Compositional variations of chromian spinels from peridotites of the Spontang ophiolite complex, Ladakh, NW Himalayas, India: petrogenetic implications

摘要

The Spontang ophiolite complex exposed along the Indus Tsangpo Suture Zone (ITSZ) represents a fragment of oceanic lithosphere emplaced after the closure of the Neo-Tethyan Ocean. The complex lying south of the ITSZ forms the highest tectonic thrust slice above the Mesozoic-Early Tertiary continental margin in the Ladakh-Zanskar Himalaya. The complex consists of a well-preserved ophiolite sequence dominated by peridotites, gabbros and ultramafic cumulates along with highly tectonized sheeted dykes and pillow lavas. The mantle suite of rocks is represented by minor lherzolites, harzburgites and dunites. Chromian spinel is brown to reddish, and its morphology and textural relationship with coexisting silicates varies with strain. Spinel occurs as blebs and vermicular exsolutions within orthopyroxene to spherical inclusions within olivine, characteristic of which is the elongate holly leaf shape. Chrome spinels are characterized by low TiO2 and high Cr2O3 indicative of their depleted nature. Cr# [= atomic ratio Cr/(Cr + Al)] in the studied spinels are characterized by a small decrease in TiO2 for a larger increase in Cr# consistent with observations for spinels aligned along the Luobusa trend of the Yarlung Zangpo Suture Zone (YZSZ) ophiolites. Variations in Cr-spinel Cr# and Mg# observed in the investigated peridotites may have resulted from a wide range of degrees of mantle melting during their evolution. Mineral and whole-rock chemistry of the Spontang peridotites is characterized by interaction between depleted magma and pre-existing oceanic lithosphere, typical of supra-subduction zone settings. The Spontang peridotites have olivine, clinopyroxene and orthopyroxene compositions similar to those from both abyssal and fore-arc peridotites and display spoon shaped REE profiles characteristic of interaction between LREE-enriched melt, derived from the subducting slab and LREEdepleted mantle residues. Equilibration temperatures calculated for the above rocks indicate that the studied samples represent typical mantle peridotites formed within the spinel stability field.