Geochemical Characteristics and Its Geological Significance of the Lower Cretaceous Zenong Group Volcanic Rocks of Geji Country, Tibet

The Zenong Group is exposed widely in the Geji area of Tibet and is composed of Duo'ai Formation, Tuo 'cheng Formation, and Langjiu Formation from bottom to top. This study carried petrography, geochemistry, and chronology on the Zenong Group volcanic rocks. It was found that these volcanic rocks show obvious eruption unconformity and pulsed magmatism. According to the major element composition, the igneous rocks of the Duo'ai, and Tuo 'cheng Formations belong to the alkaline-calc-alkaline series, while the rocks of the Langjiu Formation belong to the alkaline series. The trace elements, REE distribution, and tectonic discrimination diagram show that the Duo'ai, and Tuocheng Formation formed in the island arc environment, while the Langjiu formed in the island arc environment near the active continental margin. The average weighted zircon U-Pb age of the hornblende andesite from the Duo'ai Formation is （111.2±2.1）Ma. Combination with previous work and this study, we suggest the Zenong Group volcanic rocks are the marginal continental arc of the southward subduction of the Slainajap ocean basin represented by the Shiquanhe-Yongzhu-Namco-Jiali ophiolitic melange belt.