Reconstruction of provenance and tectonic setting of the Middle Jurrasic Yan'an Formation (Ordos Basin, North China) by analysis of major, trace and rare earth elements in the coals

Abstract

The Middle Jurassic Yan'an Formation in the Ordos Basin (North China) is an important coal-bearing succession. Twenty coal samples and four samples of silty mudstones were collected from Coal Seam 2 of the Yan'an Formation in order to determine major, trace and rare earth elements; moreover, the geochemistry of the rare earth elements, and in particular yttrium, was investigated. The major elements investigation reveals that Si, Al, Na, K, and Ti are primarily found in clay minerals. Calcium is primarily found in calcite and iron is largely controlled by sulfur. The average REE (rare earth element) contents in the coals and silty mudstones are 15.17 μg/g and 210.57 μg/g, respectively. The Upper Continental Crust-normalized distribution patterns of the REE in the coals and silty mudstones show no obvious fractionation; however, they show negative Eu anomalies and negligible Ce anomalies. Correlations between the REE and the major elements indicate that the REE in the coals and silty mudstones are both derived from eroded rocks in the hinterland and mainly controlled by the presence of detrital minerals. We investigated some diagrams of major element, trace element, and REE from the coal seam to comprehend its provenance and tectonic setting. The findings indicate that they are mainly derived from felsic and intermediate rocks in a continental island arc setting. The REE analysis shows that various provenance areas supplied clastics when coal developed in the Yan'an Formation. The findings imply that provenance analysis should be taken into account when studying the sedimentary environments of coal-bearing successions.