發(fā)布時間：2018-05-23 09:40

  本文選題：龍泉群 + 變質(zhì)巖　； 參考：《浙江大學》2017年碩士論文

【摘要】：浙西南地區(qū)出露的龍泉群,一直被認為是華夏地塊古老變質(zhì)基底的一部分。對于變質(zhì)基底形成時代和構(gòu)造屬性的研究可以為認識當時華夏地塊構(gòu)造背景以及演化過程提供重要的信息。本文以龍泉群變質(zhì)巖為研究對象,以地球化學分析和碎屑鋯石地質(zhì)年代學分析為主要手段,揭示了龍泉群原巖性質(zhì)、形成時代以及構(gòu)造背景,并對早古生代的華夏地塊構(gòu)造演化過程提出了解釋。本次研究主要取得了以下幾點認識:(1)龍泉群變質(zhì)巖主要巖性為片巖、變質(zhì)砂巖、片麻巖以及變粒巖類,以副變質(zhì)巖為主。主微量元素地球化學特征以及地球化學判別圖解顯示,龍泉群副變質(zhì)巖原巖以砂泥質(zhì)沉積巖為主,主要的原巖類型為雜砂巖,少數(shù)可能為粘土巖,原巖具有大陸島弧以及活動大陸邊緣的構(gòu)造背景;同時還在龍泉群中發(fā)現(xiàn)了斜長角閃巖,其具有富集大離子親石元素(如:Rb、Ba、K等)、虧損高場強元素(如:Th、Nb、Ta、Zr、Hf、Ti)的微量元素特征,該地化特征與島弧玄武巖十分相似。結(jié)合地球化學判別圖解結(jié)果,其原巖為島弧鈣堿性玄武巖,形成于與俯沖帶有關(guān)的大陸邊緣火山弧構(gòu)造環(huán)境中。地球化學分析指示了龍泉群形成時,該地區(qū)可能處于大洋板片向大陸俯沖的匯聚板塊邊界上,從而形成了具有大陸弧環(huán)境以及活動大陸邊緣構(gòu)造環(huán)境特征的巖石單元。(2)LA-ICP-MS鋯石U-Pb定年結(jié)果顯示,龍泉群3個副變質(zhì)巖樣品碎屑鋯石年齡特征具有高度的一致性。鋯石年齡集中出現(xiàn)在1150-900 Ma以及600-540 Ma兩個年齡區(qū)間,具有990 Ma和560 Ma的年齡峰。3個樣品最年輕的碎屑鋯石年齡分別為541 Ma、523 Ma和537 Ma,約束了龍泉群副變質(zhì)巖原巖的最大沉積年齡。因此,龍泉群的形成時代應(yīng)該晚于560 Ma,可能形成于早-中寒武世。龍泉群斜長角閃巖與陳蔡斜長角閃巖及黑云斜長角閃巖具有十分一致的原巖類型和構(gòu)造環(huán)境,可以將龍泉群斜長角閃巖原巖形成時間約束在早古生代。(3)將龍泉群變沉積巖碎屑鋯石年齡分布特征與華南地塊巖漿活動記錄以及華夏地塊新元古代晚期到奧陶紀沉積地層碎屑鋯石年齡分布特征進行對比,龍泉群變沉積巖碎屑鋯石年齡分布中缺乏華南地塊前寒武紀巖漿活動對應(yīng)的峰值,整個華夏地塊寒武紀沉積巖顯示出岡瓦納物源的年齡分布特征。寒武紀華南地塊內(nèi)部不可能作為龍泉群乃至整個華夏地塊沉積地層的主要物源,這個物源區(qū)最有可能是位于華夏地塊東南側(cè)的岡瓦納大陸。從新元古代晚期到奧陶紀,華夏地塊持續(xù)接受來自岡瓦納大陸的物源沉積。(4)龍泉群斜長角閃巖與陳蔡早古生代斜長角閃巖和黑云斜長角閃巖原巖均為島弧玄武巖類,指示了在早古生代浙江龍泉-陳蔡地區(qū)發(fā)育了 一個大陸火山弧,結(jié)合龍泉群副變質(zhì)巖構(gòu)造背景,表明當時華夏地塊北東部可能處于板塊匯聚邊緣環(huán)境下,經(jīng)歷了一期洋-陸俯沖的構(gòu)造事件。(5)根據(jù)龍泉群原巖性質(zhì)、構(gòu)造背景、形成時代以及物源區(qū)碎屑鋯石特征分析工作,結(jié)合前人的研究成果,提出了新元古代晚期到奧陶紀華夏地塊的構(gòu)造演化過程。華南地塊從新元古代晚期開始與岡瓦納大陸聚合,最初華夏地塊西部與印度北西部碰撞,此后華夏地塊東南緣不斷拼貼到了印度北部上,到龍泉群形成時的寒武紀中期,華夏地塊已經(jīng)大部分和作為岡瓦納大陸一部分的印度北部拼合到了一起,岡瓦納大陸為整個華夏地塊寒武紀沉積地層提供了主要物源。最終的聚合過程發(fā)生在華夏地塊北東部與澳大利亞之間。從寒武紀到奧陶紀,Kuunga洋向華南地塊俯沖,華夏地塊北東部處于匯聚板塊邊界環(huán)境下。隨著奧陶紀Kuunga洋的最后關(guān)閉,華南地塊和澳大利亞完成拼合。奧陶紀華南地塊與岡瓦納大陸碰撞聚合的遠程效應(yīng)傳到華南地塊內(nèi)部,引發(fā)了隨后的華南地塊早古生代晚期陸內(nèi)造山作用。
[Abstract]:The Longquan group, exposed in Southwestern Zhejiang Province, has been considered to be part of the ancient metamorphic basement of the Huaxia block. The study of the metamorphic basement formation age and structural properties can provide important information for understanding the tectonic setting and evolution process of the China massif at that time. This paper is based on the geochemical analysis of the metamorphic rocks of the Longquan group. And the geological chronological analysis of detrital zircon as the main means to reveal the nature, age and tectonic setting of the Longquan group, and explain the tectonic evolution process of the early Paleozoic Huaxia massif. (1) the main lithology of the Longquan group metamorphic rocks is schist, metamorphic sandstone, gneiss and gneiss. The main microelement geochemistry and geochemical discriminatory diagrams show that the primary rocks of the Longquan group paraceous metamorphic rocks are mainly sandy muddy sedimentary rocks, and the main types are claystone, a few may be clay rocks, the original rocks have continental island arc and tectonic setting on the active continental margin; at the same time, they are also dragons. The diagonal amphibolite has been found in the spring group, which has the characteristics of the enrichment of large ion stone elements (such as Rb, Ba, K, etc.), and loss of the trace elements of high and strong elements (such as Th, Nb, Ta, Zr, Hf, Ti), which are very similar to the island arc basalt. In the tectonic environment of the continental margin volcanic arc, geochemical analysis indicates that when the Longquan group is formed, the area may be in the boundary of the converging plate of the oceanic plate subduction to the continent, thus forming a rock unit with the continental arc environment and the tectonic environment characteristics of the active continental margin. (2) the U-Pb dating results of LA-ICP-MS zircon show that The age characteristics of the detrital zircons of the 3 Paran metamorphic rocks of the Longquan group have high consistency. The age of zircon is concentrated in 1150-900 Ma and 600-540 Ma two age intervals, with the age peak of 990 Ma and 560 Ma, the youngest detrital zircon ages are 541 Ma, 523 Ma and 537 Ma, which constrain the primary rocks of the Paran metamorphic rocks. Therefore, the formation age of the Longquan group should be later than 560 Ma, probably formed in the early Middle Cambrian. The Longquan group amphibolite, with the Chen Caixie amphibolite and the black cloud amphibolite, has a very consistent type of primitive rock and tectonic environment. The formation time of the diagonal Amphibolite of the Longquan group can be confined to the early Paleozoic. (3) The age distribution characteristics of the detrital zircon in the Longquan group metamorphic rocks are compared with the records of the magmatic activity in the Southern China massif and the characteristics of the age distribution of the detrital zircon from the late Neoproterozoic to Ordovician sedimentary strata in the Huaxia block, and the peak of the corresponding peak of the Precambrian magmatism of the Southern China block in the age distribution of the detrital zircons of the Longquan group metamorphic sedimentary rocks is short. The Cambrian sedimentary rocks in the Huaxia block show the age distribution of the Gondwana source. The Cambrian Southern China block can not be used as the main source of the sedimentary strata of the Longquan and the whole Huaxia massif. The source area is most likely located on the Gondwana continent in the southeastern side of the Huaxia block. The massif continues to accept the source deposits from the Gondwana continent. (4) the Longquan group amphibolite and the early Paleozoic amphibolite and the black cloud diagonal amphibolite are all island arc basalts, indicating that a continental volcanic arc developed in the early Palaeozoic Zhejiang Longquan Chen Cai area, which is combined with the structural background of the accessory metamorphic rocks of the Longquan group. In the eastern part of the Ming Dynasty, the northern part of the central China block may be in the edge of plate convergence and experienced a tectonic event of oceanic continental subduction. (5) according to the properties of the original rocks of the Longquan group, the tectonic setting, the formation time and the analysis of the characteristics of the detrital zircon in the source area, the late Neoproterozoic to the Ordovician Huaxia massif was put forward in combination with the previous research results. In the late Neoproterozoic Era, the Southern China block began to polymerized with the Gondwana continent. At first the western part of the Huaxia block was collided with the northern West of India. After that, the southeast margin of the Huaxia massif was continuously collaged to the north of India, to the Middle Cambrian in the formation of the Longquan group, and the Huaxia block had been mostly and as a part of a part of the Gondwana continent. The northern part of the Gondwana continent provides the main source of the Cambrian sedimentary strata in the whole Huaxia block. The final polymerization process occurs between the north and the east of the Huaxia block. From the Cambrian to the Ordovician, the Kuunga ocean subducted to the Southern China block, and the northern part of the Huaxia block is under the boundary of the converging plate boundary. The Ordovician Kuunga ocean was finally closed, the Southern China block and Australia completed the combination. The remote effect of the collision and polymerization of the Ordovician Southern China block to the Gondwana continent was transmitted to the interior of the Southern China block, causing the subsequent early Paleozoic intracontinental orogeny of the Southern China block.
【學位授予單位】：浙江大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：P542

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