北羌塘盆地中侏罗统夏里组蒸发岩锶同位素地球化学特征及物质来源

北羌塘盆地位于青藏高原的中部,属东特提斯构造域,是一个具有成盐远景的蒸发岩盆地.前人对盆地的研究多局限于层序地层、构造运动、油气成藏等,对于盆地内蒸发岩的研究,特别是其成矿流体来源的研究报道较少.文章通过对北羌塘盆地龙尾湖QY-1钻孔中硬石膏岩层锶同位素进行测定,首次对盆地中侏罗统夏里组蒸发岩成矿流体的来源进行探讨.结果表明,笔者所分析的样品的锶同位素比值变化范围为0.707475～0.709048,均值0.708331,与同时期全球海水锶同位素比值(0.706860～0.707081)相比略高,表明成矿流体来源主要是海水.结合前人对羌塘盆地构造运动与盆地演化的认识,认为陆源锶的输入造成了本区的锶同位素比值高于同期海水.此外,钻孔中的同位素组成与前人公布的全球中侏罗世(164～160.2 Ma)海水锶同位素曲线具有较好的对比性,中晚侏罗世羌塘盆地海侵海退作用的强弱是控制盆地夏里组锶同位素演化的主要因素.

Strontium isotopic geochemical characteristics and origin of evaporites from Middle Jurassic Xiali Formation in North Qiangtang Basin

The North Qiangtang Basin is an evaporite basin with a prospect of salt formation, located in the middle of the Qinghai-Tibet Plateau, belonging to the East Tethys tectonic domain. Previous studies on the basin were mostly limited to sequence stratigraphy, tectonic evolution, oil and gas accumulation, etc., while the study on evaporites, especially the data on the source of the salt forming fluids is missing. This study presents the strontium isotope analytical results of the anhydrite strata from the Longwei lake QY-1 borehole in the North Qiangtang Basin, and the source of the evaporite ore-forming fluids of the Middle-Jurassic Xiali Formation is explored for the first time. The results show that the strontium isotope ratio of evaporites in this area vary from 0.707 475 to 0.709 048, with an average of 0.708 331, which is slightly higher than the global seawater (0.706 860 to 0.707 081) du-ring the same period, indicating that the source of ore-forming fluids is mainly seawater. Based on previous understandings of the tectonic movement and basin evolution in the Qiangtang Basin, we believed that the injection of terrigenous strontium has caused the higher strontium isotope ratio in this area, which shows continental water has mixed. In addition, the strontium isotope compositions of the samples have good comparability with the global Middle-Jurassic seawater strontium isotope curve published by previous researchers. The strength of the transgression and regression in the North Qiangtang Basin during the Middle and Late Jurassic is the main factor which controls the strontium isotopic evolution of the Xiali Formation.