关键词: 硅质岩, 地球化学, 形成环境, 蛇绿混杂岩, 班公湖, 西藏

Abstract:  The ophiolite mélange is an ideal laboratory for studying the process of ocean-continent transition and plate tectonics to intracontinental tectonic transition. In this paper, the petrology and geochemical characteristics of major and rare earth elements of siliceous rocks were studied to judge the origin and forming environment of Bangong Lake ophiolite mélange in Tibet. The siliceous rocks occur as intercalated beds in Shamuluo Formation or fault-fragments in basalt, exhibit cryptocrystalline texture, bioclastic texture and massive structure, and contain abundant Late Jurassic-Early Cretaceous radiolarite. They have a SiO2 content of 73.80% to 90.44%,  and a high Al2O3 content of 3.67% to 12.33%. The ratios of MnO/TiO2, Fe2O3/SiO2, Fe2O3/TiO2, and Al2O3/(Al2O3+Fe2O3) range from 0.14 to 0.91, 31.44 to 141.88, 1.83 to 11.80, and 0.66 to 0.96, respectively. After standardization of rare earth elements in North American shale, it shows a flat rare earth distribution pattern, with Ce showing a negative anomaly overall and Eu showing a weak positive anomaly overall; δCe values is 0.77 to 2.03, δEu value is 0.99 to 2.11. The values of (La/Yb) N, (La/Ce) N, (La/Lu) N, (La/Sm) N, (Ce/Yb) N, and (Gd/Lu) N are 0.43 to 1.12, 0.51 to 1.27, 0.44 to 1.26, 0.59 to 1.12, 0.41 to 2.01, and 0.82 to 1.16, respectively. After standardization, the rare earth element chondrite shows a right-leaning rare earth distribution pattern, with overall Ce showing weak positive anomalies and overall Eu showing negative anomalies. The authors propose that the formation environment of the siliceous rocks is not a typical continental margin, but rather a certain distance from the continental margin between ocean basin and continental margin. In combination with the results of area studies, the formation environment of the Bangong Lake ophiolite mélange is preliminarily identified as a back arc ocean basin environment, which is the product of the northward subduction of the Bangong Lake-Nujiang Ocean basin in the Late Jurassic. 

Key words: siliceous rocks, geochemistry, forming environment, ophiolitic mélange, Bangong Lake, Tibet