Evaluating the Detrital Record of Continental Collision in the Songpan-Ganzi Complex, Central China, the World's Largest Pre-Cenozoic Turbidite System

松潘-甘孜地区位于青藏高原的东北角，面积相当于堪萨斯州，被一大片三叠纪的深海沉积沉积物（浊积岩）所覆盖。松潘-甘孜杂岩（SGC）是世界上最大的古浊积岩体系，其体积可与现代最大的沉积堆积孟加拉海底扇相媲美，但这些沉积物的来源却鲜为人知。围绕着SGC洋盆的大陆块体和火山弧体系可能为SGC提供了大量的沉积物。另一种可能的沉积物来源是三叠纪期间华北和华南大陆块体之间的缝合带。在三叠纪—侏罗纪东亚季风气候条件下，该缝合带在碰撞驱动的隆升作用下，迅速被侵蚀至很深的地方，地壳的消失量与SGC的沉积量相当。类似地，SGC浊积岩层序在构造上楔入两个大陆块体之间的同一时期缝合线，使人联想到现代喜马拉雅碰撞系统的时间和几何元素，包括喜马拉雅碎屑的最终存储库——孟加拉和印度河海底扇。然而，喜马拉雅系统将在未来继续演变，而早已灭绝的华北/华南/松潘-甘孜系统则包含了完整的大陆碰撞记录。本研究通过对斯坦福大学的新样品和大量现有样品（600公斤，400个样品）进行分析研究，并辅以对沉积物运输方向的实地调查，为SGC确定沉积物的来源。这项研究包括传统的砂岩岩石学分析，以及复杂的分析方法，如使用斯坦福-美国地质勘探局的敏感高分辨率离子微探针（与美国地质勘探局的J.L. Wooden合作）和斯坦福大学的拉曼光谱设备来识别指示超高压深地壳来源的矿物（例如，带有微金刚石或钴矿包裹体的锆石），这是目前华北/华南缝合带主要研究工作的重点。如果能在这些沉积物中识别出缝合线衍生的沉积物，那么就有可能追踪华北/华南缝合带的侵蚀记录，就像现代孟加拉和印度河扇沉积物记录喜马拉雅碰撞系统的侵蚀记录一样。因此，比较SGC与喜马拉雅系统的数据将有助于更好地理解碰撞系统的长期演化及其沉积记录。

The Songpan-Ganzi region, an area the size of Kansas on the NE corner of the Tibet Plateau, is underlain by a vast expanse of Triassic-age deep-marine sedimentary sediments (turbidites). The Songpan-Ganzi Complex (SGC) is the world's largest ancient turbidite system, comparable in volume to the largest modern sedimentary accumulation, the Bengal submarine fan, yet the sources of these sediment are very poorly known. Continental blocks and volcanic arc systems fringing the SGC ocean basin could have provided considerable sediment to the SGC. Alternatively, a likely source for the sediments is the nearby suture belt between the North China and South China continental blocks, which collided during the Triassic. With collision-driven uplift, this suture belt rapidly eroded to great depths in the monsoonal climate of Triassic-Jurassic east Asia, removing a volume of crust comparable to the sediment volume in the SGC. By analogy, the occurrence of the SGC turbidite sequence structurally wedged into a coeval suture between two continental blocks is reminiscent of the temporal and geometric elements of the modern Himalayan collision system, including the ultimate repositories for Himalayan detritus, the Bengal and Indus submarine fans. However, whereas the Himalayan system will continue to evolve far into the future, the long extinct North China/South China/Songpan-Ganzi system contains a full record of continental collision. This study is identifying the sources of sediment for the SGC through a program of analytical studies of new samples and extensive pre-existent collections (600 kg, 400 samples) housed at Stanford University, complemented by field investigation of sediment transport directions. The study includes conventional sandstone petrographic analyses, as well as sophisticated analytical approaches such as use of the Stanford-USGS Sensitive High Resolution Ion Microprobe (in co-operation with J.L. Wooden, USGS) and Stanford's Raman spectroscopy facility to identify minerals (e.g., zircon with microdiamond or coesite inclusions) indicative of ultrahigh-pressure deep-crustal sources, currently the focus of major research efforts in the North China/South China suture zone. If suture-derived sediment can be identified in these sediments, then it may be possible to track the record of erosion of the North China/South China suture belt in much the same way that modern Bengal and Indus fan sediments archive a record of erosion of the Himalayan collision system. Thus, comparison of data from the SGC with the Himalayan system will yield better understanding of the long-term evolution of collisional systems and their sedimentary records.