龙泉群形成时代与构造属性及对华夏地块演化的限定

发布时间：2018-05-23 09:40

本文选题：龙泉群 + 变质岩　；参考：《浙江大学》2017年硕士论文

【摘要】：浙西南地区出露的龙泉群,一直被认为是华夏地块古老变质基底的一部分。对于变质基底形成时代和构造属性的研究可以为认识当时华夏地块构造背景以及演化过程提供重要的信息。本文以龙泉群变质岩为研究对象,以地球化学分析和碎屑锆石地质年代学分析为主要手段,揭示了龙泉群原岩性质、形成时代以及构造背景,并对早古生代的华夏地块构造演化过程提出了解释。本次研究主要取得了以下几点认识:(1)龙泉群变质岩主要岩性为片岩、变质砂岩、片麻岩以及变粒岩类,以副变质岩为主。主微量元素地球化学特征以及地球化学判别图解显示,龙泉群副变质岩原岩以砂泥质沉积岩为主,主要的原岩类型为杂砂岩,少数可能为粘土岩,原岩具有大陆岛弧以及活动大陆边缘的构造背景;同时还在龙泉群中发现了斜长角闪岩,其具有富集大离子亲石元素(如:Rb、Ba、K等)、亏损高场强元素(如:Th、Nb、Ta、Zr、Hf、Ti)的微量元素特征,该地化特征与岛弧玄武岩十分相似。结合地球化学判别图解结果,其原岩为岛弧钙碱性玄武岩,形成于与俯冲带有关的大陆边缘火山弧构造环境中。地球化学分析指示了龙泉群形成时,该地区可能处于大洋板片向大陆俯冲的汇聚板块边界上,从而形成了具有大陆弧环境以及活动大陆边缘构造环境特征的岩石单元。(2)LA-ICP-MS锆石U-Pb定年结果显示,龙泉群3个副变质岩样品碎屑锆石年龄特征具有高度的一致性。锆石年龄集中出现在1150-900 Ma以及600-540 Ma两个年龄区间,具有990 Ma和560 Ma的年龄峰。3个样品最年轻的碎屑锆石年龄分别为541 Ma、523 Ma和537 Ma,约束了龙泉群副变质岩原岩的最大沉积年龄。因此,龙泉群的形成时代应该晚于560 Ma,可能形成于早-中寒武世。龙泉群斜长角闪岩与陈蔡斜长角闪岩及黑云斜长角闪岩具有十分一致的原岩类型和构造环境,可以将龙泉群斜长角闪岩原岩形成时间约束在早古生代。(3)将龙泉群变沉积岩碎屑锆石年龄分布特征与华南地块岩浆活动记录以及华夏地块新元古代晚期到奥陶纪沉积地层碎屑锆石年龄分布特征进行对比,龙泉群变沉积岩碎屑锆石年龄分布中缺乏华南地块前寒武纪岩浆活动对应的峰值,整个华夏地块寒武纪沉积岩显示出冈瓦纳物源的年龄分布特征。寒武纪华南地块内部不可能作为龙泉群乃至整个华夏地块沉积地层的主要物源,这个物源区最有可能是位于华夏地块东南侧的冈瓦纳大陆。从新元古代晚期到奥陶纪,华夏地块持续接受来自冈瓦纳大陆的物源沉积。(4)龙泉群斜长角闪岩与陈蔡早古生代斜长角闪岩和黑云斜长角闪岩原岩均为岛弧玄武岩类,指示了在早古生代浙江龙泉-陈蔡地区发育了一个大陆火山弧,结合龙泉群副变质岩构造背景,表明当时华夏地块北东部可能处于板块汇聚边缘环境下,经历了一期洋-陆俯冲的构造事件。(5)根据龙泉群原岩性质、构造背景、形成时代以及物源区碎屑锆石特征分析工作,结合前人的研究成果,提出了新元古代晚期到奥陶纪华夏地块的构造演化过程。华南地块从新元古代晚期开始与冈瓦纳大陆聚合,最初华夏地块西部与印度北西部碰撞,此后华夏地块东南缘不断拼贴到了印度北部上,到龙泉群形成时的寒武纪中期,华夏地块已经大部分和作为冈瓦纳大陆一部分的印度北部拼合到了一起,冈瓦纳大陆为整个华夏地块寒武纪沉积地层提供了主要物源。最终的聚合过程发生在华夏地块北东部与澳大利亚之间。从寒武纪到奥陶纪,Kuunga洋向华南地块俯冲,华夏地块北东部处于汇聚板块边界环境下。随着奥陶纪Kuunga洋的最后关闭,华南地块和澳大利亚完成拼合。奥陶纪华南地块与冈瓦纳大陆碰撞聚合的远程效应传到华南地块内部,引发了随后的华南地块早古生代晚期陆内造山作用。
[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.
【学位授予单位】：浙江大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P542

【参考文献】