Research Projects

Ophiolites are ancient oceanic crust obducted on land, which gives significant insights into the paleo-subduction processes (Parkinson & Pearce 1998). The Nagaland Manipur Ophiolites (NMO) lie at the eastern margin of the Indian plate, sandwiched between the Indian plate in the west and the Burmese micro-plate in the East. This ophiolite belt is dominated by mantle peridotites, ultramafic-mafic cumulates, mafic volcanics and radiolarian cherts. For several decades, Cr# (Cr/Cr+Al) in chromian spinel (Cr-spinel), hosted within ultramafic rocks, was used as a reliable tool to decipher the degree of melt extraction and to infer their geotectonic environments (Irvine, 1965, 1967). This inference assumes that the Cr-spinel core composition is homogeneous, insensitive to post-formation modification and therefore a reliable petrogenetic indicator (e.g. Dien et al., 2019). However, recent petrographic, geochemical, and isotopic studies infer post-formation modification of the mantle peridotites by fluid/melts association with subduction, rendering the paleotectonic reconstruction of ophiolites questionable. For instance, recent studies show that the composition of Cr-spinel, clinopyroxene and orthopyroxene in terms of Al (a.p.f.u.), Cr (a.p.f.u.) and fluid-mobile elements (Li, Rb, Sr, and Cs) can be modified by fluid/melt rock interaction in SSZ settings (Dien et al., 2019; Pan et al., 2022; Su et al., 2023). The evidence of these cryptic metasomatism could be observed in mineral zoning patterns in Cr-spinel, clinopyroxene and orthopyroxene and shows a positive correlation between Al2O3 and Cr2O3 contents. This compositional variation cannot be explained by a simple melting mechanism, but indicates Al-rich fluid/melt metasomatism derived from slab melting and/or mantle wedge. Hydrous minerals such as amphibole observed in many mantle peridotites across terranes are seen as direct evidence of such mantle metasomatism. Thus, in this project a careful and systematic study of micro texture, mineral chemical zonation patterns (major and trace element) will be carried out to decipher whether the signatures of partial melting are preserved or they are overprinted by fluid/melt metasomatism. In instances where the original melting residue is preserved, unaffected by melt/rock interaction, the equilibration pressure-temperature conditions of the rock under sub-solidus conditions would be estimated using THERMOCALC 3.5.1 program (Powell et al., 1998). In mantle rocks affected by metasomatism, efforts will be made to unravel the type and source of melt/fluid that interacted with the peridotite rock. The implications of this research outcome will be huge as it challenges the traditional approach of using Cr# as the sole indicator of ophiolite paleo-tectonic reconstruction. This would improve our understanding of mantle melting processes, the nature of melt/fluid-rock interaction and the paleo-tectonic setting of the ophiolites.