Abstract: LA-ICP-MS U-Pb dating and Hf isotope compositions of the zircon from granite at Puluogang Town, Namulin County, in the middle Gangdese belt, Tibet, have been studied. LA-ICP-MS U-Pb dating of zircons form two granite samples yielded weighted mean ages of  (44.3±1.1) Ma and (44.5±1.1) Ma, representing crystallization age of the granite. Geochemical data show that these samples belong to high-K calc-alkaline series, with SiO₂ contents of 60.53%-65.28%, total alkali (K₂O+Na₂O) contents of 6.53%-7.98%, Al₂O₃ contents of 15.52%-16.13%, and A/CNK values of 0.86-0.91. The granitic samples are characterized by enrichment of the light REE, clearly negative Eu anomaly, enriched in Rb, Th, U and Pb, and depleted in Nb, Ta and Ti. Two granite samples have high zircon ε_Hf(t) values of 8.8-11.1, and ¹⁷⁶Hf/¹⁷⁷Hf values of 0.282 994-0.283 060, with Hf crustal model ages of 422-557 Ma. These geological data indicate that partial melting of newly crustal materials has contributed to the magma origin of the granite. Therefore, we can draw a conclusion that the granitic pluton were probably formed by the northward subduction of new Tethys and slab rollback and break-off.

Key words: Gangdese, granite, zircon LA-ICP-MS U-Pb dating, Hf isotope, geochemistry