Geochemical characteristics of marine siliceous rocks in the western Yunnan Paleo-Tethys orogenic belt and their palaeoenvironmental implications

Many siliceous rocks representing different geologic environments are exposed in the Palaeo-Tethys orogenic belt of western Yunnan, China. In this paper, silicon and oxygen isotopes and major and trace elements including rare earth elements (REEs) of siliceous rocks collected from the region are analysed. According to geochemical discriminant proxies, such as Al/(Al + Fe + Mn), REE patterns, the Al-Fe-Mn and δ30Si-Al2O3 diagrams, most siliceous rocks in this study are found to have derived from marine biogenic silica, and a few samples were affected by hydrothermal activity. The redox-sensitive trace elements, such as V, U, Ni and Mo, of the siliceous rocks suggest that the water body of the Palaeo-Tethys oceanic basin in western Yunnan experienced great changes between oxic and anoxic environments during its evolution. The major element discriminant diagrams and the δCe, LaN/CeN and δ30Si values indicate that the basin of the Palaeo-Tethys was limited and difficult to match with an open ocean basin. The Palaeo-Tethys oceanic basin may have been delimited by several islands with poor circulation of seawater, which is called an archipelagic ocean. The δ30Si values of siliceous rocks deposited in the archipelagic ocean range from 0.3‰ to 0.8‰, which is narrower than the range of radiolarian siliceous rocks reported previously. Unlike typical shallow sea, hemipelagic and pelagic environments, the silicon isotope characteristics of siliceous rocks in the archipelagic ocean do not show a clear trend from shallow sea to deep sea.