al, 2013;Ram Mohan et al, 2008;Sarma et al, 2012;Ugarkar et al, 2000). All these rocks were intruded by granitic magma of purported subduction origin (Ram Mohan et al, 2014;Singh et al, 2021).…”

“…Graded bedding in the associated greywacke indicates an unstable turbidite-like depositional setting (also see Chakrabarthi et al, 1993;Pratihari et al, 2021;Ugarkar et al, 2000Ugarkar et al, , 2015. Ugarkar and Nyamati (2002), based on the studies on the Attikatti greywackes, inferred active continental margin sedimentation The conglomerate consists of fragments of felsic and andesitic volcanic rocks that are inferred to come from an active continental margin and island arc setting (Hedge et al, 2021;Ram Mohan et al, 2008;Ugarkar et al, 2000).…”

“…Ugarkar and Nyamati (2002) inferred an active continental margin setting for the deposition of the greywacke units in the GGB. Based on geochemical studies of associated mafic volcanic rocks of the GGB, Ram Mohan et al (2008) suggested an oceanic island arc setting, while Hedge et al (2021) argued for a continental island arc setting based on geochemical data from associated felsic volcanic rocks. In terms of provenance, Naqvi et al (1988) and Chadwick et al (2003) proposed a mixed input from the Dharwar batholiths, older gneisses, volcano-sedimentary rocks, as well as younger granitoids.…”

“…Biotite-bearing Granite (BG):Moyen et al (2003), Tonalite-trondhjemite-granodiorite (TTG): RamMohan et al (2019), Ingaldhal Felsic volcanic (IFV):Hedge et al (2021), Bababudan Group mafic volcanic (BMV), Ingaldhal Formation mafic volcanic (IMV):Kamalakannan (2014), Sargur Quartzite (SQ):Bhushan and Sahoo (2010), Banded iron formation (BIF): from unpublished average clast composition.steep slope with rapid movement downwards, triggered by an earthquake or massive flood events(Shanmugam, 2016). Although rounded clasts without any definitely identifiable indication of glacial transport like striations, suggest an aqueous medium of transportation, rare presence of faceted surfaces and dropstone-like features in the conglomerate, does not preclude glacial transport and deposition.…”