GEOCHEMICAL CHARACTERISTICS AND GEOLOGICAL IMPLICATION OF THE MUDSTONE FROM THE 7 MEMBER OF THE YANCHANG FORMA-TION IN THE WUQI-ZHIDAN AREA,ORDOS BASIN
Through analysis and testing of major trace elements and rare earth elements,eighteen mud-stone core samples from the Chang 7 Member of the Upper Triassic(T3y7)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 T3y7 mudstone in the Wuqi-Zhidan area is primarily composed of SiO2 and Al2O3,with SiO2 being the dominant component(ranging from 44.13％to 60.88％,with an average of 53.13％),followed by Al2O3(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 T3y7 deposition in the Wuqi-Zhidan area.Paleo-salinity,ana-lyzed using B element content(ranging from 449×10-6 to 1 091×10-6,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 pa-leo-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 w(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-cli-mate.Graphical analysis of δEu values(ranging from 0.45 to 1.25,with a mean of 0.94)and La/Lb-ΣREE indicates that the parent rock of the mudstone was likely oceanic tholeiitic basalt.The analysis ofδCe 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.
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