Petrology, geochemistry and sedimentary environment of Permian siliceous rocks in Yingen-Ejin basin and its adjacent areas

The geochemical characteristics and depositional environment of Permian siliceous rocks were studied based on the petrological characteristics, major elements and rare earth elements. According to the test results, Hangwula sectional siliceous rocks' Al/(Al+Fe+Mn)values are 0.68~0.78, with an average of 0.73; δCe values are 0.89~0.94, with an average of 0.90; Erduanjing sectional siliceous rocks' Al/(Al+Fe+Mn)values are 0.50~0.67, with an average of 0.60; δCe values are 0.85~0.90, with an average of 0.88. In the Al-Fe-Mn triangular diagram, samples fall into biochemically-deposited area, which indicates a biochemically-deposited origin for these rocks. Hangwula sectional siliceous rocks' Al₂O₃/(Al₂O₃+Fe₂O₃)values are 0.78~0.86, with an average of 0.83; MnO/TiO₂ values are 0.04~0.40, with an average of 0.16;(La/Ce)_N values are 1.02~1.15, with an average of 1.10; Ce_anom values are -0.041~-0.001, with an average of -0.029. Erduanjing sectional siliceous rocks' Al₂O₃/(Al₂O₃+Fe₂O₃)values are 0.82~ 0.91, with an average of 0.87; MnO/TiO₂ values are 0.05~0.47, with an average of 0.24;(La/Ce)_N values are 1.00~1.12, with an average of 1.06; Ce_anom values are -0.058~-0.033, with an average of -0.045. All these geochemical characteristics suggest that these siliceous rocks were deposited in a continental margin setting where the sea water in the period of deposition was under an anoxic condition.