【作者】 龚俊峰；

【导师】 韩宝福； 季建清；

【作者基本信息】 北京大学 ， 构造地质学， 2009， 博士

【摘要】 对喜马拉雅造山带岩体的热史演化研究不仅能为山脉的隆起历史提供精确的年代学坐标,也能为检验造山带的隆起模型及新构造样式提供基础资料。在喜马拉雅造山带中东段,系统的针对岩体热史演化的研究尚处于起步阶段,喜马拉雅山的隆升之谜也一直没有彻底解开。本文选择位于喜马拉雅中段亚东地区的亚东—哲古拉剖面以及位于东喜马拉雅构造结的雅鲁藏布江沿岸派乡—扎曲段、那木拉峰西坡以及多雄拉峰东坡等四条剖面,利用精细的阶步加温和单次全熔⁴⁰Ar/³⁹Ar年代学方法,试图构建喜马拉雅中段及东喜马拉雅构造结岩体热史演化的年代学框架,以期解开喜马拉雅造山带中东段的抬升之谜。在喜马拉雅中段亚东地区,以出露于亚东县城以北约40 km处哲古拉地区的退变质高压麻粒岩为窗口,以其中的峰期变质矿物黑云母及退变质矿物角闪石的阶步加温⁴⁰Ar/³⁹Ar年代学方法约束的高压变质作用之后的两期退变质事件年龄分别为48.5和31.1 Ma,其寄主岩石黑云母钾长花岗岩中的黑云母和钾长石⁴⁰Ar/³⁹Ar年龄分别为11.5和12.6 Ma,为后期冷却年龄。同时,高压麻粒岩中的锆石U-Pb结果为17 Ma,为其被花岗岩捕掳的年龄,其寄主岩石中的锆石U-Pb年龄集中在>1800和841 Ma,为原岩年龄信息。在亚东县城以南的两处花岗质片麻岩中,锆石U-Pb年龄分别为1149和474 Ma,为原岩和泛非变质事件的年龄,黑云母单次全熔⁴⁰Ar/³⁹Ar年龄为11.3和11.0 Ma,磷灰石裂变径迹年龄分别为7和8 Ma。亚东以南岩体与哲古拉高压麻粒岩的寄主岩石中的黑云母⁴⁰Ar/³⁹Ar年龄一致,说明在亚东—哲古拉剖面上,岩体同时冷却至黑云母对Ar同位素体系的封闭温度（³20℃）,推测之后在7⁸Ma前近同时冷却至¹10℃。根据亚东—哲古拉剖面的年代学信息,可以大致刻画高喜马拉雅结晶岩系的构造演化历史:高喜马拉雅结晶岩系的原岩在>1000 Ma前形成之后,经历了泛非变质事件的改造,之后随着印度大陆向欧亚大陆的俯冲碰撞,经历了高压变质事件,并在48.5和31.1 Ma退变质,在31.1¹7 Ma期间发生部分熔融而滞留于中下地壳15²4 km深度处,在17⁷ Ma前以通道流的方式快速折返至近地表,之后缓慢剥露于地表。东喜马拉雅构造结是气候与构造相互作用的典型地区,本文尝试使用单颗粒高精度单次全熔⁴⁰Ar/³⁹Ar法对采自雅鲁藏布江大峡谷下游墨脱县城以南约50 km处地东河段的细粒现代河沙进行年代学测试,建立东喜马拉雅构造结的热史演化序列,并通过与代表全球气候变化的18O、13C浓度变化曲线和印度大陆向北运动的速度、角度变化曲线的对比,揭示气候、构造因素与东喜马拉雅构造结热史演化的耦合,推定热史演化与气候、构造过程的关系。同时,本文对东构造结地区进行了大量地面定点样品的⁴⁰Ar/³⁹Ar年龄分析,结果表明,雅鲁藏布江沿岸派乡—扎曲段岩体及那木拉西坡共18件富钾矿物的阶步加温⁴⁰Ar/³⁹Ar年龄集中于1.3和2.5 Ma,而多雄拉东坡黑云母⁴⁰Ar/³⁹Ar年龄平均值为4 Ma左右。一方面,大量小于3 Ma的⁴⁰Ar/³⁹Ar年龄表明东构造结的大规模隆升开始于上新世中期,另一方面,多雄拉东坡与西坡⁴⁰Ar/³⁹Ar年龄的差异是由于南迦巴瓦—那木拉—多雄拉峰顶连线以西发育的北东走向掀斜构造所引起的差异性抬升和剥蚀所造成,该掀斜构造开始发育于4.0～2.2 Ma期间,且一直活动到现在,其动力源自气候与构造的共同作用。喜马拉雅中段与东构造结不尽相同的热演化历史说明二者的隆升方式和时代存在明显差异。喜马拉雅中段主要在17⁷ Ma前以通道流的方式快速隆升,之后的抬升过程相对缓慢;东喜马拉雅构造结则主要在3 Ma以来,以地壳均衡作用为背景的掀斜式抬升为主要隆升模式。二者抬升的驱动力则较为相似,都是南北向挤压作用下的气候驱动,即是构造控制下构造与气候的共同作用。更多还原

【Abstract】 Study on thermal history of Himalaya orogen provides not only accurate chronological coordinate on the cooling history, but also basic materials in verifying models related to the uplift of Himalaya. On central-east part of Himalaya orogen, systematic geochronological researches focusing on thermal evolution was neglectable, so the puzzle of how and when did Himalaya uplift has not been integrally explained. In this paper, four sections located respectively in Yadong-ZhergerLa in central Himalaya, Pai-Zaqu reach of Yalung-Tsangpo River, west slope section of NamuLa and east slope section of DuoxiongLa in Eastern Himalaya Syntaxis were chosen, and conventional and laser ⁴⁰Ar/³⁹Ar methods were used to build the framework of thermal evolution history of central-east part of Himalaya orogen, for the purpose of revealing the secret of uplif of central-east Himalaya.In Yadong district in central Himalaya, a retro-metamorphic high-pressure granulite and its country rock were focused and conventional ⁴⁰Ar/³⁹Ar tests were done. Two retro-metamorphic events in the retro-metamorphic high-pressure granulite, the ages of which were constrained by biotite and hornblende ⁴⁰Ar/³⁹Ar geochronology, happened at 48.5 and 31.1 Ma. Thereafter, two cooling ages of 11.5 and 12.6 Ma were yield, which were constrained by biotite and K-feldspar ⁴⁰Ar/³⁹Ar ages in the country rock, a biotite K-feldspar granite. At the same time, a cluster of 17 Ma zircon U-Pb age in the retro-metamorphic high-pressure granulite and two clusters of 841 and >1800 Ma zircon U-Pb ages were already published （Ji et al., 2004）, the former of which suggests the time when the high-pressure granulite was captured, while the latter of which represent when did original rocks formed. In granitic gneiss located south of Yadong, two old zircon U-Pb ages （1149 and 474 Ma） and young biotite ⁴⁰Ar/³⁹Ar ages （11.3 and 11.0 Ma） and apatite fission track （AFT） ages （7 and 8 Ma） were yield. The old zircon U-Pb ages represent the time when original rocks formed and undergone Pan- African event subsequently, while the young ⁴⁰Ar/³⁹Ar and AFT results are cooling ages. Coincident biotite ⁴⁰Ar/³⁹Ar ages yield in rocks in the south of Yadong and in the high-pressure granulite shows that on the section of Yadong-ZhergerLa, rocks were cooled to 320℃contemporarily. According to geochronological data obtained in this study, a tectonic evolutional history of rocks currently exposed on the earth surface can be made: After forming before 841 Ma, the original rocks of High Himalaya Crystalline System were reformed by Pan-African event at ⁴74 Ma, then were undergone high pressure metamorphism at ⁵5 Ma （inferred） and two retro-metamorphic events at 48.5 and 31.1 Ma respectively, thereafter, they were persisted partial molten at depth of 15²4 km during 31.1¹7 Ma, and were experienced fast extrusion event in the way of channel flow in the period of 17⁷ Ma, then were exposed to the earth surface at a lower velocity.Eastern Himalaya Syntaxis is an important area in studying climatic-tectonic reciprocity. In this paper, single-crystal-high-precision laser ⁴⁰Ar/³⁹Ar method was used to build the thermal evolutional history of Eastern Himalaya Syntaxis by tests on river sediments on the lower reach of Yalung-Tsangpo River. 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 and the diagram consisting of global deep-seaδ18O andδ13C records is done, for the purpose of revealing climatic and tectonic information on rock cooling and referring the relationship between thermal evolutional history and climate and tectonics. At the same time, plenty of geochronological data on ground samples were yield by conventional and laser ⁴⁰Ar/³⁹Ar tests. It shows that Pai-Zaqu reach of Yalung-Tsangpo River and west slope section of NamuLa yield ⁴⁰Ar/³⁹Ar age clusters of 1.3 and 2.5 Ma, while biotite ⁴⁰Ar/³⁹Ar age cluster of samples from east slope section of DuoxiongLa is about 4 Ma. Firstly, ⁴⁰Ar/³⁹Ar age clusters of <3 Ma reveal the fast cooling of eastern Himalaya Syntaxis since middle Pliocene. Secondly, the difference of ⁴⁰Ar/³⁹Ar age clusters between the west and east slope section of DuoxiongLa is the result of a titling structure, which strikes NE at the west side of Namjag-Barwa– NamuLa– DuoxiongLa, and which has been functioning since 4.0².2 Ma under the drive of both climatic and tectonic powers.The difference of thermal evolutional history between central Himalaya and Eastern Himalaya Syntaxis shows that they uplifted in different tectonic style at different period of time. In central Himalaya, massif was mainly uplifted in the tectonic model of channel flow during 17⁷ Ma at a velocity of ² mm/a, then was extruded to earth surface at a much lower velocity. In eastern Himalaya Syntaxis, massif was mainly uplifted in the structural style of titling at a background of isostasy since 4.0².2 Ma. As for the drive power, both of them have been driving by climatic and tectonic power under the dominant control of N-S tectonic compression.更多还原