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Zircon evaporation ages and geochemistry of metamorphosed volcanic rocks from the Vinjamuru domain, Krishna Province: Evidence for 1.78Ga convergent tectonics along the southeastern margin of the Eastern Dharwar Craton

Abstract:

Palaeoproterozoic intermediate to potassic felsic volcanism in volcano-sedimentary sequences could either have occurred in continental rift or at convergent magmatic arc tectonic settings. The Vinjamuru domain of the Krishna Province in Andhra Pradesh, SE India, contains such felsic and intermediate metavolcanic rocks, whose geochemistry constrains their probable tectonic setting and which were dated by the zircon Pb evaporation method in order to constrain their time of formation. These rocks consist of interlayered quartz-garnet-biotite schist, quartz-hematite-baryte-sericite schist as well as cherty quartzite, and represent a calc-alkaline volcanic sequence of andesitic to rhyolitic rocks that underwent amphibolite-facies metamorphism at ~1.61Ga. Zircons from four felsic metavolcanic rock samples yielded youngest mean 207Pb/206Pb ages between 1771 and 1791Ma, whereas the youngest zircon age for a meta-andesite is 1868Ma. A ~2.43Ga zircon xenocryst reflects incorporation of Neoarchaean basement gneisses. Their calc-alkaline trends, higher LILE, enriched chondrite-normalized LREE pattern and negative Nb and Ti anomalies on primitive mantle-normalized diagrams, suggest formation in a continental magmatic arc tectonic setting. Whereas the intermediate rocks may have been derived from mantle-source parental arc magmas by fractionation and crustal contamination, the rhyolitic rocks had crustal parental magmas. The Vinjamuru Palaeoproterozoic volcanic eruption implies an event of convergent tectonism at the southeastern margin of the Eastern Dharwar Craton at ~1.78Ga forming one of the major crustal domains of the Krishna Province. © 2012 John Wiley & Sons, Ltd.