The Kuunga Orogeny in the Eastern Ghats Belt: Evidence from geochronology of biotite, amphibole and rutile, and implications for the assembly of Gondwana

Abstract

The Eastern Ghats Belt, India, is a high grade polymetamorphic terrane; most of it was metamorphosed under high temperature to ultra-high temperature (UHT) conditions at 1000–900 Ma. This study presents new Rb–Sr biotite, 40Ar–39Ar amphibole and U–Pb rutile ages for 32 samples of HT and UHT lithologies from across the Eastern Ghats, Jaypore and Rengali Provinces. This dataset provides the first comprehensive evidence for a pervasive, region-wide amphibolite facies metamorphism at ca. 500 Ma throughout the Eastern Ghats Belt that overprinted the earlier (U)HT mineral assemblages. The lower-grade Cambrian overprint did not lead to retrogression of the high grade mineral assemblages but is recorded by different geochronometers. Rb–Sr biotite, 40Ar–39Ar amphibole and U–Pb rutile ages from the entire Eastern Ghats Belt cluster around ca. 500 Ma. U-Pb ages for zircon from six of the samples dated by Rb–Sr biotite and/or U–Pb rutile do not record this young event but yield ages that correspond to the earlier high-grade metamorphism. Combined with previously published geochronological information, the new ages show that a medium grade tectonothermal event during the Cambrian affected the whole Eastern Ghats Belt on a regional scale. This last regional metamorphism was essentially fluid-absent, as the parageneses originating from high-grade metamorphism at 1000–900 Ma are still pristine and do not show pervasive retrograde mineral reactions. This Cambrian metamorphic episode is also recorded as a high-T overprint in the high-grade gneisses of Sri Lanka, southern India and Madagascar and is the result of the assembly of Gondwana that led to the collision of Antarctica with India, which formed part of the Kuunga orogeny. The new data provide evidence for the extension of the Kuunga orogeny into eastern India and support it representing a major orogenic event in the assembly of Gondwana.