Permo-Triassic arc-like granitoids along the northern Lancangjiang zone, eastern Tibet: Age, geochemistry, Sr–Nd–Hf isotopes, and tectonic implications

Abstract

Large volumes of Permo-Triassic granitoids are exposed along the Northern Lancangjiang zone, eastern Tibet, and these rocks provide insights into the tectonic evolution of the Paleo-Tethys Ocean. We conducted detailed geological fieldwork and geochemical analysis of the Xiaochangdu and Kagong plutons that crop out along the Northern Lancangjiang magmatic belt. Zircon U–Pb data constrain the emplacement of the Xiaochangdu quartz diotites to between ~263 and ~257 Ma, and the Kagong granites and diorites to between ~234 and ~232 Ma. The Xiaochangdu quartz diorites are enriched in light rare earth (LREE) and large ion lithophile elements (LILE), depleted in high field strength elements (HFSE), have low (87Sr/86Sr)i ratios, and near-positive εNd(t) (−0.26 to 1.58) and εHf(t) (0.68–8.83) values, similar to typical subduction- related mantle-derived arc magmas. They are also characterized by high Al2O3 concentrations and low Nb/U (3.48–7.59) and Ce/Pb (3.22–4.86) ratios, indicating that their mantle source was modified by subducted pelagic sediments; Coeval granites and diorites from the Kagong pluton exhibit low A/CNK values, high LREE/HREE (heavy rare earth element) ratios, enrichment in LILE, and depletion in HFSE, also characteristic of typical arc magmas. Their variable SiO2 contents (57%– 75%), (87Sr/86Sr)i ratios, and εNd(t) (1.02–4.49) and εHf(t) (2.52–6.93) values, and relatively high zircon saturation temperatures (721–827 °C), suggest underplating of mantle-derived mafic melts beneath the lower crust. Their magmatic evolution can be explained using a MASH model.In combination with regional geological studies, our geochemical and geochronological results suggest that the late Permian Xiaochangdu and Late Triassic Kagong arc-like granitoids represent a section of a Permo-Triassic magmatic arc that was associated with the eastward subduction of the Paleo-Tethys oceanic slab beneath the Northern Qiangtang–Changdu terrane. Combined with other geological evidence, the ~263–232 Ma arc-like granitoids clearly indicate that final closure of the Paleo-Tethys ocean have not occurred until the end of the Triassic.