P-T-melt/fluid evolution of abyssal mantle peridotites from the Nagaland Ophiolite Complex, NE India: Geodynamic significance

Abstract

Spinel lherzolite from ophiolitic mélange in the Nagaland Ophiolite Complex, Indo-Myanmar ranges is used to reconstruct the P-T and chemical evolution of slab mantle in a Neo-Tethyan subduction channel. Five sequential high-T stages are identified for lherzolite based on petrography, mineral chemistry (major and trace elements), P-T pseudosection modelling and thermobarometry. (1) Large aluminous orthopyroxene + aluminous clinopyroxene + high magnesian olivine + chromiferous primary spinel grains formed during dry melting of source abyssal spinel lherzolite at T > 1150–1335 °C and P = 12–16 kbar. (2) Clinopyroxene and spinel exsolution in orthopyroxene, clinopyroxene + spinel coronae around megacrystic orthopyroxene and olivine formed during sub-solidus cooling from ~1100 to ~800 °C. (3) Neoblastic low-Al ortho- and clinopyroxene at ~800 °C formed during high-T mylonitisation, as the subducted lherzolite body accreted to the overlying wedge mantle as a metamorphic sole. Tectonism in this stage switched from being divergent to convergent. (4) Light REE-enriched, high-Mg clinopyroxene veins developed locally within the orthopyroxene megacrysts reflect the transit of a hybridised-mantle wedge melt. (5) A transient phase of high-T (T > 800 °C) hydration of clinopyroxene and locally orthopyroxene stabilised pargasite. Stages (4) and (5) relate to slab-derived melt/fluid migration through the accreted mantle peridotite. These findings provide new insights into the geodynamic evolution of the north-eastern segment of the Neo-Tethyan lithosphere as it evolved from a mid-oceanic ridge to a supra-subduction zone setting.