Petrogenesis and geodynamic significance of Neoproterozoic (∼925 Ma) high-Fe–Ti gabbros of the RenTso ophiolite, Lhasa Terrane, central Tibet

Abstract

The Precambrian origin and evolution of the Lhasa Terrane remain unclear. Here, we present new zircon U–Pb isotopic and whole-rock geochemical data for the RenTso gabbros of the central Lhasa Terrane. These data indicate that these gabbros formed during the early Neoproterozoic (∼925 Ma) and have affinities with high-Fe–Ti basaltic rocks, given that they contain low concentrations of SiO2 (39.8–49.0 wt%) and high concentrations of FeOT (11.3–23.5 wt%), TiO2 (2.19–5.70 wt%), and V (333–1204 ppm). These gabbros also have N-MORB-like trace element features such as low (La/Sm)N ratios (0.50–0.57) and the absence of negative Nb–Ta anomalies. Combining these features with the high whole-rock εNd(t) (+5.04 to +7.55) and zircon εHf(t) (+7.8 to +13.0) values for these samples indicate that the RenTso gabbros formed by the partial melting of depleted asthenospheric mantle. The Fe–Ti enrichment in these gabbros reflects conditions of low oxygen fugacity during magma fractionation. These gabbros are spatially associated with troctolite and peridotite units, indicating that together these units form a typical ophiolitic lithological assemblage. These geochemical and field observations suggest that the study area contains a subduction-unrelated-type ophiolite that most likely represents the embryonic crust of the Mozambique Ocean following continental rifting and breakup. The results therefore indicate that the Lhasa Terrane has a Precambrian affinity with the East African orogen and occupied a different position to the Qiangtang Terrane within the Rodinia supercontinent.