Microtextural evolution of chrome spinels in dunites from Mayodia ophiolite complex, Arunachal Pradesh, India: Implications for a missing link in the “two-stage” alteration mechanism

Abstract

Although occurring as an accessory phase in mafic-ultramafic rocks, the universal presence of chrome spinel in ophiolitic settings bears importance in deciphering the tectonic setting of the ophiolite. Compositional and textural modification of chrome spinel via post-magmatic processes (metamorphism and fluid-induced alteration) is widely reported from host chromitites and peridotites, however, well-documented studies from dunite in ophiolitic settings are sparse. Here we present a detailed petrographic and geochemical account of accessory chrome spinels from previously unreported massive dunite bodies of Mayodia ophiolitic complex, Arunachal Pradesh, India, that portrays a microtextural evolution during alteration/metamorphism. Although having massive appearance, the bodies are crudely foliated and deformed. Severe alteration obliterated primary spinel chemistry as well as pre-alteration mineralogical assemblages. From textural and chemical point of view, the disseminated accessory chrome spinel grains are classified into four broad groups: (1) less chloritized spinel, (2) heavily chloritized spinel, (3) patchy-zoned spinel, and (4) homogeneous spinel. The chloritized spinels are relatively coarse grained, compositionally enriched in Cr and Fe2+ (ferrous chromite). They contain Cr-rich chlorite inclusions and are surrounded by a mixed matrix of Cr-rich chlorite and Cr-rich serpentine. Unzoned, homogeneous spinels appear as aggregates and are devoid of chlorite inclusions. They are compositionally different from the chloritized spinels, being enriched in Fe3+ and depleted in Cr (ferrian chromite). The patchy-zoned spinels have a non-uniform zoning with patches of ferrous chromite floating within ferrian chromite, separated from each other by micro fracture networks. Adapting a novel technique, we reconstruct bulk rock composition based on global data compilation, which has been applied to thermodynamic modelling. Comparing our integrated results with global examples we infer that fluid-induced alteration (chloritization) caused the formation of chloritized spinels from their primary counterpart. Later oxidizing fluid interacted with the chloritized spinels and converted them to homogeneous spinel; supporting the two-stage alteration model. We interpret the patchy zoning as an arrested intermediate stage between chloritized spinel and homogeneous spinel, preserving the characters of both stages. From a regional perspective, we postulate that the chrome spinel microtextures evolved between ~780 °C and ~ 550 °C during amphibolite facies retrogression of the dunite bodies, possibly related to exhumation of Mayodia ophiolitic complex.