沉积记录约束班公湖-怒江缝合带东巧蛇绿岩的仰冲过程

Constraining the obduction process of the Dongqiao ophiolite in the Bangongco-Nujiang suture zone by the sedimentary record

班公湖-怒江缝合带(班怒带)分割了北部的羌塘地体和南部的拉萨地体,代表曾经消失的大洋--班公湖-怒江洋(班怒洋)。蛇绿岩从西部的日土到东部的丁青断续分布,是记录班怒洋演化的重要载体。长期以来关于班怒带蛇绿岩何时形成、在哪里形成、如何就位的研究薄弱。本文基于对班怒带中段东巧蛇绿岩之上的一套海陆过渡相地层(东巧组)的地层学、沉积学和物源研究,认为东巧蛇绿岩在晚侏罗世Oxfordian-Kimmeridgian之前仰冲到一个亲拉萨的被动大陆边缘之上。在此基础上,结合蛇绿岩年代学和地球化学数据,重建了东巧蛇绿岩从形成到仰冲至地表的过程,探讨了东巧蛇绿岩仰冲的动力学机制及其大地构造意义。

The Bangongco-Nujiang suture zone separates the Qiangtang terrane to the north and the Lhasa terrane to the south,representing the once-disappeared ocean—the Bangongco-Nujiang ocean.The ophiolite,which is distributed intermittently from Ritu in the west to Dingqing in the east,is important for recording the evolution of Bangongco-Nujiang Ocean.For a long time,little has been known on when and where the Bangongco-Nujiang ophiolite formed and how it was obducted.The study of stratigraphy,sedimentology and provenance of terrestrialmarine siliciclastic-cabonate deposits(the Dongqiao Formation)above the Dongqiao ophiolite in this paper suggests that the Dongqiao ophiolite was obducted onto a passive Lhasa-affinity continental margin before Oxfordian-Kimmeridgian(Late Jurassic).The process of Dongqiao ophiolite from crystallization to obduction is reconstructed,and the dynamic mechanism of Dongqiao ophiolite obduction and its tectonic implications are discussed on the basis of the ophiolite age and geochemical data.