中新世黄河水系演化重建:来自钾长石Pb同位素的约束

Miocene evolution reconstruction of the Yellow River drainage basin:Constraints from K-feldspar Pb isotopes

自剥蚀地貌中形成的剥蚀产物搬运到沉积区或汇水盆地沉积下来的过程,称为“源-汇”系统。从这些“汇”中识别“源”的信息,结合地层的沉积时代,约束黄河的形成时代、水系格局演化、流域内构造运动以及环境变迁成为黄河水系演化研究的重要内容。然而,时至今日学者们对黄河何时出现的认识并不一致,存在“中新世内流黄河和中新世外流黄河”之争。为了解决这些争议,我们运用大河物源示踪研究中新兴的碎屑钾长石Pb同位素方法,对黄河中下游沉积物开展碎屑钾长石Pb同位素分析(n=378),将其与长江中下游和华夏板块已报道的碎屑和基岩钾长石Pb同位素结果进行对比,结果表明三者之间的钾长石Pb同位素组成明显不同。台湾岛是中国东部陆架海中新世地层出露最齐全的地区。因而,我们将黄河中下游与台湾岛中新统的钾长石Pb同位素比值进行对比,结果表明二者之间不存在物源联系。结合中国东部渤海湾盆地、南黄海盆地和东海盆地中新统的物源示踪研究结果,说明黄河物质在中新世未流入上述盆地。总体而言,中国北方中新世的构造活动控制了黄河的演化过程,导致其此时处于分段演化阶段。

The process in which the denudation products formed in the denudation landform are transported to the sedimentary area or catchment basin are referred to as the“source-sink”system.Identifying the source information from the sink,combining with the sedimentary age of the strata,constraining the formation age of the Yellow River,the evolution of the drainage pattern,the tectonic movement in the basin and the environmental changes has become an important content of the study of the evolution of the Yellow River.However,scholars do not agree on when the Yellow River formed.There is a dispute between the Miocene internal flow of the Yellow River and Miocene external flow of the Yellow River.In order to resolve the dispute,we used the newly developed river provenance tracing method to analyze the detrital K-feldspar Pb isotopic compositions from the middle and lower reaches of the Yellow River(n=378),comparing the results with those from the middle and lower reaches of the Yangtze River and the Cathaysia plate.The results show that the Pb isotopic compositions of K-feldspar are obviously different among them.Taiwan island is the most complete area where Miocene strata are exposed in the eastern China shelf sea.Therefore,we compared the Pb isotopic ratios of K-feldspar from the middle and lower reaches of the Yellow River with those from the Miocene strata deposited in the Taiwan Island.The finding suggests that the Yellow River material did not enter the Taiwan Island during the Miocene.Combined with the reported provenance tracing results from the Bohai Bay,South Yellow Sea and East China Sea basins,it indicates that the Yellow River did not flow into these basins during the Miocene.In general,the Miocene tectonic activities in northern China controlled the evolution of the upper and middle reaches of the Yellow River,leading to the segmentalized evolution of the Yellow River at this time.