Middle Permian magmatism in the Tangjia-Sumdo region, Tibet: evidence for intra-oceanic subduction

The late Paleozoic tectonic evolution of the Lhasa terrane and subduction of Sumdo Paleo-Tethys ocean remains controversial due to limited data and variable Permian magmatism in central and southern Lhasa. We report new zircon U–Pb ages and Hf-isotope ratios as well as whole-rock major element, trace element, and Sr–Nd isotope data from middle Permian (266 ~ 257 Ma) arc-related magmatism, including high-Mg quartz diorite and trondhjemite in the Tangjia-Sumdo area. The quartz diorite has high SiO2 (53.92–57.90 wt.%), MgO (3.69–8.03 wt.%) and low TiO2 (0.60–1.01 wt.%) contents. It is enriched in the large ion lithophile elements (LILEs, e.g. Rb, Ba, Sr) and depleted in the high field strength elements (HFSEs, e.g. Nb, Ta, Ti). We interpreted these chemical features, in combination with uniform whole-rock (87Sr/86Sr)i (0.70375–0.70452), εNd(t) (+4.48 – +5.55), and zircon εHf(t) (+11.2 – +13.7) values, as indicative of dioritic melt derived by the interaction of fluids expelled from the subducting slab with the overlying mantle wedge in an island arc setting. The trondhjemite has high SiO2 (71.16–73.52 wt.%), Al2O3 (14.92–16.24 wt.%) and K2O + Na2O (7.34–8.27 wt.%) contents, is enriched in the light rare-earth elements (LREEs) and is depleted in Th and heavy rare-earth elements (HREEs). These rocks are characterized by positive εNd(t) (+5.91 – +6.68) and εHf(t) (+12.0 – +16.1) values and may have been derived from partial melting of subducted oceanic crust. Based on the elemental and isotopic composition of the quartz diorite and trondhjemite, we suggest that middle Permian magmatism occurred in an intra-oceanic subduction setting in the Sumdo Paleo-Tethys ocean. The Sumdo Paleo-Tethys ocean was probably a small ocean basin within the eastern Lhasa terrane and not a vast ocean basin that completely separated central Lhasa from the southern Lhasa subterrane.