鄂尔多斯盆地吴旗-志丹地区延长组长7段泥岩地球化学特征及其地质意义

GEOCHEMICAL CHARACTERISTICS AND GEOLOGICAL IMPLICATION OF THE MUDSTONE FROM THE 7 MEMBER OF THECYANCHANG FORMATION IN THE WUQI-ZHIDAN AREA,ORDOS BASIN

应用地球化学特征来识别沉积环境有着重要的地质意义。为了研究鄂尔多斯盆地吴旗一志丹地区延长组长7段泥岩地球化学特征及其地质意义,以W100、W336井钻井岩心为主,选取了18个泥岩样品,开展了主微量元素以及稀土元素的分析测试,探讨其古氧化还原性、古盐度、古水深、古气候、母岩来源等地球化学特征。研究结果表明:吴旗一志丹地区延长组长7段泥岩主量元素含量由SiO_(2)与Al_(2)O_(3)占据了主体,SiO_(2)质量分数最高(44.13%~60.88%,平均为53.13%),其次为Al_(2)O_(3)(13.47%~19.62%,平均值为17.64%);泥岩样品的V/(V+Ni)比值(0.50~0.88,平均值为0.73)和Ceanom值(-0.20~0.06之间,平均值为-0.04)反映了吴旗一志丹地区延长组长7段总体上呈还原环境;利用B元素含量(449×10^(-6)~1091×10^(-6),平均含量为796.28×10^(-6))和Sr/Ba比值(0.08~0.41,平均值为0.21)分析其古盐度,可知从长73到长72逐渐下降,从长72到长71又逐渐上升;Fe/Mn比值变化和钴元素质量分数w(Co)结果综合指示了长7段古水深整体呈深水环境;Sr元素含量(60.4×10^(-6)~370.0×10^(-6),平均值为172.24×10^(-6))及Sr/Cu比值结果(平均值为5.15)反映了古气候主要为温湿气候;利用δEu值(0.45~1.25,均值0.94)和泥岩La/Lb-ΣREE图解分析其母岩来源为大洋拉斑玄武岩,并利用δCe值(0.50~1.07,平均值0.73)分析其母岩所处的构造背景为大洋盆地-大陆边缘区之间的区域。

Through analysis and testing of major trace elements and rare earth elements,eighteen mudstone core samples from the Chang 7 Member of the Upper Triassic(T_(3)y^(7))in wells W100 and W336 in the Wuqi-Zhidan area of the Ordos Basin were selected to study their geochemical characteristics.These characteristics include paleo-reduction,paleo-salinity,paleo-water depth,paleo-climate,and parent rock source.The results indicate that the major element content in the T_(3)y^(7) mudstone in the Wuqi-Zhidan area is primarily composed of SiO_(2) and Al_(2)O_(3),with SiO_(2) being the dominant component(ranging from 44.13%to 60.88%,with an average of 53.13%),followed by Al_(2)O_(3)(ranging from 13.47%to 19.62%,with an average of 17.64%).The V/(V+Ni)ratio(ranging from 0.50 to 0.88,with a mean of 0.73)and the Ceanom value(ranging from-0.20 to 0.06,with a mean of-0.04)of the mudstone samples indicate a generally reductive environment during the T_(3)y^(7) deposition in the Wuqi-Zhidan area.Paleo-salinity,analyzed using Belement content(ranging from 449×10^(-6) to 1091×10^(-6),with an average of 796.28×10^(-6))and Sr/Ba ratio(ranging from 0.08 to 0.41,with an average of 0.21),suggests a gradual decrease in paleo-salinity from Chang 73 to Chang 72,followed by a gradual increase from Chang 72 to Chang 71.The Fe/Mn ratio and cobalt mass fraction zw(Co)indicate that the paleo-water depth of T3y7 was generally in a deep-water environment.The Sr element content(ranging from 60.4 ×10^(-6) to 370.0×10^(-6),with a mean of 172.24×10^(-6))and Sr/Cu ratio(averaging 5.15)reflect a predominantly warm and humid paleo-climate.Graphical analysis of 3Eu values(ranging from 0.45 to 1.25,with a mean of 0.94)and La/Lb-ZREE indicates that the parent rock of the mudstone was likely oceanic tholeitic basalt.The analysis of Ce values(ranging from 0.50 to 1.07,with a mean of 0.73)reveals that the tectonic background of the parent rock lies between an oceanic basin and a continental margin.