西藏孔隆—达果地区古近纪火山—沉积盆地时空演化

对冈底斯带的研究历来聚焦于岩浆弧，对弧间盆地的较少关注导致火山—沉积序列缺乏精细化研究。冈底斯带古近纪地层划分方案是基于并沿用东段林周、南木林地区的层序格架，即林子宗群与日贡拉组垂向叠置不整合接触，在带上其他地区适用时常产生矛盾，制约了基础地质及资源评价工作。通过系统实测孔隆—达果地区古近纪地层剖面，选取剖面中火山岩进行LA-ICP-MS锆石U-Pb测年，以详实的同位素年代学数据搭建精细年代地层格架，以沉积学、地层学研究分析充填演化过程，恢复火山—沉积盆地古地理。结果显示冈底斯造山隆升剥蚀并被扇沉积体系记录的过程，从晚白垩世早期断续持续至古近纪；以火山岩和/或以沉积岩为主的盆地，发育时限均下延至约70 Ma，暗示岩浆作用与隆升剥蚀对雅鲁藏布洋俯冲的响应几乎同时启动；火山—沉积盆地发育贯穿了整个增生造弧事件，以印亚大陆初始碰撞后的沉积间断为界，分为70~56 Ma和56~40 Ma两期，火山岩与沉积岩同时发育，以时空上的负消长关系占主导地位，表现为剖面上交互或夹层，并受喷发中心、沉积中心的横向迁移约束，产生了地层发育时限的空间变化；受晚白垩世末—古近纪雅鲁藏布洋北向俯冲及印亚大陆碰撞过程影响，持续的造山隆升及岩浆活动的周期性强弱变化约束了盆地发育样式，火山—沉积序列在区域上延展不稳定，垂向序列产生多样性。因此，本文提出层型剖面上火山岩与碎屑岩垂向叠置序列关系不能普适地代表整个冈底斯带，同期火山岩与沉积岩存在空间上快速相变过渡，应使用更为精细年代格架下的空间展布关系，指导冈底斯带弧间盆地地层划分，探讨印亚大陆碰撞的火山—沉积响应过程。

Spatiotemporal Evolution of Paleogene Volcano-sedimentary Basin in Konglong-Dago Area,Southern Tibet

Extensive studies on the Gangdese Belt have focused on the magmatic arc， but none are reported on inter-arc basins. Previous Cenozoic stratigraphic classification of the Gangdese Belt based on studies in Linzhou （near Lhasa） and in the Namling area （near Xigaze）， has suggested unconformable contact between the lower Linzizong group and upper Rigongla Formation， due to the lack of detailed studies of volcano-sedimentary sequences. However， several areas of conflict arise when the above classification is applied to other areas in the Gangdese Belt， including the Konglong–Dago area， disadvantaging geological research in the region. The present study focused on the volcano-sedimentary basin in Konglong–Dago area， adopting systematic measurement at geological cross-sections， LA-ICP-MS zircon U-Pb dating of volcanic rocks， and sedimentological and stratigraphical investigations. The chronostratigraphic framework was established， and the filling patterns and spatiotemporal evolution were determined. The results suggest that the process of Gangdese orogenic uplift and denudation recorded in the fan delta continued intermittently from early in the Upper Cretaceous until the Paleogene. The lower limits of the formation were dated to ca. 70 Ma in the volcanic rock- and/or sedimentary rock-dominant basin， implying that magmatism and uplift denudation in response to the Yarlung Zangbo Ocean were initiated almost simultaneously. The formation of a volcano-sedimentary basin took place throughout the arc hyperplasia. This occurred in two phases: 70-56 Ma and 56-40 Ma. A negative correlation between growth and decline of volcanic and sedimentary rocks， rather than vertical superimposition， dominated the spatiotemporal evolution， manifested as interbedding and intercalations in the cross-sections. This was constrained by relatively lateral migration of eruption centers and depocenters， such that both volcanic rock- and sedimentary rock-dominated strata developed spatial variation during their formation. From the Upper Cretaceous to the Paleogene， the Konglong–Dago area was influenced by the northward subduction of the Yarlung Zangbo Ocean and the Indo-Asian continent collision. Periodic variation in the intensity of this continuous orogenic uplifting and collision constrained development patterns in the basin. Hence， we propose that the usual model of a vertically superimposed sequence between the volcanic and sedimentary rocks based on typical geological cross-sections cannot be universally applied to the whole Gangdese Belt. Rapid spatial phase transition between volcanic and sedimentary rocks was occurring during the same periods. A more elaborate chronostratigraphic frame is therefore required to explain the spatial relationships and stratigraphic classifications seen in inter-arc basins in the Gangdese Belt， and the volcano-sedimentary response to the Indo-Asian continental collision.