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
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.