Genesis of coronae and implications of an early Neoproterozoic thermal event: a case study from SE Chotanagpur Granite Gneissic Complex, India

Abstract

AbstractPartial equilibrium textures such as corona provide information on changing pressure–temperature (P-T) conditions experienced by a rock during its geological evolution. Coronae layers may form in single or multiple stages; understanding the genesis of each layer is necessary to correctly extract information regarding the physicochemical conditions experienced by the rock. Mafic rocks from SE Chotanagpur Granite Gneissic Complex, India, show the presence of multi-layered coronae at olivine–plagioclase contact with the mineral sequence: olivine | orthopyroxene | amphibole + spinel | plagioclase. Textural studies indicate that the coronae formed during metamorphism in a single stage due to a reaction between olivine and plagioclase. Reaction modelling shows that the corona formation occurred in an open system and experienced a minor volume loss. Pseudosection modelling and thermobarometry suggest that the P-T conditions related to corona formation are 860 ± 50°C and 7 ± 0.5 kbar. A μMgO-μCaO diagram shows that the layers in coronae formed in response to chemical potential gradients between the reactant minerals. A combination of field observations and the P-T conditions of coronae formation suggest a fluid-driven metamorphism. Correlation with extant geological information indicates that the corona-forming event is possibly related to the accretion of India and Antarctica during the assembly of Rodinia.