Insights Into The Petrogenesis Of Depleted Mantle Dunite From The Central Part Of The Nagaland-Manipur Ophiolites, North East India

This communication presents results of mineral and whole-rock geochemistry of rarely occurred dunites in the central part of the Nagaland-Manipur Ophiolites (NMO), North East India, and discusses their genesis and tectonic evolution. These rocks are characterized by low concentration of average CaO (0.58 wt%), Al2O3 (0.42 wt%) and Sigma REE (1.24 ppm), but high Mg# (0.91-0.92) and Cr# (0.61-0.73) values in chromian spinels. They exhibit a U-shaped REE pattern depleted in MREEs, which is equivalent to dunite composition, possibly part of a restite peridotite which underwent through extensive partial melting. The estimated degree of partial melting based on chromian spinel Cr# ranged from 20.04% to 20.70%. Low concentration of TiO2 (0.10-0.16 wt%) in chromian spinel in these dunites confirms no evidence of metasomatism. Therefore, we propose that dunites in the NMO represent the remnants of residual mantle wedge which underwent extensive partial melting in a subduction zone. Absence of metasomatism indicates no melt-wall rock interaction during the process of mantle melting and final obduction on the surface.