Zircon U-Pb dating, petrogenesis and tectonic significance of the Middle Jurassic granodiorite in Zhiruo area, western Lhasa block.

The Jurassic magmatism in the western Lhasa block of the Qinghai-Tibet Plateau is of great significance for discussing the subduction evolution history of the Neo-Tethys Ocean. This paper reports LA-ICP-MS zircon U-Pb age, zircon Hf isotope and whole-rock geochemical composition of granodiorite in Zhiruo area, western Lhasa block. The results show that the weighted average zircon ²⁰⁶Pb/²³⁸U ages of the two granodiorite samples are 165.9±1.3 Ma and 160.3±1.2 Ma, respectively, belonging to the Middle Jurassic. The samples have the characteristics of high SiO₂, Al₂O₃, rich alkali, low P₂O₅, TiO₂, and the aluminum saturation index A/CNK is 0.9~1.3. The sample (S19T13) belongs to the weakly peraluminous shoshonite series I-type granite, and the sample (S21T12) belongs to the quasi-aluminum medium potassium calc-alkaline I-type granite. The content of rare earth elements in the samples is generally between (98~186) × 10^-6, with obvious negative Eu anomaly (Eu/Eu* value of 0.18~0.28), loss of high field strength elements such as Nb, Ta, P, Ti, enrichment of large ion lithophile elements such as Rb, Th, U, Pb and light rare earth elements, with the characteristics of arc magmatic rocks. The granite diorite has depleted zircon Hf isotope composition (ε_Hf(t)= -15.2~-13.0), and the crustal model age (T_DM^C) is 2030 ~ 2171 Ma, indicating that the magmatic material is derived from the partial melting of the ancient crust, and the fractional crystallization dominated by hornblende occurred during the ascending process, and finally formed a series of intermediate-acid rocks in Zhiruo area. Comprehensive studies have shown that the formation of this set of rocks may be related to the northward subduction of the Neo-Tethys Ocean.