1. Institute of Mineral Resources, Chinese Academy of Geological Science, Beijing 100037, China;
2. MNR Key Laboratory of Metallogeny and Mineral Assessment, Beijing 100037, China;
3. College of Earth Sciences, Jilin University, Changchun 130061, China;
4. Geochemistry Exploration Brigade of Sichuan Bureau of Exploration and Development of Geology and Minerals Resources, Deyang 618000, Sichuan, China;
5. College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China

A Middle-Late Jurassic magmatic belt has been newly identified in the southern margin of the Bangonghu-Nujiang junction zone, its forming ages is not Early Cretaceous that was defined by the predecessors. This discovery provides new information for re-defining the subduction polarity and process of the Bangonghu-Nujiang Tethys Ocean. Taking the Geji pluton in Ali area of Tibet as a research target, the authors camed out studies on rock geology and petrography, zircon U-Pb dating, geochemistry and whole rock Sr-Nd isotopes. The results show that Geji pluton consists of quartz diorite, in which mafic microgranular enclaves (MMEs) are well developed. The zircon U-Pb ages of the Geji quartz diorite and MMEs are 153 ±2 Ma and 150 ±2 Ma, respectively, which are consistent within test error, representing contemporaneous magmatic event. Geochemical characteristics show that Geji quartz diorite belongs to a series of calc-alkaline metaluminous rock, whereas the MMEs are tholeiite series metaluminous rock. The Geji quartz diorite and MMEs have similar REEs and trace elements distribution patterns, but MMEs contain lower content of Nb, Ta, Zr, Hf and higher total rare earth. The (⁸⁷Sr/⁸⁶Sr)_i values of quartz diorite are between 0.714 2 and 0.714 4, ��_Nd(t)=-12.0~-14.0, and the MMEs have the (⁸⁷Sr/⁸⁶Sr)_i values of 0.712 9~0.714 4 and the ��_Nd(t) values of -11.4~-13.8. Combined with petrography and geochemical characteristics, the host quartz diorite and MMEs from Geji pluton are believed to be the results of mixing of mafic magma derived from enriched mantle with felsic magma derived from ancient crust in different proportions. Based on regional tectonic evolution, Geji pluton may be formed in the background of the arc-continent soft collision between Shiquanhe archipelagic arc-basin system and ancient Lhasa block, associated with the southward subduction of Bangonghu-Nujiang Thethys ocean.