松辽盆地南部海力锦地区上白垩统姚家组砂岩物源示踪及其构造背景综合研究:来自岩石地球化学及锆石U-Pb年代学的制约

Provenance and tectonic setting of the Upper Cretaceous Yaojia Formation sandstones in the Hailijin area,southern Songliao Basin:Constraints from petrogeochemistry and zircon U-Pb chronology

针对可地浸砂岩型铀矿潜在铀储层预测、铀源评价而言,明确赋矿砂体的物源是关键。海力锦铀矿床位于松辽盆地南部,赋矿砂体为上白垩统姚家组灰色砂岩。系统采集了该矿床内16个钻孔中的26件样品,首次对姚家组灰色、褐红色、褐黄色砂体展开了碎屑岩岩石学、地球化学及锆石年代学分析工作。测试结果表明,这套多色砂岩在铀工业矿孔、矿化孔及无矿孔中整体表现较为一致,均具高SiO_(2),相对较低的Al_(2)O_(3)、Fe _(2)O _(3)、MgO、MnO、Na_(2)O、TiO _(2),地球化学分类属于岩屑砂岩、亚岩屑砂岩类,指示源岩可能为酸性岩类;化学蚀变指数(CIA)值为62.93~83.46(平均74.08),成分变异指数(ICV)值为0.4~1(平均0.6),推测其经历过半干旱-半潮湿气候背景下的中等化学风化作用影响;根据源岩属性判别其母岩主要为长英质(酸性)岩石,且源区构造背景属于被动大陆边缘环境;碎屑锆石U-Pb测年显示谐和年龄主要集中分布在3个年龄区间,分别为180~110,320~230,2536~1738 Ma。结合岩石地层划分、盆缘岩浆岩年代以及区域构造演化特征,推断海力锦地区姚家组下段沉积物最有可能来源于大兴安岭南缘的林西地区。

[Objective]Tracing the provenance of host clastic rocks is key for understanding uranium reservoir projection and uranium source evaluation of alluvial uranium ore.[Methods]The Hailijin uranium ore,of which the host rock is the Upper Cretaceous Yaojia Formation,is located in the southern part of Songliao Basin,northeastern China.This paper performed detailed petrography,whole-rock geochemistry and detrital zircon U-Pb geochronology of sandstone in the Yaojia Formation.[Results]Twenty-six samples collected from 16 drill holes display high contents of SiO_(2) and relatively low contents of Al_(2)O_(3),Fe _(2)O _(3),MgO,MnO,Na_(2)O and TiO _(2),which plot in the area of lithic sandstone to sublithic sandstone,thus indicative of source rocks as the felsic components.The chemical index of alteration values range from 62.93 to 83.46,while the index of compositional variability values range from 0.4 to 1,which corresponds to moderate chemical weathering in a semiarid to semihumid climatic setting.The geochemical discriminant diagram shows that the tectonic background of the Yaojia Formation is a passive continental margin.Zircon U-Pb ages are mainly distributed at 180-110 Ma,320-230 Ma and 2536-1738 Ma.[Conclusion]Combined with stratigraphy,magmatic records around the Songliao Basin and regional tectonic evolution,this study suggests that the sediments of the Yaojia Formation in the Hailijin were sourced from the Linxi area of the southern Great Xing′an Range.