Petrogenesis and Geological Significance of the Jasacuo Monzogranite, Western Gangdese Belt, Southern Tibet: SIMS Zircon U-Pb Chronological and Whole-Rock Geochemical Constraints

Early Cretaceous magmatism in the western segment of the Gangdese belt is less well constrained than that in the central and eastern segments. This study presents petrography, whole-rock geochemistry, and SIMS zircon U–Pb geochronology for the Jasacuo monzogranite in Zhongba County, southern Tibet. Zircons are euhedral and show oscillatory zoning; 17 concordant analyses yield a weighted mean 206Pb/238U age of 101.4 ± 0.8 Ma (MSWD = 1.01), indicating crystallization in the late Early Cretaceous. The rocks are characterized by high SiO2 (63.73–77.11 wt.%), high K2O, low MgO, TiO2, and P2O5, and A/CNK values of 0.92–1.08, indicating metaluminous to weakly peraluminous, high-K calc-alkaline compositions with I-type affinity. Chondrite-normalized REE patterns show LREE enrichment and negative Eu anomalies, whereas primitive-mantle-normalized trace-element patterns display enrichment in Rb, U, Th, and Pb and depletion in Ba, Nb, Sr, Zr, and Ti. These features indicate that the Jasacuo monzogranite is an evolved felsic intrusion generated in a subduction-related continental-arc setting associated with northward subduction of the Neo-Tethyan oceanic lithosphere. The magma was dominated by crustal components and underwent significant fractional crystallization, mainly involving feldspar, with minor biotite and amphibole.