Compositional signatures of clinopyroxenes from the mafic rocks of the Nagaland ophiolite (NE India): Inferences for melting of diverse source regions in a transitional arc-type setting

Abstract

The Nagaland-Manipur ophiolite belt (NMOB) represents remnants of the Neo-Tethyan Ocean that evolved during the accretion of the Indian and Myanmar lithospheric plates. We studied clinopyroxenes of cumulate gabbro and basalt rocks from the northern section of the belt to decode the mechanism that controlled their petrogenesis and geotectonic setting. Clinopyroxenes (augite-diopside) exhibit compositional zoning and significant variations in their major and trace element contents. Zoned clinopyroxene from a single volcanic rock sample (2d-3-6-11) show higher concentrations of TiO₂ (1.44 to 5.13 wt%) and Al (0.128–0.359 apfu) and lower Si (1.658–1.889 apfu) compared to clinopyroxenes from the other mafic rocks. The mg# [100 × Mg/(Mg + Fe²⁺)] values of clinopyroxenes range between 60.1 and 85.7. The trace element contents and the chondrite-normalised REE patterns of clinopyroxenes imply diverse geochemical affinities of both depleted mantle typical of tholeiite rocks and a less depleted LREE-enriched melt for the Nagaland mafic rocks. Pressure and temperature of crystallisation were estimated using single clinopyroxene geothermobarometry and were found to vary between 1–8 kbar and 840–1179 °C, respectively. In discrimination diagrams, the zoned clinopyroxenes show an alkaline composition consistent with a within-plate environment. On the other hand, clinopyroxenes from the rest of the mafic samples show a sub-alkaline composition typical of magmas produced in arc-related settings. Our study indicates the existence of diverse magma sources in an island arc-to-back arc setting during the formation of magmatic rocks in the NMOB.