Analysis of trace elements and rare earth elements（REEs）in sedimentary rocks is an effective way to reconstruct the depositional environment and the tectonic settings, due to very high sensitivity of elements to the variation of depositing environment. In this study, twenty-five samples of mudstone, siltstone and fine-grained sandstone were collected from different parts and layers of the Late Triassic Yanchang Formation in the Helan Mountain. These samples were analyzed on major and trace elements and REEs in order to give clues of the depositional environment and tectonic settings. The results show that the Yanchang Formation generally deposited in an oxidation-reducing transitional fresh water environment under a warm-humid paleoclimate. Chondrite-normalized REE patterns show that the rocks are enriched in light REEs and depleted in heavy REEs, with negative Eu anomaly and weak Ce anomaly. Both La/Yb-ΣREE and La/Th-Hf discrimination diagrams and the Gd/Yb ratios indicate that the source rocks of the Yanchang Formation are mainly composed of post-Archean felsic rocks, with slight contribution of basic rocks and sedimentary rocks. According to discrimination diagrams of La-Th-Sc, Th-Sc-Zr/10 and Th-Co-Zr/10, source materials of the Yanchang Formation were likely deposited in a continental island arc setting and clastic materials from an active continental margin were partly added during the late depositional period.
Peng Zhichao Zhao Hongge Li Yanan Lei Linlin Fu Xinghui Li Meng. Geochemical characteristics of the Late Triassic Yanchang Formation in the Helan Mountain area and their geological significance[J]. Chinese Journal of Geology, 2018, 53(3): 908-921.