Chromite Composition and Platinum-Group Element Distribution in the Proterozoic Chimalpahad Anorthosite Complex, South India: Implications for Magmatic Processes and Discrimination of Tectonic Setting

Abstract

AbstractThe Proterozoic Chimalpahad Anorthosite Complex (CAC) is a deformed and dismembered complex in South India comprising of anorthosite-leucogabbro-gabbro-ultramafic rocks. It is located in the Nellore-Khammam Schist Belt (NKSB) within the contact zone between the Eastern Dharwar Craton (EDC) and the Eastern Ghats Belt (EGB). The chromitites occur as small dismembered lensoidal bodies within the ultramafic rocks. Presence of micrometric inclusions of iridium-like platinum-group minerals (IPGM), such as laurite (RuS2), erlichmanite (OsS2), irarsite (IrAsS), and Ir-Os-Ru alloys, is reported for the first time in the CAC chromites. The chromite cumulates preserve magmatic character (Cr#: 0.61 to 0.77; Mg#: 0.29 to 0.55), while disseminated chromites and rims of some grains (Cr#: 0.59 to 0.89; Mg#: 0.10 to 0.25) show postmagmatic modifications. The chromites are characterised by higher Cr and Al contents and lower Fe and Ti contents. The mineral chemistry of chromites such as Cr# and Mg# contents of Al2O3 (10.05–20.07 wt.%) and TiO2 (0.1 to 0.25 wt.%) typically suggests that the CAC chromitites and their ultramafic hosts were generated from the S-undersaturated boninite melt in a magmatic arc in suprasubduction tectonic setting. A general negative slope of bulk-rock PGE distribution with upward convex pattern is observed for the CAC chromitites in the chondrite-normalized plot. These geochemical and mineral chemical characters in addition to location of the chromitites and the CAC in the tectonic zone between Eastern Dharwar Craton and Eastern Ghats Mobile Belt point towards ophiolitic affinity of the CAC chromitites. Laurite (RuS2) grains are trapped close to the cores of chromites, whereas erlichmanites (OsS2) are trapped away from the cores. Textural relations indicate primary orthomagmatic nature of laurite and Or-Ir-Ru alloys and their precipitation prior to or concomitant with chromite from an S-undersaturated melt at ~1200–1300°C and log fS2 varying between −2 and −1.3. Ru-Os disulfide and erlichmanite crystallized subsequently with a gradual decrease in temperature in the differentiating ultramafic magma. Precipitation of irarsite and base-metal sulfides can be attributed to local and rapid fluctuations of temperature, fS2, and fAs during the formation of the chromitites due to influx of fresh batch of melts or crystallization in an open magmatic system. The compositional variations of laurite-erlichmanite record heterogeneous physicochemical environment during their crystallization. In addition to zoning, preservation of IPGMs in the CAC chromites points towards fast cooling of the magma possibly associated with rapid exhumation of the chromitites and their ultramafic hosts from the roots of the subcontinental lithospheric mantle (SCLM) towards their final emplacement into the crust. Occurrence of the CAC along a linear extension with the Kondapalli Ultramafic Complex and deformed remnants of ophiolites (Kandra and Kanigiri complexes) along the eastern margin of the Eastern Dharwar Craton (EDC) bordering the Eastern Ghats Belt (EGB) indicates this zone to be a Proterozoic convergent margin.