右江盆地及邻区中晚二叠世硅质岩Sr同位素组成及其古海洋意义

The Permian seawater record is characterized by a large decline in 87Sr/86Sr values during the Early-Middle Permian, reaching the lowest 87Sr/86Sr value in the late Middle Permian followed by a gradual rise in the Late Permian. This is one of the most rapid changes in the Sr isotopic record of the entire Phanerozoic. The causes of this long-term trend change and the Capitanian minimum have not been fully understood. Importantly, the Emeishan volcanic eruption coincided with the Capitanian minimum, indicating a possible genetic link. Therefore, more researches are needed to assess the role of the eruption and erosion of the Emeishan volcanic rocks in the evolution of the seawater 87Sr/86Sr values during this period. The Youjiang Basin is adjacent to the Emeishan large igneous province (ELIP) and extensive Middle-Late Permian cherts developed well in this basin and environs. We plan to analyse the Sr isotopic compositions as well as origin of the Middle-Late Permian cherts in the Youjiang Basin and environs, to ascertain the spatial and temporal evolutional pattern of the ambient seawater Sr isotopic compositions of the ELIP, and provide some new information about the role of the ELIP in the evolution of the seawater 87Sr/86Sr values during the Middle-Late Permian and its possible mechanism.

二叠纪全球海水的Sr同位素组成存在一次明显的波动，表现为从早中二叠世海水87Sr/86Sr值持续降低直到中二叠世末期出现显生宙以来最小的87Sr/86Sr值，之后海水的87Sr/86Sr值持续升高。目前，对该时期海水Sr同位素组成的变化及最小值的认识，还存在多种不同的解释。前人研究表明，峨眉山大火成岩省在时间上与中二叠世末期Sr同位素最小值在时间上具有较好的耦合性，暗示二者之间可能存在一定的联系。然而，目前对峨眉山火山岩的喷发与剥蚀对该时期海水Sr同位素组成变化的影响还缺少足够的认识。右江盆地紧邻峨眉山大火成岩省，盆地内及邻区发育中晚二叠世硅质岩。本项目拟通过对右江盆地及邻区中晚二叠世硅质岩Sr同位素的研究，结合硅质岩成因分析，探讨这一时期峨眉山大火成岩省周围海水Sr同位素组成的时空分布规律，为探索峨眉山大火成岩省在中晚二叠世海水Sr同位素组成变化中所扮演的角色及其可能的机制提供新的认识。

中-晚二叠世之交海水的Sr同位素组成存在一次明显的波动，表现为从早二叠世到中二叠世卡匹敦期海水87Sr/86Sr值持续降低直到出现显生宙以来最小的87Sr/86Sr值（~ 0.70685），之后到早三叠世海水87Sr/86Sr值持续升高。对该时期海水Sr同位素组成变化以及最小值的解释，还存在多种不同的解释。本项目分析了硅质岩的成因以及形成于不同构造背景下硅质岩的主量元素、微量元素、稀土元素、Si同位素和Sr-Nd同位素组成特征。项目对右江盆地周缘中晚二叠世硅质岩、玄武岩夹层中灰岩Sr同位素以及峨眉山大火成岩省火山活动时限开展了相关研究。研究结果将Sr同位素最小值约束在中二叠世卡匹墩期晚期J. prexuanhanensis - J. xuanhanensis牙形石带。峨眉山大火成岩省的火山活动时限持续到了257.8Ma。峨眉山大火成岩省可能在中晚二叠世之交全球Sr同位素变化中扮演了重要的角色。