Neoproterozoic back-arc and arc-type magmatisms in the Rutong and Shiquanhe region, west of Tibet: Tectonic implications for the early-stage evolution of the Bangong-Nujiang suture zone and North Lhasa terrane

Due to limited exposure of the Precambrian rocks in the Tibetan Plateau, their affinities and evolutions are inadequately studied. Here we report the newly identified Neoproterozoic metabasites (including metagabbros and metadolerites) at the western end of the Bangong-Nujiang suture zone (BNSZ) and the west part of the North Lhasa terrane. The Rutong Dong (RTD) metagabbros and Shiquanhe Nan (SQN) metadolerites yield zircon U-Pb ages of 838 Ma and 748 Ma, respectively. The RTD metagabbros are tholeiitic and exhibit both MORB- and arc- like geochemical characteristics, and we consider these to have formed in a back-arc basin setting. Combined with positive zircon epsilon Hf(t) Hf (t) (+6.3 +6.3 to +13.8) and whole-rock epsilon Nd(t) Nd (t) (+4.4 +4.4 to +5.3) values, the geochemical features of the RTD metagabbros indicate that they originated from a depleted mantle source enriched by subductionrelated components (e.g., Th and U). The SQN metadolerites are mainly of transitional character (between tholeiitic and calc-alkaline), as well as representing typical calc-alkaline basalts. Their arc-like geochemical characteristics, as well as low positive whole-rock epsilon Nd(t) Nd (t) (+1.7 +1.7 to +3.4), and positive zircon epsilon Hf(t) Hf (t) (+4.3 +4.3 to +5.9) values, indicate generation by partial melting of an enriched subduction components modified mantle in an arc- type setting. Based on previous research, we propose that the North Lhasa terrane likely separated from India at ca. 925-860 Ma by the opening of the Mozambique Ocean. As the subduction zones were active at ca. 838-730 Ma in this ocean, a series of subduction-related magmatic events occurred, that is recorded in this study of the Rutong-Shiquanhe region, and other parts of the BNSZ and North Lhasa terrane. Consequently, we suggest that the BNSZ not only preserves information of the Meso-Tethys but also retains geological records of the Neoproterozoic Mozambique Ocean.