青藏高原拉萨地体东部早中生代变质作用与构造意义

发布时间：2018-05-12 08:59

本文选题：拉萨地体 + 变质作用　；参考：《中国地质大学(北京)》2017年博士论文

【摘要】：拉萨地体位于青藏高原的南部,夹持于印度-雅鲁藏布江缝合带和班公-怒江缝合带之间,其地质构造演化历史与特提斯洋的多期次俯冲、消减和陆-陆碰撞有关,是青藏高原的重要组成部分。拉萨地体处于青藏高原重要的构造部位,对揭示青藏高原的形成与演化过程至关重要。拉萨地体的东部出露多处中高级变质岩系,包含了多种类型的变质岩石,如片岩、片麻岩、石英岩、大理岩和斜长角闪岩。位于拉萨地体最东段的东久和巴松措两套变质岩系产出于古生代沉积岩中,由片岩、副片麻岩、正片麻岩和少量钙硅酸岩组成。本文对这两套变质岩系进行了系统的岩石学、地球化学、锆石U-Pb年代学和相平衡模拟研究,确定了这些变质岩的原岩可能为奥陶纪的沉积岩,此后经历了早中生代的变质作用。基于岩相学、矿物化学和相平衡模拟研究,我们确定了东久变质岩经历了高角闪岩相的变质作用,变质作用P-T轨迹为近等温降压型。峰期变质温压条件为745°C和9.6Kbar,中压角闪岩相退变叠加的温压条件为600~630°C和5~6Kbar。LA-ICP-MS原位锆石U-Pb定年结果显示锆石变质边的年龄为194~192Ma,表明东久变质岩系的变质时代为早侏罗世。拉萨地体东部巴松措变质带经历了中压角闪岩相变质作用(T=665°C;P=4.9Kbar),变质作用峰期温压条件落在20-40°C/Km的中压相系地温梯度范围内,属于典型的“巴罗型”变质带。变质岩的原岩为一套沉积岩,包括泥质岩、杂砂岩、灰岩和火山碎屑岩等。变质岩中锆石核部记录了2520 Ma~511 Ma的年龄,峰值位于1499 Ma、1161 Ma和557 Ma,变质沉积岩的物质源区经历了不同时期的岩浆作用且沉积年龄不早于511 Ma;变质锆石记录了201~190 Ma的变质年龄,表明该变质带的变质作用发生在早侏罗世。巴松措-东久变质带与西部的近同期形成的松多高压榴辉岩带、旁那高蓝闪片岩带和念青唐古拉地区的角闪岩相变质带一起构成了一条东西延伸约500Km长的拉萨地体中部变质构造带。同时,230Ma~182Ma的连续变质年龄显示拉萨地体中部经历了一个晚三叠世到早侏罗世由西向东的渐次碰撞造山和地壳加厚过程。本次研究为青藏高原东部晚古生代到早中生代的形成与演化的再造,为复合造山作用及动力学的研究提供了重要的限定。
[Abstract]:The Lhasa terrane lies in the southern part of the Qinghai-Tibet Plateau, between the India-Brahmaputra suture zone and the Bangong-Nujiang suture zone. Its history of tectonic evolution is related to the multiple subduction and subduction of the Tethys ocean and land- land collision. It is an important part of the Qinghai-Tibet Plateau. Lhasa terrane is an important tectonic position in Qinghai-Tibet Plateau, which is very important to reveal the formation and evolution of Qinghai-Xizang Plateau. The eastern Lhasa terrane contains a variety of metamorphic rocks, such as schist, gneiss, quartzite, marble and amphibolite. Two sets of metamorphic rocks, Dongjiu and Bassongcao, which are located in the easternmost member of Lhasa terrane, are formed in Paleozoic sedimentary rocks and are composed of schist, paragneiss, orthogneiss and a small amount of calcium silicate. The two sets of metamorphic rocks have been systematically studied in petrology, geochemistry, zircon U-Pb chronology and facies equilibrium simulation. It is concluded that the source rocks of these metamorphic rocks may be sedimentary rocks of the Ordovician, and have undergone early Mesozoic metamorphism since then. Based on the studies of petrography, mineral chemistry and phase equilibrium simulation, we have determined that the Dongjiu metamorphic rocks experienced metamorphism of high amphibolite facies, and the P-T locus of metamorphism is a near isothermal depressurization type. The peak metamorphic temperature and pressure conditions are 745 掳C and 9.6 Kbar. the temperature and pressure conditions of middle pressure amphibolite facies retrograde superposition are 600 掳C and 630 掳C and 5~6Kbar.LA-ICP-MS in-situ zircon U-Pb dating results show that the metamorphic edge of the zircon is 1942 Ma, indicating that the metamorphic age of the eastern Jurassic metamorphic series is early Jurassic. The Bassongcao metamorphic belt in the east of Lhasa terrane has undergone medium pressure amphibolite facies metamorphism (T _ (665) 掳C ~ (1) P ~ (10) K ~ (barb). The temperature and pressure conditions of the metamorphism peak period fall within the range of 20-40 掳C/Km of the medium pressure facies system geothermal gradient, which belongs to the typical "Barrow type" metamorphic belt. Metamorphic rocks consist of a set of sedimentary rocks, including argillaceous rocks, complex sandstone, limestone and pyroclastic rocks. The zircon core age of 2520 Ma~511 Ma was recorded in metamorphic rocks, with peak values at 1499 Ma, 1161 Ma and 557 Ma. The metamorphic sedimentary rocks underwent magmatism in different periods and their sedimentary ages were not earlier than 511 Ma.The metamorphic zircons recorded metamorphic ages of 201 ~ 190Ma. The metamorphism of this metamorphic zone occurred in the early Jurassic. Bassongcao-Dongjiu metamorphic belt and the Songduo high-pressure eclogite belt formed in the near same period in the west, The Panna high blue diorite schist belt and the amphibolite facies metamorphic belt in Nianqingtang Gula area constitute a metamorphic tectonic belt in the middle of Lhasa terrane which extends from east to west to about the length of 500Km. At the same time, the continuous metamorphic age of 182Ma indicates that the central Lhasa terrane underwent a gradual collision orogeny and crustal thickening from west to east from late Triassic to early Jurassic. This study provides important limits for the formation and evolution of late Paleozoic to early Mesozoic in the eastern Qinghai-Xizang Plateau and for the study of complex orogeny and dynamics.
【学位授予单位】：中国地质大学(北京)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P588.3;P542

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