Harzburgite-dunite transition in the Nidar Ophiolitic Complex, Ladakh, NW India and its effect on olivine crystallographic fabric

Abstract

The Nidar Ophiolitic Complex (NOC) of Ladakh, north India, is a well-preserved remnant of the Tethyan oceanic lithosphere. We performed mineral chemistry and microstructural analyses on five spinel-harzburgite and three spinel-dunite samples from its mantle section which show varying degree of serpentinization. The five harzburgite samples are divided into two types based on the mineral chemistry of olivine (Mg#=0.92-0.94) and spinel (TiO2=0-0.38 wt.%, Al2O3=8.67-27.46 wt.%, Cr#=0.50-0.81). These data, together with dynamic recrystallization textures, indicate high-temperature (≥ ∼1000 °C) origin for the peridotites. The first type includes three harzburgites with spinel and olivine chemistry indicating a spreading ridge to Supra-Subduction Zone origin, with A-type olivine Crystallographic Preferred Orientation (CPO) and a transtensional to simple shear regime of fabric formation. The second type consists of two harzburgites with boninitic affinity, showing D- and E-type olivine CPO and a simple shear regime of fabric formation. Mineral chemistry of the three dunites suggests melt-rock interaction and an island arc setting, with two showing A-type and one E-type olivine CPO and a transtensional regime of fabric formation. Transtensional regime inferred from vorticity analysis helps envisage an extensional setting for the lithospheric mantle. Dehydration of subducting oceanic slab, in a simple shear regime, led to hydration and partial melting of the mantle wedge, changing A-type fabric into E-type. Continued slab-pull resulted in further melting, dehydrating the remaining mantle and creating D-type fabric. This study demonstrates formation of replacive dunite from depleted harzburgite owing to melt-rock interaction and transformation of associated olivine CPO.