Abstract: The Gezhang area of Namling County, Tibet is structurally located in the southern part of the middle Gangdese belt (i.e., south-middle Lhasa block) in the Geji-Shenzha back-arc basin. In this paper, we studied the volcanic rocks exposed in the Gezhang area through petrology, LA-ICP-MS zircon U-Pb dating and elemental geochemistry researches and Lu-Hf isotope analysis. The results show that the volcanic rocks in the Gezhang area comprises mainly a set of acidic volcanic clastic rocks and minor volcanic lava. The rock assemblage consists of primarily rhyolitic tuff and ignimbrite (both contain breccia crystal fragments) and small amounts of rhyolite and basalt. Typical magmatic zircon developed in volcanic rocks. Through LA-ICP-MS U-Pb dating of zircons from rhyolite and rhyolitic ignimbrite samples, we obtained rock ages of 133±1.7 Ma and 128±1.5 Ma, confirming the volcanic rocks are formed in the Early Cretaceous (K1). Major elemental analysis shows high SiO2 (69.4%75.8%), K2O (2.93%5.58%) and Al2O3 (12.7%14.6%) and low TiO2 (0.20%0.34%) and Na2O (0.05%2.5%) contents. The volcanic rock is weakly peraluminous to peraluminous (A/NK=0.921.83, A/CNK=1.063.59) and of high potassium calc-alkaline series. The rare earth element (REE) partition mode shows strong light REE-enriched right-sloping type. The (La/Yb)N ratio is 12.3 to 22.0, with obvious Eu negative anomalies (Eu/Eu*=0.470.62). Trace elements generally exhibit positive anomalies of Rb, Th and K, and negative anomalies of Ba, Ta, Nb, Sr, P and Ti. Volcanic rocks have εHf(t) values concentrated between -12.4 and -7.2 and two-stage crust Hf model ages (tDM2) concentrated between 1.5 and 1.7 Ga, equivalent to the geological age of the Nyainqentanglha Group (AnZNq). The Early Cretaceous volcanic rocks in the Gezhang area, therefore, should result from partial melting of the Nyainqentanglha Group (AnZNq) at the base of middle Lhasa block with minor mantle participation, and formed in the continental-margin magmatic arc environment created by the southward subduction of the Bangong-Nujiang plate.

Key words: Early Cretaceous volcanic rocks, zircon U-Pb ages, elements and Hf isotope composition, petrogenesis, Gezhang area, Tibet