Tectonic attribute of the Shiquanhe-Namco ophiolitic belt: Insights from geochemical characteristics of siliceous rocks in the Asa mélange zone, central Tibet

Mesozoic siliceous outcrops with ophiolite were found in Shiquanhe-Namco Ophiolitic Mélange Zone (SNMZ) and Bangong-Nujiang suture zone (BNSZ), which can reveal the sedimentary environment Bangong-Nujiang Ocean and relationship between BNSZ and SNMZ. In this paper, research on the siliceous rocks from the Asa area, located in the central segment of SNMZ, is reported, including field work, petrology, as well as major- and trace-element analysis. Moreover, the genesis and sedimentary environments of these siliceous rocks have also been analyzed. The Asa siliceous rocks consist of two types: red siliceous and black siliceous, which are exposed as rock blocks in the mélange belt. The SiO₂ content in red siliceous rocks is lower than that of pure siliceous rocks, while the red siliceous rocks are relatively enriched in Al. The rare earth element patterns of the red siliceous rocks are similar with flat model with a weak negative Ce anomaly after North American Shale standardization (NASS). The red siliceous rocks belong to the siliceous rocks of normal sedimentary genesis. So, the red siliceous rocks were formed in a continental margin basin environment with the addition of terrigenous materials in the source area. The black siliceous rocks are enriched in Fe and Mn, with a strong negative Ce anomaly after North American Shale standardization (NASS). The black siliceous rocks are of hydrothermal origin, influenced by basic pyroclastic materials and formed by hydrothermal activity near the continental margin. The geochemical characteristics of the siliceous rocks indicate that siliceous rocks in Asa ophiolitic mélange were formed in a limited oceanic basin environment close to the continental margin, which is consistent with siliceous rocks from other ophiolites in SNMZ. Unlike SNMZ, BNSZ not only contains siliceous rocks formed in the continental margin environment but also preserves siliceous rocks deposited in the pelagic environment. Therefore, the difference in the siliceous depositional environments between BNSZ and SNMZ suggests that SNMZ does not represent a tectonic nappe of BNSZ but is an in-situ ophiolitic mélange belt originated from a limited oceanic basin.