Geochemical dichotomy of Tethyan oceanic mantle and the supercontinent connection: Insights from Os isotopic signatures of mantle peridotites from Naga Hills ophiolite

Abstract

Naga Hills Ophiolite (NHO) represents a thrusted section of relict Neo-Tethyan oceanic lithosphere comprising distinct crustal and upper mantle lithologies that preserve imprints of different stages of evolution of ancient ocean basins and subsequent accretion onto continental margins during late Cretaceous-Eocene collision involving Indian and Eurasian Plates. This study presents bulk geochemistry and Re-Os isotopic compositions for the mantle peridotites of NHO to address their petrogenetic aspects, implications for Neo-Tethyan upper mantle heterogeneity and thermo-tectonic evolution of Neo-Tethyan oceanic lithosphere through subduction-accretion-collision processes. Geochemical characteristics of the mantle peridotites, as reflected from their chondrite normalized REE patterns, binary relationship between fractionation depletion indices, low Re/Os, suggest that these are mantle residues resulting from ? 5-20 % of melt extraction from a spinel peridotite source that experienced decompression melting in a fore-arc extensional setting in response to subduction initiation. Isotopically, these mantle peridotites are subchondritic with 187Os/188Os (0.1218-0.1266) and ?Os values of ?3.73 to ?0.09 comparable with the depleted MORB source. LILE-LREE enrichment, HFSE depletion, and U-shaped chondrite-normalized REE patterns demonstrated by the studied samples with Re addition collectively corroborate multistage petrogenetic processes in compliance with geodynamic transition from extensional to compressional regime in a suprasubduction zone environment and partial melting of a chemically heterogeneous source mantle. This compositional heterogeneity can be equated with mantle depletion by melt extraction and refertilization by influx of slab-dehydrated fluids and percolation of boninitic melts. The non-radiogenic Os isotope signatures, negative ?Os values and wide spectrum of melt extraction ages for these mantle peridotites correspond to (i) multiple melt production and melt extraction events prior to the opening of Neo-Tethyan seaway (ii) detachment and incorporation of ancient, depleted SCLM fragments into oceanic mantle concurrent with subduction driven ocean basin closure-congregation of Gondwana Supercontinent and emergence of Neo-Tethyan embryonic ocean in response to disintegration of Gondwana Supercontinent.