Archean tectonics and crustal evolution around Shimoga greenstone basin western Dharwar craton constraints from geology geochronology and geochemistry

Abstract

This thesis explores the tectonic evolution and crust-mantle interactions of the Shimoga newlinegreenstone belt and its adjoining basement within the western Dharwar craton (WDC), southern newlineIndia. Through a comprehensive multidisciplinary approach, encompassing field observations, newlinepetrography, geochronology, and geochemistry, the research provides new insights into newlineNeoarchean crustal growth, mantle processes, transient tectonics, and evolving surface newlineenvironments. The study focuses on understanding the formation of habitable continental crust newlineduring the Archean by examining lithological assemblages, structural relationships, magmatic newlinehistories, and sedimentary records. The Shimoga region exposes three key litho-tectonic units: newline(1) TTG-type granitoid basement gneisses forming the Honnali dome, (2) ultramafic rocks and newlinepolymictic Kaldurga conglomerates, and (3) bimodal volcano-sedimentary sequences newlinecomprising mafic and felsic volcanics interbedded with cherts, greywackes, argillites, newlinecarbonaceous shales, and banded iron formations (BIFs). Field evidence suggests a tectonic newlinetransition from a rift setting, marked by ultramafic emplacement and deposition of the newlineconglomerates (~2700 Ma), to a continental arc system, as seen in the arc-related features of newlinethe greenstone volcanics. Petrographic analyses indicate greenschist to lower amphibolite newlinefacies metamorphism across all units. TTG gneisses are largely trondhjemitic, with newlinegeochemical signatures indicating their origin from hydrous melting of thickened arc crust. newlineClasts of the Kaldurga conglomerate share similar petrographical and geochemical newlinecharacteristics with the granitoid basement, suggesting derivation from composite crustal newlinesources. Ultramafic rocks show depleted REE and negative Nb-Ti anomalies, consistent with newlinederivation from a heterogeneous but dominantly depleted mantle source. Geochronologic data, newlineincluding new U Pb zircon and Sm Nd isochron ages, constrain the formation of the TTG newlinebasement between 3431 3163 Ma, deposition of the Kaldurga conglomerate around 2700