Geochemistry and geochronology of gabbros from the Asa Ophiolite, Tibet: Implications for the early Cretaceous evolution of the Meso-Tethys Ocean

Abstract

The formation of the Shiquanhe–Namco Ophiolite Mélange Zone (SNMZ) and its relationship to the Bangong–Nujiang Suture Zone (BNSZ) in central Tibet are key to understanding the Mesozoic tectonic evolution of the Meso-Tethys Ocean. This study presents zircon UPb geochronological, whole-rock geochemical, and LuHf isotopic data for gabbros from the Asa Ophiolite of the central SNMZ. Four gabbros yielded zircon UPb ages of 117.3 ± 0.6, 117.3 ± 0.5, 115.3 ± 0.5, and 114.9 ± 0.6 Ma. Whole-rock geochemical analyses indicate that these rocks are tholeiitic, enriched in light rare earth and large-ion lithophile elements, and depleted in Nb and Ta. These gabbros have geochemical affinities to island arc and mid-ocean ridge basalts, suggesting that they originated in an oceanic back-arc basin. Whole-rock geochemical data and isotopically juvenile εHf(t) values suggest that the gabbros were derived from a depleted mantle source that was enriched by slab-derived fluids and partial melts of sediment. Our data, combined with previous work, suggest that the BNSZ records the Early Cretaceous subduction of the Meso-Tethys oceanic crust and that the SNMZ represents a back-arc basin. During the Early Cretaceous, the Meso-Tethys Ocean remained open and contained an intra-oceanic subduction system and back-arc basin.