Crystallization Evolution of Accessory Minerals in Palaeoproterozoic Granites of Bastar Craton, India

The Malanjkhand and Dongargarh granitoids are compared to study the role of accessory minerals (apatite, zircon, titanite and titanomagnetite) to understand crystallization evolution of Palaeoproterozoic granitic magmatism in the Bastar Craton, central India. Two varieties of titanite (magmatic and hydrothermal types) are observed in the Palaeoproterozoic granitoids. Occurrence of zircon, apatite, titanite and titanomagnetite as accessory phases in the Palaeoproterozoic granitoids indicates that the early stage of crystallization of granitic magma was saturated with Zr, P and Ti. Petrography and mineral equilibria reaction suggest that magmatic titanite in the Palaeoproterozoic granitoids was formed due to hydration of amphiboles and biotites which favours high f O₂ and f H₂O conditions. Apatite composition suggests that it was formed in the granitic magma at nearliquidus to near-solidus temperature (900–1000°C) which increased fugacity ratio log(fH₂O/fHF) and also controlled the halogen budget during magma crystallization. Relatively high halogen content in the apatites from the Malanjkhand Granitoid (MG) indicates that the parental magma was enriched in F and Cl compared to Dongargarh Granitoids (DG), implying a dominant role of halogens in metal transportation and hydrothermal mineralization. In MG, apatite was the earl iest phase to be crystallized in granitic magma followed by zircon and titanite whereas, in DG, crystallization of zircon was followed by apatite and titanite. Two contemporaneous plutons, DG and MG, represent a unique Palaeoproterozoic granitic magmatism wherein early progressive crystallization is dominated by accessory mineral saturation in a relatively static environment with constant magma composition.