陇中盆地始新统-中新统物源的碎屑锆石U-Pb年龄研究

The initiation of the present East Asian monsoon system is one of the outstanding issues in Asian paleoenvironment evolution. The loess-paleosol successions preserved on the Chinese Loess Plateau indicate the appearance of monsoonal system in East Asia were by 25 Ma. The basal part of loess deposits, however, was often scattered and discontinuous due to lack of a suitable landform and/or erosion at the initial stage. Thus, its may be younger than the inception of the East Asian monsoon, and its precise age is still controversial. Alternatively, the Tertiary basins in the Chinese Loess Plateau with continuously long sedimentary successions covering this critical interval can also preserve eolian materials, thus provide important archives for studying the inception of East Asian monsoon. In this proposal, two well dated fluvial-lacustrine records, covering the peroid of late Eocene to early Miocene from the Xining and Lanzhou Basins in the Longzhong Basin, were selected to study the timing of eolian materials occurred in the basin sediments by detrital zircon provenance analysis. We will systematically investigate the detrital zircon age spectra of the fine sediments in these long records to detect the timing of their provenance changes. This provenance change may be related to the contribution of eolian materials derived from Asian interior or local materials derived from surrounding mountain ranges. This will be addressed by comparing the age spectra of the fine sediments with local derived fluvial sand layers from these two basins, and comparing with our previous results from the Miocene loess in Qinan. Our results will provide an exact timing for the onset of the East Asian monsoon system.

东亚季风的起源是新生代东亚环境演化最重要的地质事件。指示季风环流的黄土高原风成红土-古土壤序列的底界年龄揭示其起源年代不晚于25 Ma，但黄土底界在沉积初期易受到堆积地貌和侵蚀等因素的影响，其年龄可能晚于季风环境格局出现的真实年代，导致目前对东亚季风起源的确切年代仍存在不同认识。陇中盆地始新世-中新世的河湖相地层沉积连续，覆盖了季风的起源年代，并包含指示东亚季风环流的粉尘物质，是研究东亚季风起源的关键地质记录。本项目拟选取陇中盆地的西宁盆地和兰州盆地两条已有详细古地磁定年的晚始新世-早中新世河湖相沉积序列为研究对象，通过碎屑锆石U-Pb年龄分析，查明陇中盆地不同时期细颗粒沉积物的物质来源，并与研究序列中来自近源的河流相砂层以及申请人前期获得的秦安中新世风成红土的碎屑锆石年龄谱对比，通过两个平行序列的相互印证确定陇中盆地粉尘物质开始大量出现的时间，揭示东亚季风环境格局起源的确切年代。

东亚季风的起源是新生代东亚环境演化最重要的地质事件之一，但目前对东亚季风起源的确切年代以及触发因素仍存在较大争议，一些研究认为东亚季风起源于古近纪/新近纪之交，而另一些研究认为东亚季风起源于晚始新世（~40 Ma）。来源于亚洲内陆地区的粉尘物质受东亚冬季风传输，在黄土高原地区堆积，其起始年代可反映东亚季风的起源，由于风成红土的底界在沉积初期易受到堆积地貌和侵蚀等因素的影响，其年龄可能晚于季风起源的真实年代。本项目选取位于粉尘堆积区的陇中盆地始新世-中新世的洪积、冲积、湖积地层为研究对象，通过碎屑锆石U-Pb年龄物源示踪方法，研究了陇中盆地不同时期细颗粒沉积物的物质来源，并与沉积序列中来自近源的河流相砂层以及秦安中新世风成红土的碎屑锆石年龄谱对比，探讨了陇中盆地粉尘物质开始出现的时间。结果表明：（1）陇中盆地西宁地区的晚始新世和早渐新世沉积物主要来自周边的造山带，不支持Licht等（2014，2016）提出的亚洲季风在晚始新世就已出现的观点；（2）在古近系/新近纪之交，陇中盆地天水地区周边山系受该时期青藏高原隆升的影响而发生了显著抬升，导致天水盆地新近纪沉积物的物源发生了明显转变，天水盆地内中新世细颗粒湖相沉积样品的碎屑锆石与秦安中新世风尘堆积类似，并包含有18 Ma的年轻锆石，指示着亚洲内陆粉尘物质在早中新世进入了陇中盆地，是东亚冬季风和亚洲内陆干旱区出现的标志。上述研究结果揭示出陇中盆地在古近纪/新近纪之交经历了青藏高原的隆升和物源的显著转变，并开始接受亚洲内陆的粉尘物质，表明该时期的青藏高原的隆升导致了东亚季风和亚洲内陆干旱区的起源，支持古气候模拟研究所揭示的东亚季风起源的机制。