Zircon U-Pb Ages and Geochemical Characteristics of Youqiumi Porphyry Pluton in Cimabanshuo Area, Tibet

The ore-bearing porphyry body has favorable geological conditions for porphyry copper mineralization in Cimabanshuo area, Tibet. Therefore, it is of great significance to study the geological background and genesis of diagenesis and mineralization in this area. In this paper, combined with their new understanding and new discoveries in the field works, the authors carried out a detailed research of zircon U-Pb dating and petrogeochemistry, for Youqiumi pluton in Cimabanshuo area.The results show that the granodiorite-poryphy yields an LA-ICP-MS zircon U-Pb age of 15.2±0.8 Ma to 15.2±0.7 Ma, namely the formation age of Miocene. Chemical analyses show that the granodiorite-poryphy has a characteristic of high SiO₂ (65.08%-66.85%), high K₂O (3.85%-4.58%), rich K₂O+Na₂O (7.87%-8.90%), depleted MgO (1.51%-1.84%), meta-aluminum (A/CNK=0.88-0.95). Chondrite-normalized REE distribution pattern diagrams display right-incline and weakly negative Eu anomalies, differentiated obviously (La/Yb)_N=36.60-47.43. Enriched strongly in compatible elements such as Rb, Th, U, K and depleted in high field strength elements (HFSEs) such as Nb, Ta, Ce, P, Ti. Specifically, Youqiumi granitic pluton shows typical adakitic signatures:high Sr (674×10^-6 to 876×10^-6) and low Yb (0.560×10^-6 to 0.757×10^-6) and low Y (7.97×10^-6 to 9.98×10^-6), contents with high Sr/Y ratios (73.84-109.98). However, the whole-rock Sr, Nd isotopic compositions of granodiorite-poryphy have initial ⁸⁷Sr/⁸⁶Sr ratios and ε_Nd(t) values are 0.707 878 and -8.26 respectively, with ancient Nd two-phase model ages of 1 503 Ma, which indicate that the magma sources may have contributed to partial melting of previous juvenile lower crust.Combined with igneous rocks in the studied area, it is proposed that Youqiumi granitic pluton was formed in post-collisional extension setting of India-Asia contional collision orogenic belt.