Geochemical Characteristics And Tectonic Evolution Study In The Amdo And Dengqen Segments Of Bangong-Nujiang Suture Zone

Bangong—Nujiang suture zone(BNSZ) has important implications for tectonic evolution of the Tibetan Plateau. However, the development time and tectonic evolution model of the Tethys ocean in the Amdo and Dengqen segments of Bangong—Nujiang suture zone exist intense debat; In addition, the BNSZ is also a lack of reported subduction sensitive island arc volcanic rocks. In this study, the Amdo and Dengqen ophiolites and associated island arc volcanic rocks in the middle—west BNSZ are investigated by detailed field geological survey and studied by indoor mineralogy, petrology. Using LA–ICP–MS zircon U–Pb geochronology and zircon Lu–Hf isotope in situ testing,Whole-rock analysis of major and trace elements and Sr–Nd isotope, in combination with regional geological results,to explore the framework of space-time’s evolvement and the tectonic environment for the Amdo and Dengqen ophiolites, reveal the meaning of island arc volcanic rocks for the subduction of the Bangong—Nujiang Tethys ocean(BNTO). The paper make the following main conclusions:(1) The zircon U–Pb age of Amdo ophiolite diabase is 183.4 ± 2.5Ma, the zircon U–Pb ages of Dengqen ophiolite gabbro and gabbro are 175.3 ± 2.5Ma and 163.1 ± 1.8Ma, respectively, which show Bangong—Nujiang Tethys ocean in the Amdo and Dengqen segments mainly developed in the middle—late Jurassic.(2) In Dengqen area, a combination of oceanic island is found, its geochemical characteristics are similar to the typical ocean island basalts, Sr–Nd isotopes indicate that magma derive from the depleted mantle. Combined with regional information, We speculate that its age is likely to early Cretaceous, which indicates Tethys ocean in the Dengqen segment is not closed in the early Cretaceous.(3) The element geochemical characteristics of Amdo and Dengqen ophiolites show SSZ–type features, Sr–Nd isotopic and zircon Hf isotope in situ indicate the magma of Amdo and Dengqen basic rocks derive from depleted mantle.(4) In Amdo area, the geochemical characteristics of intermediate-acidic volcanics show island arc features, Sr–Nd isotopic and zircon Hf isotope in situ show their magma derive from lithospheric mantle. The zircon U–Pb age of dacite is 179.7 ± 4.4Ma, these provide direct evidence of the volcanic rocks for the northward subduction of the Bangong—Nujiang Tethys ocean in the early Jurassic.(5) By analyzing element geochemical characteristics of Daru Tso volcanics, a set of high-Mg andesites is identified, and the zircon U–Pb age of high–Mg andesite is 163.3 ± 1.7Ma, these provide direct evidence of the volcanic rocks for the southward subduction of the Bangong—Nujiang Tethys ocean in the middle—late Jurassic.(6) Bangong—Nujiang Tethys ocean exists intra-oceanic subduction during the the Jurassic(~190–163Ma) and double–direction subduction from the Late Jurassic to the Early Cretaceous(~163–108Ma).