Melt-rock interaction and fractional crystallization in the Moho transition Zone: Evidence from the cretaceous Naga Hills Ophiolite, North-East India

Abstract

The Moho Transition Zone (MTZ) within the suboceanic lithosphere records an episode of multiple magmatic exchanges between the underlying mantle and the overlying oceanic crust. This study investigated a fossilized MTZ from the Naga Hills Ophiolite (NHO) by characterizing its lithology and describing the petrogenesis of various rock types. Like the other well-studied MTZs around the world, the NHO MTZ has an association of dunite, wehrlite, clinopyroxenite and gabbroic rocks. The dunite formed by crystal fractionation from an olivine-normative basaltic melt that successively crystallized olivine-gabbro, gabbro and oxide-gabbro. Our study of the wehrlite provides evidence of melt-mantle interactions that lead to the formation of replacive dunite, later impregnated with clinopyroxene saturated melt. The melt percolated through the replacive dunite and re-equilibrated with the olivine to some extent. The clinopyroxenite formed simultaneously with the wehrlite as a crystal mush in pockets from clinopyroxene saturated melt. In summary, the major and trace element mineral chemistry of the NHO MTZ rocks documents their evolution through melt-rock interaction and fractional crystallization. Clinopyroxene played an important role in the evolution of NHO MTZ, particularly the formation of wehrlite and clinopyroxenite. There is no direct evidence for the involvement of water in the formation of the MTZ rocks. However, the presence of primary hydrous phases in the evolved rock types and the signatures of early crystallization of clinopyroxene prior to plagioclase in the initial cumulates, discernible from a positive Sr anomaly, suggest the hydrous nature of the parent melt. Therefore, the NHO possibly represents a fragment of oceanic lithosphere that had its origin above a subduction zone and was later incorporated within a mountain belt during terminal closure of the Neo-Tethys Ocean.