酒泉盆地青西凹陷下沟组湖相热水沉积岩锶同位素地球化学特征

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

以地质背景、物质组分和岩石组构分析为基础,对酒泉盆地青西凹陷下沟组湖相热水沉积钠长石—铁白云石岩进行了Sr同位素地球化学研究。结果表明,热水沉积岩87Sr/86Sr比值变化范围为0.71225～0.71781,平均值0.71561,远高于同期海水和下沟组玄武岩,略高于代表早白垩世湖水锶同位素组成的藻灰岩,但低于基底壳源硅铝质岩,反映热流体不可能为单纯的湖水或幔源岩浆水;其中直接化学结晶沉淀的纹层状泥微晶热水沉积岩锶同位素比值变化范围较小,可代表均一化的热卤水池流体锶同位素组成特征;指示不同喷流口位置的"水爆角砾岩"锶同位素组成变化较大,但同一位置的热水角砾与胶结物锶同位素组成基本一致,说明两者应属于具相同成因意义的同期热水喷流沉积产物;综合青西凹陷下沟组湖相热水沉积岩锶同位素地球化学特征,初步推测早白垩世湖底热流体可能为富集硅铝质基底岩石高放射成因Sr的深循环下渗湖水与少量上升幔源岩浆水构成的混合热流体,可为青西凹陷早白垩世湖底热流体性质的确定以及热流体循环动力学模型和热水沉积模式的建立奠定基础。

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 87Sr/86Sr 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 87Sr/86Sr ratio of seawater and basalt of corresponding period and a little higher than the S7Sr/S6Sr ratio of the algal limestone, which represents the strontium isotopes composition of lake water in early cretaceous, but lower than the S7Sr/S6 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 87Sr/S6Sr ratio is less, which represent the composition of St-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 sublaeustrine 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 sublaeustrine hydrothermal fluid nature, circulatory geodynamic model of the hydrothermal fluid and hydrothermal sedimentary mode.