SHRIMP U-Pb ages of zircons from mafic granulites of the Eastern Ghats Belt, SE India: Implications for the evolution of the palaeoproterozoic arc crust

Abstract

We determined SHRIMP U-Pb ages of zircons from three mafic granulite samples of the polymetamorphosed, multiply-deformed Kondapalli-Ongole domain of the Eastern Ghats Belt (EGB), SE India. Our goals are: (1) to evaluate the origin of zircons in the high-grade mafic granulites; and (2) to clarify the petrotectonic evolution of magmatic and metamorphic rocks along the SE margin of India. The zircons record three distinct Paleoproterozoic U-Pb age groups: igneous protoliths of the mafic granulites apparently crystallized at ∼1715 Ma, and underwent granulite-facies high-temperature metamorphism at ∼1676 Ma, with a subsequent fabric-defining garnet-granulite grade metamorphic overprint at ∼1610 Ma. Based on textures, cathodoluminescence images, and geochemistry, we identified three zircon types among the ∼1610 Ma zircons: Type I and II zircon grains recrystallized or newly grown in equilibrium with feldspar and garnet, respectively; Type III zircons crystallized from aqueous anatectic melt in quartzofeldspathic veins. Some zircons of all the three types show alteration effects. The ∼1715 Ma event likely represents formation of the Kondapalli-Ongole magmatic arc crust, whereas the ∼1610 Ma high-grade metamorphic event reflects suturing of the arc to proto-India and cratonization of the EGB. The interval between the formation of U-poor, low Th/Yb zircons (equilibrated with garnet) and U-rich, high Th/Yb zircons (equilibrated with fluid-rich melt) is geologically insignificant, suggesting that tectonic accretion of the EGB, its final suturing, and orogenic collapse took place rapidly.