西藏札达地区上白垩统-下始新统达机翁组:对冈底斯弧前盆地演化的制约

冈底斯弧前区域地层沉积记录,对新特提斯洋消亡和印度-亚洲碰撞过程的研究具有十分重要的意义。位于西藏南部札达地区的达机翁组,北邻冈底斯岩浆弧,南靠雅鲁藏布江缝合带。岩石组成主要包括砾岩、岩屑砂岩、泥页岩和灰岩等。沉积环境分析认为达机翁组形成于扇三角洲相环境。火山灰锆石U-Pb定年、碎屑锆石最年轻年龄以及底栖有孔虫化石组合共同约束达机翁组的形成时代为晚白垩世-始新世早期(即ca.73～49Ma)。物源区分析结果表明达机翁组物源类似于区域上分布的日喀则弧前盆地沉积,直接以北侧冈底斯岩浆弧为主要物质源区。通过与区域弧前沉积对比,为冈底斯弧前盆地海相地层时代提供制约,结果显示新特提斯洋在亚洲大陆南缘的弧前海退存在东西方向上的穿时性,即海水自东向西逐渐退出,并最终在～49Ma退出冈底斯-拉达克弧前区域。

Upper Cretaceous-Lower Eocene Dajiweng Formation in the Zhada area, southern Tibet: Implications for the Trans-Himalayan forearc baisn evolution

The marine strata in the Gangdese forearc area play a significant role in the study of the closure of Neo-Tethyan Ocean and the subsequent India-Asia collision. The Dajiweng Formation exposed in the Zhada area, southern Tibet, is bounded by the southern Gangdese magmatic arc to the north and the Indus-Yarlung Zangbo suture zone to the south. It is mainly composed of conglomerate, sandstone, mudstone and limestone. Sedimentary analysis revealed that the Dajiweng Formation had been deposited in the fan-delta environment. A combination of the tuff zircon U-Pb dating, youngest detrital zircon ages and large benthic foraminiferal assemblage yielded that the Dajiweng Formation formed during the Late Cretaceous-Early Eocene period (i.e., ca.73~49Ma). The provenance data showed that the Dajiweng Formation is similar to that of the Xigaze forearc basin sediments, which were mostly derived directly from the Gangdese magmatic arc. In contrast to the Gangdese-Ladakh forearc deposits, the forearc marine regression was diachronous along the strike, i.e. the sea water regressing gradually from east to west and disappearing at ~49Ma in the western Gangdese-Ladakh forearc area.