本文選題：拉薩地體 + 變質(zhì)作用��； 參考：《中國地質(zhì)大學(北京)》2017年博士論文

【摘要】：拉薩地體位于青藏高原的南部,夾持于印度-雅魯藏布江縫合帶和班公-怒江縫合帶之間,其地質(zhì)構(gòu)造演化歷史與特提斯洋的多期次俯沖、消減和陸-陸碰撞有關(guān),是青藏高原的重要組成部分。拉薩地體處于青藏高原重要的構(gòu)造部位,對揭示青藏高原的形成與演化過程至關(guān)重要。拉薩地體的東部出露多處中高級變質(zhì)巖系,包含了多種類型的變質(zhì)巖石,如片巖、片麻巖、石英巖、大理巖和斜長角閃巖。位于拉薩地體最東段的東久和巴松措兩套變質(zhì)巖系產(chǎn)出于古生代沉積巖中,由片巖、副片麻巖、正片麻巖和少量鈣硅酸巖組成。本文對這兩套變質(zhì)巖系進行了系統(tǒng)的巖石學、地球化學、鋯石U-Pb年代學和相平衡模擬研究,確定了這些變質(zhì)巖的原巖可能為奧陶紀的沉積巖,此后經(jīng)歷了早中生代的變質(zhì)作用�；趲r相學、礦物化學和相平衡模擬研究,我們確定了東久變質(zhì)巖經(jīng)歷了高角閃巖相的變質(zhì)作用,變質(zhì)作用P-T軌跡為近等溫降壓型。峰期變質(zhì)溫壓條件為745°C和9.6Kbar,中壓角閃巖相退變疊加的溫壓條件為600~630°C和5~6Kbar。LA-ICP-MS原位鋯石U-Pb定年結(jié)果顯示鋯石變質(zhì)邊的年齡為194~192Ma,表明東久變質(zhì)巖系的變質(zhì)時代為早侏羅世。拉薩地體東部巴松措變質(zhì)帶經(jīng)歷了中壓角閃巖相變質(zhì)作用(T=665°C;P=4.9Kbar),變質(zhì)作用峰期溫壓條件落在20-40°C/Km的中壓相系地溫梯度范圍內(nèi),屬于典型的“巴羅型”變質(zhì)帶。變質(zhì)巖的原巖為一套沉積巖,包括泥質(zhì)巖、雜砂巖、灰?guī)r和火山碎屑巖等。變質(zhì)巖中鋯石核部記錄了2520 Ma~511 Ma的年齡,峰值位于1499 Ma、1161 Ma和557 Ma,變質(zhì)沉積巖的物質(zhì)源區(qū)經(jīng)歷了不同時期的巖漿作用且沉積年齡不早于511 Ma;變質(zhì)鋯石記錄了201~190 Ma的變質(zhì)年齡,表明該變質(zhì)帶的變質(zhì)作用發(fā)生在早侏羅世。巴松措-東久變質(zhì)帶與西部的近同期形成的松多高壓榴輝巖帶、旁那高藍閃片巖帶和念青唐古拉地區(qū)的角閃巖相變質(zhì)帶一起構(gòu)成了一條東西延伸約500Km長的拉薩地體中部變質(zhì)構(gòu)造帶。同時,230Ma~182Ma的連續(xù)變質(zhì)年齡顯示拉薩地體中部經(jīng)歷了一個晚三疊世到早侏羅世由西向東的漸次碰撞造山和地殼加厚過程。本次研究為青藏高原東部晚古生代到早中生代的形成與演化的再造,為復合造山作用及動力學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 擄C and 9.6 Kbar. the temperature and pressure conditions of middle pressure amphibolite facies retrograde superposition are 600 擄C and 630 擄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) 擄C ~ (1) P ~ (10) K ~ (barb). The temperature and pressure conditions of the metamorphism peak period fall within the range of 20-40 擄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.
【學位授予單位】：中國地質(zhì)大學(北京)
【學位級別】：博士
【學位授予年份】：2017
【分類號】：P588.3;P542

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