Charactreistics of the Late Triassic Jiangai Granite Mass and the Slab Break-Off in Central Qiangtang，Tibet

The tectonic setting and petrogenesis mechanism of the Late Triassic magmatic activity in the Longmu Co-Shuanghu Suture zone in central Qiangtang are still controversial. As aimed, it is significance to research the later tectonic evolution of the suture zone though isotopic chronology and geochemistry for the Jiangaidarina granite mass (hereinafter referred to as Jiangai mass). Located in the south of Longmu Co-Shuanghu Suture zone, Jiangai mass consist of biotite granodiorite and granite porphyry, and obtain three zircon U-Pb LA-ICP-MS ages of 212.6 $\pm$ 4.9 Ma、215.4 $\pm$ 2.7 Ma and 210.4 $\pm$ 2.0 Ma. These ages indicat that the Jiangai mass are Late Triassic magmatic activity instead of the Early Jurassic. Geochemically, the rocks are high in SiO2 and Al2O3，and wide range of K2O (2.60%~4.99%) and Na2O + K2O (3.98%~7.31%), with the A/CNK values of 0.91~1.50. The rocks contain amphibole minerals and P2O5 is negatively correlated with SiO2. The above characteristics indicate a series of high K-potssium, aluminum- peralumnous, and I-type granite rocks.Furthermore, the Jiangai mass are also characterized by low REE, relatively heavy LREE, enrichment LILE(Rb, Ba, Th, U, K) and depletion of HFSE(Sr, P, Ti), with the slightly negative Eu anomaly (0.56~0.87). The Hf (t) (-10.3~-2.4 and 10.2) and tDM2 ages (average \mbox{1 663 Ma and 453 Ma) of Jiangai parent magma mixing with the partial melts from remelting of ancient lower crust and mantle-derived material. Taking the regional geological setting into consideration, we infer that, triggered by the break-off of the Longmu Co-Shuanghu oceanic slab subducted beneath the North Qiangtang plate, the potassium basalt of lower crust South Qiangtang Block partial remelting, and the mantle-derived material transform the sediments at the same time. Followed by the amphibole, plagioclase and other crystallization process, the Jiangai mass formed eventually. The mechanism of Jiangai mass is mainly related to the subduction slab break-off. Meanwhile, it is proved the Longmu Co-Shuanghu Suture zone has been completely closed before the Late Triassic, and the Late Triassic is the post-collision tectonic~stage.