雅鲁藏布江缝合带日喀则群的蛇绿岩质海底扇及其板块构造意义

OPHIOLITIC SUBMARINE FANS AND THEIR PIATETECTONIC SETTINGS IN THE XIGAZE GROUP ALONG THE YARLUNG ZANGBO SUTURE ZONE IN XIZANG

关键词:  雅鲁藏布江缝合带  日喀则群  蛇绿岩质海底扇

Key words:  Yarlung Zangbo suture zone  Xigaze Group  Ophiolitic submarine fans

摘要:

沿雅鲁藏布江缝合带分布的日喀则群(K₂TK)砂泥质细粒复理石中发育着一种独特的沉积单元——蛇绿岩质砂砾质海底扇。这些扇体规模小,内扇和中扇相序典型。无论是内扇还是中扇,水道都特别发育,而且水道游荡作用明显。盛行砂砾质高密度浊流和粘性碎屑流沉积,沉积物粒度粗,含大量卵石级以上的粗碎屑,具有近源、快速堆积的特点。其碎屑组分为世界现代和古代海底扇沉积所罕见,主要是蛇绿岩碎屑,并含少量岛弧火山-岩浆岩碎屑和老地层的碎屑。这些扇体具有活动边缘型海底扇的典型特征,但又有其特殊性,它们发育在雅鲁藏布洋盆闭合、印度板块与藏北板块开始碰撞时的残留复理石盆地边缘。板块碰撞导致蛇绿岩质杂岩体和洋壳物质逆冲、抬升形成一个外弧造山带。该外弧造山带成为这些扇体的主要物源区。同时,强烈的逆掩和纳布作用也可将部分岛弧岩和老地层带到残留盆地边缘,一起作为来源母岩。

Abstract:

The ophiolitic sandy and gravelly submarine fans are developed as a kind of distinctive sedimentary unit in the fine-grained flysch of the Xigaze Group along the Yarlung Zangbo suturc zone in Xizang. The characteristics of these submarine fans are as follows. (1) They are relatively small in scale. (2) The channel deposits are especially well developed in both upper and middle fans, and their sequences are very clear. (3) The channel are frequently migrated and braided. (4) The sandy and gravelly high-density turbidity current deposits and the cohesive debris flow deposits are prominant in these submarine fans. (5) The sediments are coarse-grained and contain a large amount of cobbles and boulders, indicating that they are relatively proximal and rapidly accumulated. (6)Much of the sediments are derived mostly from ophiolites and only a small part from island-arc volcanic-magmatic rocks and older sedimentary strata, which may make these submarine fans rare in both modern and ancient equivalents in the world.
These distinctive submarine fans were developed on the margins of the residual flysch basins during the closing of the Yarlung Zangbo Ocean and the onset of the collision of the Indian plate with the North Xizang plate. The collision of these plates resulted in the overthrusting and uplifting of the ophiolitic complexes and the oceanic crust, and thus in the formation of an outer arc orogenic zone which serve as the main provenance of these fans bodies. Meanwhile, intense overthrusting and napping may also introduce part of island-arc rodk and older strata to the margins of the residual basins which are interpreted as an additional source of sediments.