西藏泽当蛇绿岩玄武岩SHRIMP锆石U-Pb年龄 及其地质意义

雅鲁藏布江缝合带中各蛇绿岩体的准确定年对待提斯洋演化和青藏高原隆升的研究具有重要意义.泽当蛇绿岩是雅鲁藏布江缝合带东段最大的蛇绿岩块体,关于其形成年龄目前仍存在不同的认识.通过SHRIMP锆石U-Pb测年得到蛇绿岩中玄武岩的形成年龄为154.9Ma±2.0Ma(95％置信度,MSWD=0.98).蛇绿岩中的玄武岩是洋脊扩张的产物,其形成年龄代表了扩张事件的时间,也代表了蛇绿岩的形成时代.结合已有的雅鲁藏布江缝合带蛇绿岩的形成年龄,该年龄进一步反映出雅鲁藏布江缝合带蛇绿岩形成时间具有东早西晚的特点.泽当蛇绿岩与含有埃达克质英云闪长岩的泽当岛弧火成岩基本为同期形成的.地球化学特征显示定年的玄武岩形成于俯冲带之上,且具有指示洋内俯冲环境的地球化学特征.因此,泽当SSZ型蛇绿岩可能形成于洋内俯冲机制.

Zircon U-Pb dating of basalt from Z(e)tang ophiolite in Tibet and its geological implications

The precise dating of ophiolites from the Yarlung Zangbo suture zone can provide an important constraint on the geological evolution of the Neo-Tethys and the uplift of the Tibetan Plateau. Zêtang ophiolite is the largest outcrop distributed in the eastern Yarlung Zangbo suture zone, but so far there has been no consensus concerning its formation time. This paper reports newly obtained SHRIMP U-Pb dating of zircons from the basalt of this ophiolite mélange, i.e., 154.9Ma±2.0 Ma (95% conf., MSWD=0.98). The age could represent the spreading time of the ancient oceanic crust. In comparison with ages of other ophiolites in the Yalung Zangbo suture zone, this age indicates that the formation of ocean crust was earlier in the east than in the west. The Zêtang ophiolite has essentially the same formation age as the tonalities-bearing island arc rock assemblage, indicating the intra-Tethyan subduction. Based on the geochemistry of basalts as well as the characteristics of the setting and the boninite, the authors hold that Zêtang ophiolite was derived from an intra-oceanic subduction.