关键词: 鄂尔多斯盆地, 东胜地区, 直罗组砂岩, 地球化学, 物源分析

Abstract:

The provenance analysis of sandstones are essential for the distribution and composition of sandbodies, reservoir properties and the sources of uranium, in the uranium deposits exploration. In this paper, detailed petrological and geochemical studies were carried out on the sandstones of Middle Jurassic Zhiluo Formation from 8 drilling cores in the northeastern Ordos Basin, in order to determine their provenance and tectonic setting. The samples are coarse greywacke with moderate to high SiO₂ content  and Al₂O₃/SiO₂ ratio, a high content of K₂O and a low content of Fe₂O₃, MgO and TiO₂, which implies that these sandstones are in felsic，lacking of mafic materials. The rocks are enriched in large ion lithophile elements (LILEs, such as K, Rb, Sr and Ba), and depleted in high field strength elements (HFSEs, such as Nb, Ta, Ti and P). These features indicate that the source region have affinity of subductionrelated tectonic setting. Similarly, other discrimination diagrams, such as (TFe₂O₃+MgO)-TiO₂,  (TFe₂O₃+MgO)-Al₂O₃/SiO₂, Th-Sc-Zr/10, Th-Co-Zr/10, further suggest that the Zhiluo Formation sandstones were deposited as detritus from a continental island arc or active continental margin. Together with other characteristics, such as enrichment LREE, relatively flat HREE patterns, without or slightly Eu anomalies, flat patterns with positive Eu anomalies in the PAAS-normalized rare element plots, allowed us to conclude that the rocks are strongely related to the Phanerozoic granitoids from the northern margin of North China craton.

Key words: Ordos Basin, Dongsheng district, Zhiluo Formation sandstone, geochemistry, provenance analysis