Geochemical characteristics, source area properties and tectonic significance of clastic rocks of Xilinkobo Formation in the south of Baiyunshan ophiolite melange belt, Beishan, Inner Mongolia

To reveal the polarity of the subduction of the Beishan ocean, the authors made a geochemical analysis of the sandstone of Xilinkebo Formation in Baiyunshan area of Beishan orogenic belt, Inner Mongolia.The results show that the rocks are mainly composed of feldspar quartz sandstone and feldspar lithic sandstone, with a small amount of lithic quartz sandstone, and all of them show weak metamorphism.The main mineral components are mainly quartz, plagioclase and K-feldspar.Average percentage content of major element oxides are SiO₂ 80.70%, Al₂O₃ 8.07%, MgO 1.08%, CaO 1.48%, TFe₂O₃ 1.86%, K₂O 1.12% and Na₂O 1.75%.The REE patterns of curves are consistent, with LREE/HREE being 11.32~16.54, 13.83 on average, and (La/Yb)_N being 7.38~12.39, 10.43 on average.Eu anomaly value is 0.57, LREE are obviously enriched relative to HREE, and REEs fractionate evidently.The CIA value of clastic sedimentary rocks in Xilinkebo Formation suggests that the clastic rocks of provenance in the study area were subjected to moderate chemical weathering under warm and wet conditions.Their parent rocks probably originated from acidic volcanic rocks or granites.Combined with the rock assemblage characteristics, tectonic combined characteristics and geochemical characteristics of Xilinkebo Formation, it is considered that the source rocks were formed in a tectonic environment of passive continental margin.It is revealed that the provenance might have come from the Tarim block, indirectly reflecting the polarity of northward subduction of Beishan Ocean.