东喜马拉雅构造结气候构造作用下热史演化的40Ar/39 Ar年代学记录

Cooling history constrained by detrital ^(40)Ar/^(39) Ar geochronology in Eastern Himalaya Syntaxis: Implications for climatic and tectonic records.

以喜马拉雅山系为典型实例,究竟是气候作用还是构造作用引起山体隆升的问题已经成为地球系统科学研究中的重要前沿问题。无论是气候因素还是构造因素引起山体隆升,二者都与一个共同的地表过程——剥蚀作用相关,剥蚀作用对山体中地质体的影响可以用岩石矿物经历的热史演化来描述,所以,在造山作用研究中,山体或山脉的热史演化是揭开地质体经历地质过程、山体隆升研究的重要途径。利用河砂组成矿物来研究流域的地质过程和构造演化已经成为现代地质科学的重要手段。本文采集了雅鲁藏布江下游墨脱县以南约50 km处地东河段内的现代河砂,对其中的角闪石、白云母、黑云母及钾长石等四种矿物进行了高精度单颗粒激光^(40)Ar-^(39)Ar年代学测试,并进行了概率统计。地东河段河砂中富钾矿物^(40)Ar-^(39)Ar年代学统计结果显示,大峡谷流域的热史演化可以确定有多个阶段,分别可以识别出70～69、61～60、43-42、35～34、26～25、25～23、22～20、20～18、17～14、12～11、8～6、5～4及<2Ma等13个热史演化阶段。通过将上述热史信息与印度大陆与欧亚大陆碰撞角度和碰撞速率变化曲线的对比,可以确定70～69、61～60、43～42、35～34、22～20和12～11Ma等6个阶段的年代学信息是两大陆碰撞角度和碰撞速率变化事件在东喜马拉雅构造结热史上的记录;通过与全球深海氧、碳同位素记录曲线的对比,可以认为26～25、25～23、17～14、8～6、5～4和<2Ma等6个阶段的年代学信息是气候变化在东喜马拉雅构造结热史上的记录。东喜马拉雅构造结地质体热史演化是构造与气候相互作用的结果。

As one of the most typical examples, Himalaya orogen is the focus in studying an important and front dispute in Earth science. It is a paradox problem that the uplift of mountains is caused by climatic process or by tectonic movement. Both climatic and tectonic uplift are linked to surface erosion, the influence of which to geological system can be described by means of the cooling history of minerals or rocks. So the cooling history of a mountain range is the key to uncover the geological process of rising mountains. Research on minerals from river sediments is one of the most useful methods in modern geological sciences to reveal the geological process and the tectonic evolution. For the purpose described above, newly deposited fine-grain river sand was sampled at Didong reach, which locates around the lower reach of Yalung-Zangbo River, to obtain high precision Ar/ Ar geochronological data on single crystals of hornblende, muscovite, biotite and K-feldspar collected from the sample, and a statistical diagram of age probability is shown in the contrast figure. The high precision ^(40)Ar/^(39)Ar geochronological data on single crystals of minerals from sediments at Didong reach reveals the cooling history of rocks in the drainage area of Yalung-Zangbo Great Canyon. According to the statistical results, the episodes of cooling history of the drainage area of Yaluang-Zangbo Great Canyon can be identified as 70～69、61～60、43～42、35～34、 26～25、25～23、22～20、20～18、17～14、12～11、8～6、5～4 and <2Ma respectively. The contrast of age statistical results against the figure showing the changes in rate and angle of convergence during India plate has been colliding against Eurasian plate since Late Mesozoic shows that probably the ages of 70～69、61～60、43～42、35～34、22～20 and 12～11 Ma are records of tectonic events of changes in rate and angle of convergence of the two continent, while the contrast of the age statistical results against the diagram consisting of global deep-sea δ^(18) O and δ^(13) C records suggests that the ages of 26～25、25～23、17～14、8～6、5～4 and <2Ma seem to be the records of global climate change. The ^(40)Ar/^(39)Ar geochronological data published in this paper reveals the interaction of tectonic and climatic in rock uplifting in Eastern Himalaya Syntaxis.