Geochemical Characteristics and Their Indication to Weathering and Provenance of Pisha Sandstone from Lower Jurassic Yan'an Formation in Jungar Banner,Inner Mongolia Autonomous Region
[Objective]The weathering degree and provenance of the Pisha sandstone from the Lower Jurassic Yan'an formation in Jungar,Inner Mongolia Autonomous Region was analysed in order to provide a basis for further research on the origins and epigenetic evolution of the Pisha sandstone,and the prevention and control of soil erosion.[Methods]We conducted field investigation and geochemical analyses,including major,trace,and rare earth element analyses,and calculated the chemical alteration index(CIA),plagioclase alteration index(PIA),light-to-heavy rare earth element ratio(LREE/HREE),and related trace and rare earth element relationships in the Pisha sandstone.[Results]① The geochemical classification diagram showed that the Pisha sandstone in Jungar comprised shale,subarkose,and lithic sandstone facies.The Pisha sandstone was mainly composed of SiO2 and Al2O3.The content of most of the major elements was lower than that in the average upper crust(UCC).The CIA value ranged from 64.95 to 86.57,with an average of 77.46.This was consistent with the characteristics of the Pisha sandstone from the Middle Triassic Ermaying Formation near the study area,reflecting a common history of rock weathering.② The siltstone and mudstone facies had a higher trace element content than the sandstone facies,with Ba being the most abundant element,followed by Zr.The paleotemperature of the deposition of the Pisha sandstone was 10.67-22.78 ℃,with an average value of 17.68 ℃;the PIA value was 70.48-97.13,with an average of 89.76.③ The rare earth element distribution pattern in the Pisha sandstone was consistent with that in the UCC,showing the characteristics of light rare earth enrichment,heavy rare earth depletion,and weak negative Eu anomalies.The total rare earth(ΣREE),light-to-heavy rare earth ratio(LREE/HREE)and(La/Yb)N values of the siltstone and mudstone facies were higher than those of the sandstone facies.This difference was mainly due to the attraction of most of the trace elements released by rock weathering during epigenetic processes by clay minerals,or due to hydraulic sorting.[Conclusion]The Pisha sandstone from the Lower Jurassic Yan'an Formation in Jungar,Inner Mongolia,consists of terrestrial clastic sedimentary rocks with a low compositional maturity and sorting,sourced from an area that had experienced moderate to strong chemical weathering.The Pisha sandstone formed under arid to semi-humid conditions.The rare earth element distribution pattern combined with the discrimination diagram of major and trace elements indicates that the Pisha sandstone from the Yan'an Formation originated from Hercynian-Indosinian granites in the Yinshan-Daqingshan area,in the Northern Ordos Basin.
万方数据
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