Zircon LA-ICP-MS Dating and Geochemical Characteristics of I-type Granitoids from the Yanhu Area, West Segment of the Bangongco-Nujiang Suture (Western Tibet): Petrogenesis and Implications for the Southward Subduction of the Tethyan Ocean

ABSTRACT The Yanhu granitoids are located in the west segment of the Bangongco-Nujiang suture in the western Tibetan Plateau. The main rock types of the granitoids are diorite porphyry, quartz diorite, granodiorite, granite and granite porphyry. Here, their zircon LA-ICP-MS U-Pb ages and petrogeochemical data are reported. Three groups of magmatic events can be distinguished from the Yanhu area: group 1 includes samples AK01 and ZK01 of diorite porphyry, and sample D3658 of quartz diorite that yield mean zircon U-Pb ages of 121.0 ± 2.7 Ma, 116.6 ± 2.0 Ma and 116.0 ± 3.9 Ma, respectively; group 2 includes sample D0050 of diorite porphyry, samples D1393 and D3660 of granodiorite and sample D3065 of granite porphyry that yield mean zircon U-Pb ages of 104.9 ± 2.0 Ma, 105.4 ± 3.8 Ma, 104.2 ± 1.9 Ma and 104.2 ± 1.9 Ma, respectively; group 3 includes sample D3093 of granite that yields mean zircon U-Pb ages of 93.6 ± 1.5 Ma. The zircon LA-ICP-MS U-Pb ages suggest that the Yanhu granitoids were emplaced at 121.0 – 93.6 Ma, representing Cretaceous magmatism in the west segment of the Bangongco-Nujiang suture. The granitoids are composed of SiO2 (56.57 to 76.98 wt.%), Al2O3 (12.20 to 17.90 wt.%), Na2O (3.61 to 4.98 wt.%), K2O (2.06 to 4.71 wt.%) and CaO (0.27 to 5.74 wt.%). The Yanhu granitoids exhibit enrichment in LREE (light REE) and LILE (large ion lithophile elements) such as Rb, Th, U, Pb and K and depletion of HREE (heavy REE), P, Ti, Nb, Ta and Zr. Their A/CNK ratios of 0.85-1.06 are <1.1, implying that they are high-K, metaluminous-weakly peraluminous I-type granites. The Yanhu granitoids were generated mainly by partial melts of the meta-igneous lower crust and some arc-related materials. The Yanhu granitoids probably formed in VAG and syn-COLG tectonic settings related to the southward subduction of the Tethyan Ocean. Diorite porphyry and quartz diorite magmatism from 121.0 Ma to 116.0 Ma may be associated with the southward Bangongco–Nujiang Tethys oceanic crust subduction. Diorite porphyry, granodiorite, and granite porphyry magmatism from 105.4 Ma to 104.2 Ma may be associated with the rising asthenosphere induced by the slab breakoff. Granite magmatism from 93.6 Ma may be related to the crustal thickening induced by the final amalgamation of the Lhasa Terrane and the Qiangtang Terrane.