The latest Jurassic protoliths of the Sangsang mafic schists in southern Tibet: Implications for the spatial extent of Greater India

Abstract

Field observations, petrology, zircon geochronology, whole-rock geochemistry, and Sr-Nd isotopes were used to reveal the lithology, age, and tectonic setting of the protoliths of the Sangsang mafic schists in the Yarlung Zangbo Suture Zone (YZSZ), central southern Tibet. The mafic schists occur as exotic blocks within the accretionary complex of the YZSZ. Relic amygdaloidal features indicate the schist protoliths were volcanic rocks. The mineral assemblage mainly comprises riebeckite + magnesioriebeckite + chlorite + sericite + albite + relic clinopyroxene. The youngest group of zircon ages constrains the formation time of the protoliths to 149.2 ± 2.2 Ma (i.e., latest Jurassic). Abundant Paleozoic and older zircons suggest the protolith volcanic rocks were erupted onto a continental terrane. Whole-rock geochemical and Sr-Nd isotopic data indicate the protoliths were of ocean island basalt affinity. The mafic schists mostly have high-Ti, alkaline, basaltic compositions with 43.57–46.93 wt% SiO2, 3.27–7.24 wt% Na2O + K2O, and 4.04–4.69 wt% TiO2. The schists are enriched in light rare earth elements relative to heavy rare earth elements, and have (La/Yb)N = 5.85–8.53 and small positive Ta and negative Zr-Hf anomalies. (87Sr/86Sr)i values vary from 0.7044 to 0.7055, while (143Nd/144Nd)i ranges from 0.512670 to 0.512727, with εNd(t) values of +4.4 to +5.5. The protoliths of the mafic schists were probably formed in a within-plate extension setting associated with mantle plume upwelling and melting of continental lithosphere. This setting was related to the Late Jurassic continental breakup of Argoland off the northern margin of east Gondwana, and thus marked the paleoposition of the northern edge of Greater India before the breakup of India from east Gondwana during Early Cretaceous. The N-S extent of Greater India at that time was ~2400 km. This further indicates how the India–Eurasia continental convergence has been accommodated.