从尼玛地区地质新资料看中特提斯洋的构造演化

Tectonic evolution of the Meso-Tethyan Ocean: An example from the Nyima region in Xizang

关键词:  尼玛  中特提斯洋  多岛弧盆系  西藏

Key words:  Nyima  Meso-Tethyan Ocean  archipelagic arc-basin system  Xizang

摘要:

班公湖怒江结合带南缘存在一套中、晚侏罗世稳定浅海碎屑岩沉积,属残余海盆地沉积,表明尼玛地区的俯冲消减机制在中侏罗世以后已结束。结合带南侧三叠系确哈拉群为一套半深水-深水沉积,是陆架边缘沉积序列,代表结合带打开之初的较早期沉积。确哈拉群之上不整合覆盖了一套中侏罗世钙碱性岛弧火山岩系,是班公湖怒江结合带在早侏罗世向南俯冲对应的滞后弧火山岩。在结合带南侧80~100km范围内分布着一条东西长超过100km的中晚侏罗世后碰撞强过铝花岗岩带,属班公湖怒江结合带向南俯冲碰撞作用的后碰撞阶段产物。综合认为,尼玛地区中特提斯洋是在三叠纪打开、中侏罗世以前向南俯冲闭合的。结合区域上该结合带闭合时间有早有晚、俯冲方向有南有北的事实,提出中特提斯是一个具有众多互不相通的、时代早晚各不相同的小洋盆共同组成的多岛洋,其间存在许多大小不一、运动方向和性质各不相同的地体。不同时期、不同方向的弧弧碰撞、弧陆碰撞造山(造陆)机制是解释中特提斯洋发展演化诸多问题的理想模式。

Abstract:

The occurrence of the Middle-Late Jurassic shallow-marine clastic deposits as the residual sea-basin deposits on the southern flank of the Bangong-Nujiang suture zone shows that the subduction and consumption regimes should be ceased after the Middle Jurassic. The Triassic Quehala Group on the southern flank of the suture zone as the abyssal-bathyal deposits is believed to be the passive continental margin deposits and represents the earlier deposits following the formation of the suture zone. The Middle Jurassic calc-alkaline island-arc volcanic rock series unconformably overlain upon the Group are considered as the lag arc volcanic rocks in response to the southward subduction of the suture zone during the Early Jurassic. The 100-km long granite belt within the extent of 80 to 100 km on the southern flank of the suture zone is interpreted as the post-collisional products formed in response to the southward collision of the suture zone during the Middle-Late Jurassic. It is postulated that the Meso-Tethyan Ocean in the Nyima region, Xizang was opened during the Triassic, and enclosed by collision before the Jurassic. The Meso-Tethyan Ocean is an archipelagic ocean composed of many isolated oceanic basins of different geological ages. The mechanisms of arc-arc collision and arc-continent collision and mountain building (continent building) may be interpreted as the ideal models for the interpretation of the tectonic evolution of the Meso-Tethyan Ocean.