Tectonic setting and petrogenesis mechanism of Late Triassic magmatism in Central Qiangtang, Tibetan Plateau: Take the Xiangtaohu pluton in the Hongjishan region as an example

The tectonic setting and petrogenesis mechanism of Late Triassic granitoids, which widely exposed in the central Qiangtang Terrane, Tibet, remain unconstrained so far. A better understanding of such issues will help us to reveal tectonic evolution processes of the Longmu Co-Shuanghu-Lancang suture zone during the Late Triassic. This paper reports petrological characteristics, zircon U-Pb age, whole-rock geochemistry of analyzed samples from Xiangtaohu pluton, which is one of the representative Late Triassic pluton in the central Qiangtang Terrane. In situ zircon U-Pb LA-ICP-MS dating on the magmatic genetic zircons yielded an age of the 210.9 +/- 3.9Ma and 212.6 +/- 1.5Ma, representing the emplacement age of the pluton. Geochemically, the rocks are high-K calc-alkaline to calc-alkaline series, and rich in alkali (K2O + Na2O = 5.58% similar to 7.55%) and K (K2O/Na2O = 1.05% similar to 1.63%), with the A/CNK values of 1.03 similar to 1.09, which fall into the range of slightly peralumnous rocks. The rocks are also characterized by enrichment of LREE and LILE (Rb, Ba, Th, U, K and La) and depletion of Nb, Ti, Zr and P, which slightly negative Eu anomaly (Eu/Eu* = 0.61 similar to 0.87). The elemental ratios of the Xiangtaohu pluton, such as Th/U (5.19 similar to 9.36), Nb/Ta (8.13 similar to 10.06), combined with the contents of high Sr (average 389 x 10(-6), low Yb (average 1.87 x 10(-6)) and HREE-smooth pattern (Ho-N approximate to Yb-N), indicate the partial melting of magmatic rocks from lower crust, with amphibole eclogite as the restite. Macroscopic characteristics such as ubiquitous mafic microgranular enclaves (MMEs), long prisms of apatite and captured quarts in MMEs indicate that the origin of the pluton is closely related to magma mixing. Combined with the previous studies and correlative geological events,it is concluded that the Late Triassic magmatism in Central Qiangtang were formed in a post-collision extensional tectonics in response to slab break-off of the subducted Longmu Tso-Shuanghu Tethyan oceanic lithosphere beneath the North Qiangtang subterrane.