Temporal-spatial distribution and evolution implication of the composite intrusion in the Xiuwacu W-Mo deposit,SE Tibetan Plateau

Located in the south section of the Yidun terrane (SYT), the Xiuwacu W-Mo deposit occurs in a composite intrusion composed by the Late Triassic, late Cretaceous and Cenozoic intrusions. The study on the magmatic evolution process of the composite intrusion is poor, limiting the understanding of the deposit and the relation ship between regional tectonic events and magmatic activities. Basing on previous research results, this paper carries out geochronological and geochemical analysis of the biotite granite in the east ore section, and monzogranite, granite porphyry, and aplite in the west ore section, so as to explore the magmatic evolution process of the Xiuwacu composite intrusion and its indicative significance to regional tectonic background. With a crystallization age of biotite granite of 205.4 ±4.3 Ma, the Xiuwacu composite intrasion belongs to high potassium shoshonitic aluminous series, enriched in light rare earth (LREE) and large ion lithophile elements(LILE) Rb, Nd and Sm, depleted in heavy rare earth (HREE) and high field strength elements (HFSE) Nb, Ta, Zr and HF. It is with an obvious negative Eu anomaly and a high oxygen fugacity (fo₂ =-22.8~-6.8, average-12.2). The positive ε_Hf(t) values are from 0.0 to 3.9, the ε_Nd (t) are -2.11 to -1.18, the ⁸⁷Sr / ⁸⁶Sr are from 0.707248 to 0.708070, and the zircon Hf two-stage model age is 1.32~1.68 Ga. The monzogranite, granite porphyry and aplite in the west ore section have the same material source and very similar geochemical characteristics. The age of granite porphyry is 80.2 ± 1.5 Ma. Compared with the biotite granite, it is of a stronger negative Eu anomaly, a lower oxygen fugacity (fo₂ = -33.2 ~ -3.2, average -23.0), a lower temperature, a negative ε_Hf(t) value (-8.4 ~ -2.1) and ε_Nd(t) value (-8.59 ~ -5.14) a higher ⁸⁷Sr / ⁸⁶Sr value (0.709987 ~ 0.713559) and an older zircon Hf two-stage model age (1.62 ~ 2.36 Ga). The results show that the Late Triassic biotite granite was formed by fractionation crystallization due to the Gantze-Litang oceanic westward subduction in an open-system. The parental magma mainly comes from depleted mantle and a small amount of subduction oceanic crust material. The late Cretaceous intrusion mainly comes from the ancient lower crust and a small amount of mantle derived material under the regional tensile environment. The magmatic fractionation crystallization under the relatively closed-system is affected by the assimilation and contamination of surrounding rock in the process of magmatic evolution. It is of great significance for revealing the relationship between regional tectonic and metallogenic events.