雅鲁藏布江蛇绿岩的形成与日喀则弧前盆地沉积演化

雅鲁藏布江蛇绿岩被时代连续的日喀则群沉积覆盖及其形成时代(120-110Ma)与冈底斯弧开始发育的时代(115-100Ma)十分相近的事实使人们有理由提出:雅鲁藏布江蛇绿岩是否代表着印度板块与拉萨地块间的特提斯-喜玛拉雅洋残迹的疑问。根据近期的研究,笔者认为雅鲁藏布江蛇绿岩不是形成于三叠纪的特提斯-喜玛拉雅洋的残迹,而是特提斯-喜玛拉雅洋向拉萨地块俯冲的初期(阿普第-阿尔必期),由俯冲作用在冈底斯弧前地区引发的海底扩张作用形成的一种俯冲带上叠型蛇绿岩(supra-subduction zone ophiolites).至森诺曼期,弧前海底扩张作用停止,雅鲁藏布江蛇绿岩开始向南仰冲,在其南侧形成增生杂岩楔。仰起的蛇绿岩开始向日喀则弧前盆地提供蛇绿质碎屑,如冲堆组。森诺曼期-土仑期,盆地接受了一套深水复理石沉积,沉积物源部分来自南部边缘脊的蛇绿质碎屑,而大部分则来自北侧的弧火山岩和岩浆岩碎屑。森诺期-路坦丁期,盆地逐渐变浅,接受了浅海-滨海沉积,物源均来自北部的岩浆弧。至始新世末期,发育在盆地南侧的增生杂岩楔与印度板块发生碰撞,日喀则弧前盆地闭合。

ORIGINAL TECTONIC SETTING OF THE TSANGPO OPHIOLITE ANDSEDIMENTARY EVOLUTION OF THE XIGAZE FOREARC BASIN

It is generally accepted that the Tsangpo ophiolite zone is a typial suture between Indian and Asian plates, and had formed in the tectonic setting of a major ocean ridge. However, the accumulated evidences have demonstrated that the formation age(120-110Ma)of the Tsangpo Ophiolite is almost the same as the starting development age(115-110Ma)of the Gangdise Arc, and the Tsangpo Ophiolite had a continuous sedimentary relationship with the sedimentary formation in the Xigaze forearc basin. The facts raise a question that did the Tsangpo Ophiolite form in a major ocean ridge? Is it the suture between the Indian Plate and the Lhasa Terrain? Based on our studies, it is suggested that(1)the Tethys-Himalayan Ocean between the Indian Plate and the Lhasa Terrain could have formed during the Triassic Period, because some of the Triassic ophiolite blocks have been found in the accetionary complex which lies just south of the Tsangpo Ophiolite;(2)During the Aptian stage of Early Cretaceous Period, the Tethys-Himalayan Ocean started northward subduction. The subduction resulted in the sea-floor spreading directly above the subducted oceanic lithosphere at the forearc region during the initial stages of subduction prior to the development of the Gangdise Arc, and the "Tsangpo Ophiolite" and "Tsangpo marginal sea" were formed with the sea-floor spreading;(3)At the Senomanian stage of Late Cretaceous Period, the sea-floor spreading stopped, and the oceanic crust of the "Tsangpo marginal sea" obducted southward to constitute a ridge which enclosed the forearc region.(4)Since the Senomanian stage of Late Cretaceous Period, the "Tsangpo marginal sea" evoluted into the Xigaze forearc basin. During the early period(the Senomanian stage)of development of the basin, the ophiolitifeous slide sediments from the rigde constituted by the obducting oceanic crust and radiolarian cherts were deposited on the south margin of the basin, and on the north part of the basin the sediments mainly came from the Gangdise Arc; During the middle period(Turonian stage)of development of the basin, the turbidite fan was well developed in the whole basin, Gnd the sediments came mainly from the northern Gangdise Arc; During the late period(Senonian to Lutetian stage)of development of the basin, the basin evolved into a littoral environment, and two zones of terrestrial deposit were formed in the northern margin and sorthen side of the basin.(5)At the Oligocene epoch, the collision between the Indian Plate and the accetionary complex wedge probably occurred.