雅鲁藏布江缝合带区域性构造—沉积特征

Regional tectono-sedimentary feature of the Yarlung Zangbo suture zone

沿雅鲁藏布江缝合带的蛇绿岩套出露状态可分为3种类型,如在岗仁波齐和白朗地区完整出露大套基性-超基性岩体,层状堆晶岩,辉绿岩质岩席、岩墙,以及层状深海沉积建造。而在仁布地区的蛇绿岩露头规模迅速收敛,仅剩少量地幔岩体表现为串珠状排列的小岩株向东延伸并逐渐尖灭。在拉萨地区蛇绿岩套则完全缺失,相应位置出露一套同时代中-酸性火山岩建造。为探讨雅鲁藏布江蛇绿岩带的大地构造性质,笔者选择蛇绿岩套出露巨厚、迅速收敛和完全缺失3种不同的构造位置开展构造—沉积剖面研究。初步结果显示:白朗大规模蛇绿岩套核心区保留基底构造窗,且局部可直接观察到深海沉积建造与上下围岩复理石的沉积整合接触关系。仁布地区的镁铁-超镁铁岩体具有保留完好的原生侵位构造形迹,岩体与围岩常呈侵位时的牵引平行或者低角度斜交接触关系,地幔岩与围岩接触带保留典型烘烤边等热接触变质带(晕)等。两剖面的岩石均处于区域性脆性-韧脆性变形和低绿片岩相变质作用,缺乏统一的区域性构造极性,因而整体属于原地系统,并非经历板块俯冲—碰撞—反逆冲的外来残余洋壳岩片。拉萨地区在蛇绿岩带向东延线位置出露侏罗—白垩系中-酸性火山建造,大剖面南北两侧的中-新生代沉积建造的岩性组合,构造样式和沉积环境演变过程具有明显的相似性和时间上的宏观对应性,应当记录了同一个盆地的完整演化旋回。上述构造—沉积地质特征说明雅鲁藏布江蛇绿岩套并不代表中生代近万公里宽阔洋壳的残余构造岩片,新特提斯洋实际上属于一条中生代不均一洋壳化的弧后裂陷盆地。

The ophiolites along the Yarlung Zangbo suture zone outcroped in 3 types,large maficuhramafic body,stratified cumulative, diabasic dike and sheet swarm, and deep sea sedimentary formation in the Gangrenboqi and Bailang regions. The opniolitic complex constrict rapidly in the Renbu region, only pearl-chain shaped stocks extend eastwarded then disappeared. There is the lack of ophiolite in Lhasa region, but a set of intermediate to acid volcanic sediments out cropped in the relevant position of the ophiolite corresponding in the western. We carried out the geological tectono-sedimentary investigations in locations of such 3 types. The preliminary data show that the original emplacement structures exist in both Bailang and Renbu ophiolite outcropts. A tectonic window inside of the Bailang ophilite body is identified, and the parallel even conformable contact relationship can be observed locally between ophiolite bodies and flysch layers on both side of the zone. The country flysch layers kept low angle even parallel with the stocks surfaces in Renbu region with their thermal optalic metamorphic structures. All rocks underwent low green-schist metamorphism and were then fractured without regional tectonic polarity, and no any large shear zone in north-south direction exists between country layers and mafic rock bodies. The geologic information show that the whole system is in situ, but not foreign resulted from subduction nor collision of the plates. The Mesozoic sedimentary facies and lithology are similarin both north and south segments of the Lhasa profile, and its sedimentary evolutionare corresponding in time. These recorded a history of the evolution of the intact basin. All geologic information above does not warrant a postulate that the Yarlung Zangbo ophiolite zone represents the relic crust of a great ocean thousands km in width, the Neotethys should be in fact back-arc basin collapse with its juvenile narrow oceanic crust.