内蒙古准格尔旗下侏罗统延安组砒砂岩地球化学特征及其对风化、物源的指示

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

[目的]研究砒砂岩的风化程度和物源特征,为砒砂岩的深入研究及岩土侵蚀防治提供基础理论依据。[方法]选取内蒙古准格尔旗下侏罗统延安组砒砂岩为研究对象,通过野外调查及试验研究,利用主量元素、微量元素及稀土元素地球化学方法对砒砂岩化学蚀变指数(CIA)、斜长石蚀变指数(PIA)、轻重稀土元素比(LREE/HREE)及相关微量、稀土元素关系进行了分析。[结果](1)准格尔旗下侏罗统延安组砒砂岩以SiO_(2)和Al_(2)O_(3)为主,大部分主量元素含量低于平均上地壳标准化值(UCC);地球化学分类图显示砒砂岩主要为页岩、亚长石砂岩和岩屑砂岩;砒砂岩CIA值为64.95~86.57,平均值为77.46。这与研究区周边的中三叠统二马营组特征较为一致,反映经历共同的岩石风化历史。(2)砒砂岩中粉砂岩和泥岩中各微量元素含量均高于砂岩,其中Ba含量最高,Zr次之;砒砂岩形成古温度为10.67~22.78℃,平均17.68℃,PIA值为70.48~97.13,平均值为89.76。(3)砒砂岩稀土元素配分模式与UCC一致,整体呈现轻稀土富集、重稀土亏损、弱负铕异常(δEu)特征;粉砂岩和泥岩总稀土(∑REE)、轻重稀土比(LREE/HREE)、(La/Yb)_(N)值均高于砂岩。这种分异主要是由于表生作用中岩石风化溶出的主要微量元素大多被黏土矿物吸附或水力分选所致。[结论]内蒙古准格尔旗下侏罗统延安组砒砂岩为成分成熟度、分选程度较低的陆源碎屑沉积岩,源区经历了中等—强烈的化学风化;砒砂岩形成时为干旱—半湿润的古气候条件;砒砂岩稀土元素配分模式结合主、微量元素判别图指示延安组砒砂岩来自鄂尔多斯盆地北部阴山—大青山地区的海西—印支期花岗岩类。

[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 SiO_(2) and Al_(2)O_(3).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(∑REE),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.