藏南柳区砾岩的沉积环境及物源分析:对雅鲁藏布缝合带古近纪隆升的约束

沿雅鲁藏布江缝合带分布的柳区砾岩是喜马拉雅造山作用过程中重要的沉积记录。然而,目前对该套地层的构造属性仍存在不同的认识,因为尚未发现来自冈底斯中酸性的火山岩砾石,部分学者认为其是在印度和洋内岛弧碰撞形成的。本次工作对柳区出露的柳区砾岩进行了详细的剖面实测、沉积学观察和物源区分析。地层由厚层的砾级到巨砾级的砾岩以及相对较薄层的砂岩和泥岩组成,砾石包括硅质岩、基性-超基性岩、石英砂岩、岩屑砂岩以及板岩和千枚岩。砾岩分选差,磨圆差,颗粒支撑和基质支撑均发育,根据岩相组合判断其形成于冲积扇和辫状河环境。较大的砾径以及极低的结构成熟度表示为近源堆积,暗示雅鲁藏布江蛇绿岩带为该套砾岩的重要源区,而特提斯喜马拉雅带为板岩和片岩的主要源区。岩屑砂岩的碎屑颗粒统计结果显示岩屑的含量为82%~85%,其中沉积岩屑为主(82%~95%),石英颗粒以单晶石英为主。碎屑锆石U-Pb年龄有453~579Ma和737~889Ma二个主要的范围,而缺少200~400Ma的锆石年龄。上述观测都说明日喀则弧前盆地、雅鲁藏布蛇绿岩带和特提斯喜马拉雅为柳区砾岩的重要物源区。由于柳区砾岩内部含有日喀则弧前盆地提供的物源,所以柳区砾岩是印度-欧亚板块碰撞之后沉积的。而柳区砾岩内各成分的变化反应源区对物源贡献的变化,同时记录了造山带隆升的历史,具体表现为印度-欧亚板块碰撞后,首先雅鲁藏布江蛇绿岩带和日喀则弧前盆地相对较快隆升,并遭受剥蚀,为柳区砾岩的沉积提供初始的物源,随着印度板块的俯冲,特提斯喜马拉雅带开始隆升,成为了柳区砾岩的物源,主要提供板岩和千枚岩。进一步的俯冲使得蛇绿岩带大幅度隆升而阻碍了日喀则弧前盆地和冈底斯继续提供物源,使得柳区砾岩上段石英砂岩中缺少火山岩石英和再旋回的石英颗粒。

Sedimentary environment and provenance of the Liuqu conglomerate, southern Tibet: New constraints on the Paleogene uplift of Yarlung-Tsangpo suture zone

The Liuqu conglomerates, which has been recognized as the sedimentary records of the Himalayan orogenic processes, locates at the south of the Yarlung-Zangbo suture zone. However, its tectonic setting is still debated. Some authors argued that the Liuqu conglomerate is the product of collision between India and an intra-oceanic arc, because no acidic volcanic gravels derived from Gangdese has been found. Here we report results from sedimentology and detrital zircon U-Pb ages from a measured section of the Liuqu Group. This group mainly consists of thick granule to boulder conglomerate, relatively thin sandstone and mudstone. The major ingredients of gravel are siliceous rocks, basic-ultrabasic rock, quartz sandstone, lithic sandstone, phyllite and slate. Conglomerates are poorly sorted and rounded, grain-supported and matrix-supported. According to facies associations, the Liuqu conglomerate formed in alluvial fan and braided river environment. Larger gravel and low structure maturity are expressed as proximal accumulation, suggesting that the Yarlung-Zangbo ophiolite belt is an important source area of this group. The Tethyan Himalaya belt is main source for slate and phyllite. Modal framework-grain composition of lithic sandstone shows that the proportion of rock debris is 82%~85%, which is dominated by the sedimentary rock debris (82%~95%). Quartz particles are given priority to single crystal quartz (Qm). Detrital zircon U-Pb ages display two peaks of 453~579Ma and 737~889Ma, but lack ages of 200~400Ma. Therefore, the Xigaze forearc basin, the Yarlung Zangbo ophiolite and Tethyan Himalaya are important sources for the Liuqu Group. The Liuqu conglomerate deposited after India-Asia collision because it contains ingredient from Xigaze forearc basin. The provenance changes imply the uplift history of their source areas: the Yarlung-Zangbo ophiolite belt and Xigaze forearc basin began to uplift after India-Asia collision. As the Indian plate subducted, Tethyan Himalayan belt began to uplift, and eventually became the source, supplying slate and phyllite of Liuqu conglomerate. Continuous subduction made ophiolite belt uplifted to a higher elevation, hindering debris from Xigaze forearc basin and the Gangdese arc transported to the south, which resulted in the absence of volcanic quartz and recycled quartz particles in the quartz sandstone in the upper part of Liuqu Group.