剑川盆地新生界碎屑锆石物源示踪及其对古金沙江演化的指示

The formation of the Jinshajiang “First Bend” in the upper Yangtze River represents the birth of the east-flow Yangtze drainage, which is one of the key points to study the evolution of the Yangtze River. The Jianchuan Basin located in the south of the “First Bend” keeps the sedimentary records of the “First Bend” formation and evolution history. Based on the geomorphology, the previous studies illustrated that the Paleo-Jinshajiang flew south and passed through the Jianchuan basin, then formed the “First Bend” after being captured. Whether this hypothesis is true still need the geology and sedimentology evidences to prove. The Jianchuan Basin develop relative integrity Cenozoic sedimentary sequences. According to the field investigation, there exist a stratum associated with large fluvial systems, the source of which is likely distal. Whether this ancient fluvial system can represent the south-flowing paleo-Jinshajiang needs needs further study. This project is going to carry out the research of the stratigraphy, chronology and sedimentology of the Cenozoic deposits in the Jianchuan Basin, and to study the provenance of the sequences by using multiple methods, including the element, isotope and detrital zircon U-Pb dating. Our purpose is to distinguish the provenance change of the spatial and temporal, and to confirm the geological evidence of the south-flow paleo-Jinshajiang. Meanwhile, to discuss the time of the paleo-Jinshajiang rechannel to the east (the formation of the “First Bend”). It will provides us the important geological archives of the evolution of the Yangtze River and the regional tectonic-geomophic ecolution of the Eastern Tibet.

长江上游金沙江“第一弯”的形成标志着长江东流水系的“诞生”，是研究长江演化的关键问题之一。位于第一弯南部的剑川盆地，保存了第一弯形成和演化的沉积记录。前人基于地貌学的研究认为古金沙江曾经南流，经过剑川盆地，后被袭夺东流形成如今的“第一弯”，事实是否如此，仍需要地质学与沉积学的证据。剑川盆地发育较完整的新生代沉积。野外观察发现，其中不乏具有大型远源特征的河流沉积体系。该套沉积是否能够代表南流的古金沙江仍有待论证。本项目拟在已有研究的基础上，对剑川盆地新生代沉积开展地层学、年代学和沉积学研究，通过碎屑锆石U-Pb年龄及元素、同位素地球化学的物源示踪方法，综合判断物源时空变化规律，明确古金沙江水系南流的地质证据，探讨金沙江水系改流东向（第一弯形成）的时限，从而为长江演化和青藏高原东南缘区域构造地貌演化提供依据。

长江上游金沙江“第一弯”的形成标志着长江东流水系的“诞生”，是研究长江演化的关键问题之一。位于第一弯南部的剑川盆地，保存了第一弯形成和演化的沉积记录。前人基于地貌学的研究认为古金沙江曾经南流，经过剑川盆地，后被袭夺东流形成如今的“第一弯”，事实是否如此，仍需要地质学与沉积学的证据。剑川盆地发育较完整的新生代沉积。野外观察发现，其中不乏具有大型远源特征的河流沉积体系。该套沉积是否能够代表南流的古金沙江仍有待论证。本项目在已有研究的基础上，对剑川盆地新生代沉积开展地层学、年代学和沉积学研究，通过多种物源示踪方法，明确古金沙江水系南流的地质证据，探讨金沙江水系改流东向（第一弯形成）的时限，从而为长江演化和青藏高原东南缘区域构造地貌演化提供依据。.项目通过野外考察及火山岩测年，得到~35 Ma的上限年龄，对剑川盆地新生代地层序列进行了重新厘定；通过碎屑锆石U-Pb年龄分布特征，判定松潘-甘孜褶皱带与扬子地块西缘对剑川盆地沉积物贡献较大，且宝相寺组和金丝厂组沉积物与金沙江具有密切的关系；通过元素、Sr-Nd同位素地球化学的物源示踪方法，判定金丝厂组沉积物与金沙江具有一定的相似性。综合物源变化规律及地层年代，可认为金沙江在至少35 Ma之前的始新世曾向南流，经过剑川盆地，形成金丝厂组地层沉积，之后在石鼓地区改道东流。.