PETROLOGY, GEOCHEMISTRY AND GEOCHRONOLOGY OF GABBROS FROM THE SHIQUANHE OPHIOLITIC MéLANGE, WESTERN TIBET: IMPLICATIONS FOR AN INTRA-OCEANIC SUBDUCTION ZONE WITHIN THE NEO-TETHYAN OCEAN

Ophiolites are useful tools for reconstructing the tectonic history of a region because they indicate the presence of sutures and the closure of ancient oceanic basins. The Shiquanhe-Yongzhu-Jiali ophiolitic mélange belt (SYJMB) is located in central Tibet where it extends for at least 1000 km from Shiquanhe in the west to Chayu in the east. Dismembered rock units in the mélange have ages from the Late Triassic to the Early Cretaceous, and are in fault contact with pre-Ordovician, Permian, and Jurassic–Early Cretaceous blocks. Some earlier workers thought the SYJMB was obducted southward during closure of the Bangong-Nujiang Ocean, whereas others suggested that it represents a late Jurassic to early Cretaceous back-arc basin associated with the Bangong-Nujiang suture zone (BNSZ). Because most previous studies focused on the eastern and middle portions of the SYJMB, little is known regarding the tectonic environment and geodynamic significance of its western portion. In this study, we focused on the petrology, whole-rock geochemistry, and geochronology of gabbros within the mélange in order to investigate the evolution of the belt.

All of the gabbros in the mélange have undergone greenschist to amphibolite facies metamorphism. U-Pb analyses of zircons from a cumulate gabbro yielded a weighted mean age of 193.2±4.5 Ma，MSWD=0.81. Whole-rock geochemistry suggests that these gabbros have island arc and mid-ocean ridge basalt affinities. Most analysed samples have generally flat chondrite-normalized rare earth element (REE) patterns showing slight depletion in light REE (LREE), and N-MORB-normalized incompatible element diagrams indicate depletion in high field strength elements (HFSE) (Nb, Ta) and enrichment in large ion lithophile elements (LILE). The geochemical data 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 subducted sediment.

Based on our data and previous studies, we propose that an intra-oceanic subduction system and back arc basin existed in the Neo-Tethyan Ocean of central Tibet during the Early Jurassic, resembling modern, active, intra-oceanic subduction systems in the western Pacific.