Relicts of Neo-Tethyan mantle wedge in the Indo-Burma Range, India: Record of carbonate metasomatism and Neo-Tethyan mantle evolution

Several different geochemical signatures, i.e., mid-oceanic ridge (MOR) and supra subduction zone (SSZ), are frequently reported from ophiolite belts. Such bi-modal geochemical signatures are generally interpreted in terms of formation in two contrasting tectonic settings: divergent and convergent settings with associated petrogenetic processes. Whilst MOR-like and SSZ-like geochemical signatures are well understood in general terms, their combined occurrence in the peridotite component of ophiolite belts is not fully understood. Here, we describe the geochemically comparable Nagaland and Manipur ophiolites which are part of a same belt located in the Indo-Burma Range, India, and represent part of the eastern Tethys regime. In this study we explore the mechanisms which are responsible for this dual geochemical signature in a contiguous ophiolite belt formed during the closure of eastern Neo-Tethys. The existing and new whole-rock Nd isotopic signatures in the serpentinized peridotites from the Manipur ophiolite reveal that the dual geochemical signatures observed in the peridotites are due to patchy metasomatism of the mantle wedge. Thus, the entire mantle section peridotite in the Nagaland and Manipur ophiolites represents a relic of that Neo-Tethyan mantle wedge and this is also supported by the occurrence of high Cs/Th and low U/Th in the serpentinised peridotites. Further, variation of La_N/Yb_N, Sm_N/Hf_N, Ti/Eu, Zr/Hf, Ca/Al and Mg# observed in the secondary and primary clinopyroxenes of the studied peridotites can be explained by an influx of carbonate-rich fluid derived from subducted pelagic limestone. Elemental zoning and associated modeling of clinopyroxenes also clarify that the mantle metasomatism and different degrees of partial melting in the mantle wedge were responsible for the heterogeneity of the Neo-Tethyan mantle preserved in the Nagaland and Manipur ophiolites of the Indo-Burma Range.