By analyzing the petrological and geochemical characteristics of the clastic rocks of the Sifangtai Formation in the south end of the Changyuan trap in the Songliao Basin, the compositional characteristics and the tectonic background of the provenance area of the Sifangtai clastic rocks in the study area were discussed. The Sifangtai Formation sandstone consists mainly of feldspar sandstone with medium structure and composition maturity. The detrital particles are relatively low in quartz (Q), rich in debris (L) and feldspar (F). The average contents of quartz, debris and feldspar are 42.10%, 37.83%, and 20.07% respectively, and the average Q/(F+L) ratio is 0.80. The heavy mineral assemblage including garnet, zircon, magnetite, and epidote indicates that the source rocks are mainly acidic magmatic and metamorphic rocks. Dickinson discriminant diagram indicates that the sandstone mainly derived from recirculation orogenic belt and magmatic arc. Clastic rocks of the Sifangtai Formation have high Si content and low Fe and Mg contents. The rocks have SiO2 contents of 60.42%-83.39%, low K2O/Na2O ratios (0.91-1.52) and (TFe2O3+MgO) values (1.11%-9.14%), and are characterized by high Rb, Sr, Ba, Pb, and U, low Sc, V, Cr, Co, Ni and Th, Zr, Hf, and Nb, with ΣREE of 66.6×10-6 to 236×10-6. The clastic rocks have chondrite normalized REE patterns resemble that of the upper crust, showing light rare earth enrichment, flat heavy rare earth, and medium negative Eu anomalies. The chemical alteration index CIA is 46.72-64.49, with an average of 56.54, and the component variation index ICV is 0.99-1.39, with an average of 1.12, indicating that the provenance area of the clastic rocks experienced weak weathering. The provenance is mainly the first deposition in the structural zone without sedimentary recycling characteristics. Tectonic environment discrimination diagrams and characteristic ratios of major elements and trace elements show that the tectonic environment of the Sifangtai Formation is active continental margin and continental island arc. The discrimination diagrams show that the source rocks are mainly felsic volcanic rocks, mixed with medium acid or basic volcanic rocks. Based on the regional tectonic evolution, it is believed that the clastic materials of the Sifangtai Formation at the southern end of the Changyuan trap mainly came from the Mesozoic granites, volcanic and metamorphic rocks developed in the tectonic mixed zone of Zhangguangcailing and Eastern Jihei. © 2021, Science Press. All right reserved.
