Petrology, geochemistry and geochronology of gabbros from the Zhongcang ophiolitic mélange, central Tibet: Implications for an intra-oceanic subduction zone within the Neo-Tethys Ocean

In order to investigate the evolution of Shiquanhe-Yongzhu-Jiali ophiolitic mélange belt, the gabbros from new discovered Zhongcang ophiolitic mélange are studied through petrology, whole-rock geochemistry, zircon U-Pb dating and Lu-Hf isotope. The gabbros investigated in this paper contain cumulate gabbro and gabbro dike, and they have undergone greenschist-amphibolite facies metamorphism. The chondrite normalized rare earth element (REE) patterns of most of these rocks show flat types with slightly light REE (LREE) depletion and the N-MORB normalized incompatible elements diagrams indicate depletion in high field strength elements (HFSE) (Nb, Ta) and enrichment in large ion lithophile elements (LILE). These gabbros have island arc and mid-ocean ridge basalt affinities, suggesting that they were originated in an oceanic back arc basin. Whole rock geochemistry and high positive ɛNd(t) values show that these gabbros were derived from ∼30% partial melting of a spinel lherzolite mantle, which was enriched by interaction with slab-derived fluids and melts from sediment. U-Pb analyses of zircons from cumulate gabbro yield a weighted mean age of 114.3±1.4 Ma. Based on our data and previous studies, we propose that an intra-oceanic subduction system and back arc basin operated in the Neo-Tethy Ocean of central Tibet during Middle Jurassic and Early Cretaceous, resembling modern active intra-oceanic subduction systems in the western Pacific.