Abstract: The Jigongcun molybdenum mine is located in the southern part of the GangdiseXiachayu late YanshanHimalaya magmatic arc zone. In this paper, we studied the intermediate-acid rocks widely distributed in the mining area and carried out a systematic investigation on field geological survey, petrology, LA-ICP-MS zircon U-Pb dating and geochemical characteristics of these rocks. The findings indicate that the Jigongcun intermediate-acid rocks are mainly quartz diorite and some granodiorite. The rocks LA-ICP-MS zircon U-Pb age of 51.8±0.5 Ma suggests they were formed in the Eocene (E2) period. They have low K (K2O content ranges 1.66%2.44%) and high Na (Na2O content ranges 3.58%4.06%) contents and belong to calc-alkaline-high potassium calc-alkaline series with quasi-aluminum-weak aluminous characteristics. The ΣREE contents vary greatly (53.6117)×10-6 with light REE enrichment, represented by right-inclined REE distribution mode, and (La/Yb)N value of 3.907.31. No obvious Ce and Eu anomalies were observed (Ce/Ce*=0.921.17; Eu/Eu*=0.961.15), while the trace-element spider diagram shows positive Rb, Th, U and K and negative Ba, Nb, Ta, P and Ti anomalies. The Jigongcun intermediate-acid rocks belong to I-type granite, and formed from the partial melting of the lower basaltic crust due to thickening of the crust, in the subduction and collision environment of the Indo-Eurasian plate.

Key words: intermediate-acid intrusive rocks, zircon U-Pb dating, rock geochemistry, tectonic setting, Jigongcun molybdenum mining area, Tibet