Origin and evolution of ferrogabbro-anorthosite suite of rocks from the Neoproterozoic Koraput alkaline complex, India: Implication for the Rodinia breakup

Abstract

The origin and evolution of the Koraput Alkaline Complex (KAC), situated in the western part of the Eastern Ghats Granulite Belt (EGGB), India, have long been debated. The complex consists of a variety of rock types including ferrogabbro, foid gabbro (or essexite), hornblendite, nepheline syenite, syenite, and some anorthositic variants. Comprehensive petrology, geochemistry and geochronology of the rock types carried out in this study provides a critical assessment of their petrogenetic linkage with past supercontinent cycle. Using quantitative geochemical modelling, we explain geochemical variability of the lithounits from a high aluminous parental melt, equivalent to fine-grained ferrogabbro representative of the complex. The geochemical modelling using alpha-MELTS indicates that anorthosite, essexite, and hornblendite are the product of fractional crystallisation whereas, ferrogabbro formed by equilibrium crystallisation from the residual melt. On the other hand, nepheline syenite and syenite are the product of assimilation-fractional crystallisation (AFC) from a hybridized melt assimilating the continental crust within which the parental magma was emplaced. Whole rock geochemistry of parental magma, along with Hf isotopic ratios of the zircons of nepheline syenite and anorthosite, suggest subcontinental lithospheric mantle (SCLM) source for the KAC primary melt in an intraplate tectonic setting. The primary melt had a high Mg# (71), and was formed at a mantle potential temperature and pressure of 1570 °C and 3.45 GPa respectively. U-Pb dating of zircon from nepheline syenite and anorthosite suggests that the KAC was emplaced between 741.8 ± 2.9 and 732.2 ± 2.8 Ma, which bears implication for the Rodinia supercontinent breakup.