Characteristics of Strontium Isotopic Geochemistry of Sublacustrine Hydrothermal Sedimentary Rock of Xiagou Formation in Qingxi Sag, Jiuquan Basin

Key words: 
• Sr isotope /
• lacustrine hydrothermal sedimentary rock /
• hydrothermal fluid /
• lower Cretaceous /
• Jiuquan Basin

Abstract: Based on the analysis of geologic background, hydrothermal material components and rock fabric, we carry out the study of Sr isotopic geochemistry on the lacustrine hydrothermal sedimentary albiteankerite rock of Xiagou Formation, Lower Cretaceous in Qingxi Sag, Jiuquan Basin. The 87Sr／86Sr ratio of hydrothermal sedimentary rock in the study area is 0.712 25～0.717 81 and the average value
0.715 61 is much higher than the 87Sr／86Sr ratio of seawater and basalt of corresponding period and a little higher than the 87Sr／86Sr ratio of the algal limestone, which represents the strontium isotopes composition of lake water in early cretaceous, but lower than the 87Sr／86Sr ratio of underlying siliconaluminum rock. It shows that the thermal fluid is not simplex lake water or mantle magmatic water. The laminated mudmicrocrystallite hydrothermal sedimentary rock is the product of hydrothermal mineral chemical precipitation and the variation range of its 87Sr／86Sr ratio is less, which represent the composition of Srisotope of thermal fluid in homogeneous hot brine pool. The composition of Srisotope of "water bursting breccia rock" which indicates different jet orifice location has major variation, but the composition of Srisotope of hydrothermal breccia and cementation are almost consistent in the same jet orifice, which shows that both of them are the same period and the same resource eruptive flow sediments. From strontium isotope geochemistry characteristics of hydrothermal sedimentary rock in Qingxi sag, Jiuquan Basin, it is primarily speculated that early Cretaceous sublacustrine hydrothermal fluid in the area is the mixed hydrothermal fluid which is constituted of deep circulating lake water and a little of mantlederived magmatic water and full of high radiogenic Sr of underlying siliconaluminum basement rock. The preliminary report and study of strontium isotope composition of lacustrine hydrothermal sedimentary rock will lay foundation for the further research of the early Cretaceous sublacustrine hydrothermal fluid nature, circulatory geodynamic model of the hydrothermal fluid and hydrothermal sedimentary mode.