Geochemistry and tectonic significance of the Gongzhu peridotites in the northern branch of the western Yarlung Zangbo ophiolitic belt, western Tibet

Abstract

The Gongzhu ophiolite is situated in the northern branch of the western Yarlung Zangbo ophiolitic belt. This massif consists of a strongly dismembered ophiolitic sequence dominated by mantle peridotites. The peridotites comprise lherzolite with low- to moderately-depleted mineral and bulk rock compositions. The degree of partial melting deduced from Cr# values of the Gongzhu peridotites varies between 7% and 10%. The mineral and whole rock compositions of the Gongzhu peridotites are comparable to those of abyssal peridotites. The chondrite normalized REE compositions of the peridotites typically display U-shaped or spoon-shaped patterns, and primitive mantle-normalized PGEs patterns show Ir depletion relative to Os and Ru, and Pt enrichment relative to Rh and Pd. On the basis of the petrological, mineralogical and geochemical data, we concluded that the Gongzhu peridotites either formed in the back-arc setting of an intra-oceanic subduction system or the Gongzhu and Dajiweng peridotites both formed in the in the same incipient forearc/proto-forearc environment of an intra-oceanic subduction zone.