江南造山带西段新元古代下江群年代地层标定与盆地演化

发布时间：2018-05-19 16:51

本文选题：物源分析 + 锆石U-Pb年代学　；参考：《中国地质大学》2015年博士论文

【摘要】：下江群是分布在江南造山带西段(黔东南地区)的一套前寒武纪浅变质陆源碎屑岩夹火山碎屑岩组合。其地层时代归属与划分,特别是区域地层对比一直未能得到很好解决；此外,下江群的沉积演化、盆地性质以及盆地动力学机制长期存在较大争议。本文主要运用造山带沉积地质学的研究思路和方法,对黔东南地区下江群开展沉积地质学、碎屑岩全岩地球化学、砂岩碎屑组分、碎屑锆石微量元素和U-Pb年代学等多方面研究,获得了如下主要认识：(1)下江群是一套具复理石特征的陆源碎屑岩夹火山碎屑岩建造。岩性主要以粉砂质板岩、变余粉砂岩、变余粉-细砂岩、凝灰质板岩、凝灰质变余粉砂岩和沉凝灰岩等为主。沉积构造有水平层理、平行层理、粒序层理、交错层理、块状层理、均质层理、脉状和透镜状层理等。事件沉积有滑塌滑移事件沉积和浊流事件沉积。属于滨海-浅海-半深海沉积体系。(2)运用LA-ICP MS测年技术,测得江南造山带西段四堡群河村组顶部的碎屑岩和下江群中乌叶组第一段顶部的含凝灰质碎屑岩、清水江组底部与中部的沉凝灰岩、平略组中上部的含凝灰质碎屑岩及隆里组中下部的碎屑岩中锆石最小年龄组的加权平均年龄分别为819.8±6.4 Ma和779.5±4.7 Ma、764.0±6.3 Ma 与(756.8±7.6 Ma、756±13 Ma)、733.9±8.8 Ma及725±10 Ma。以这些年龄数据约束下江群地层沉积时限约为815～720 Ma。其中,甲路组一段沉积时限约为815～805 Ma；甲路组二段沉积时限约为805～800 Ma；乌叶组一段沉积时限约为800～780 Ma；乌叶组第二段至番召组沉积时限约为780～770 Ma；清水江组沉积时限约为770～745 Ma；平略和隆里组沉积时限约为745～720 Ma。结合江南造山带地区报道的地层年龄或与地层具明显先后关系的岩浆岩年龄,开展了下江群时期的地层划分与对比。本文年龄数据表明下江群属于新元古代,显示江南造山带基底的构造背景与属性与“格林威尔运动”无关。(3)通过对下江群的沉积相和沉积环境分析,认为下江期沉积演化经历了以下几个阶段：①剥蚀夷平和填平阶段(甲路组一段)；②盆地初始伸展阶段(弧后伸展)(甲路组二段)；③盆地持续伸展阶段(弧后伸展)(乌叶组和番召组时期)；④盆地差异性隆升阶段(清水江组时期)；⑤盆地萎缩和快速消亡阶段(平略组至隆里组时期)。(4)下江群的砂岩碎屑颗粒组成统计显示,砂岩整体分选磨圆较差-中等,少数分选磨圆较好,含较多的岩屑和长石颗粒以及凝灰质杂基,部分发生硅化、绢云母化和泥化等浅变质作用,见云母类、锆石等副矿物。长石风化蚀变明显,火山灰具有蚀变微晶和霏晶结构。主要组分石英总量(7.7%～89.3%)、单晶石英(5.5%～78.9%)、多晶石英(2.1%-38.6%)、长石总量(2.1%～34.3%)、斜长石类(2.1%～27.0%)、碱性长石类(1.1%-36.8%)、岩屑总量(4.9%～78.6%)、沉积岩屑+变质岩屑(4.4%～73.1%)和火山岩屑(0.0%～65.9%)。Q-F-L、Q-M-Lt和Qp-Lv-Ls图解显示下江群砂岩物源较为复杂,主要来自再旋回造山带物源及岩浆弧物源。表明物源区主要为与弧有关的再旋回造山带。图解中具有明显弧造山带物源,显示下江群时期江南造山带西段存在岩浆弧,结合区域资料认为该岛弧在广西龙胜一带,下江群处于弧后盆地沉积环境。(5)全岩地球化学研究中,主量元素含量及比值、稀土微量元素含量及比值、稀土元素配分模式和主量、微量元素判别图解等多种、多重参数判别结果表明,下江群的碎屑沉积物形成于活动大陆边缘环境。(6)作者将下江群和四堡群中碎屑岩和沉凝灰岩样品所有年龄数据划分为723Ma-745 Ma、745 Ma-775 Ma、775 Ma-815 Ma、815 Ma-825 Ma、825 Ma-875 Ma和875Ma等6个年龄组段,进行锆石微量元素图解判别。六个组别的投图结果基本相似。在Th/U-Nb/Hf 和 Th/Nb-Hf/Th图解上几乎全部落入岩浆弧/造山带环境,而远离板内/非造山带环境。在Hf-Th/Yb、Hf-U/Yb、Y-Th/Yb和 Y-U/Yb图解上落入大陆花岗质岩石的锆石区域,其中有1/2左右数据点落入大陆地壳锆石和大洋地壳锆石的混合区,个别落入大洋地壳锆石区域。由此认为江南造山带西段新元古代锆石可能形成于岛弧俯冲环境,而非板内环境,其岛弧俯冲环境至少持续到新元古代时期下江群(760Ma±)。(7)扬子地块东南缘(江南造山带)东段的双桥山群,西段的梵净山群、冷家溪群和四堡群的碎屑锆石具有相似的年龄谱峰值。扬子地块东南缘西段的下江群、丹洲群以及板溪群和东段的下江期碎屑锆石年龄谱峰值也具有相似性,区别在于两个较老的年龄峰值存在差异,表明它们的物源区总体相似但存在一定的差异性。而四堡群及其相当层位与下江群及其相当层位的碎屑锆石年离谱峰值比较,显示四堡群的1600 Ma和1950 Ma年龄峰值更为明显,而下江群的2500 Ma和2000 Ma峰值较为明显,反映物源区的变化或是物源区剥蚀层位的变化。二者与华夏地块岩浆岩和扬子地块内部岩浆岩年龄谱峰值存在明显差异,而与扬子地块周缘岩浆岩年龄谱峰值相似,揭示物源区主要是扬子地块周缘增生带。而大量同沉积碎屑锆石的出现反映来自活动的火山物质。来自扬子地块内部物源信息弱,没有来自华夏地块物源信息,暗示华夏地块与扬子地块新元古代四堡期和下江群尚未完全拼贴一体,二者之间存在深海(海沟)相隔。江南造山带西段下江期可能处于“沟—弧—盆”体系。(8)清水江组是下江期物源转换的关键时期。下江群早期(甲路组至清水江组)与下江群晚期(平略组和隆里组)物源的碎屑锆石年离谱比较,前者具相对较多的老锆石年龄,后者则几乎未见老锆石年龄,暗示下江群早期物源可能主要来自扬子地块周缘四堡群及其相当层位的沉积区和扬子地块基底,而晚期物源转变为下江群早期沉积物,较少有来自四堡期的物源。另外,清水江组的沉凝灰岩年龄770～745 Ma与区域上广西龙胜、湖南古丈的枕状玄武岩和基性岩浆岩年龄(760 Ma)在误差范围内一致,暗示清水江组的源区可能来自广西龙胜和湖南古丈一带。扬子地块北缘、西缘及雪峰山地区的770～745 Ma的岩浆活动明显,但是由于扬子地块相隔,它们无法为南缘提供物源(如果提供物源必然有更多的扬子地块内部物源信息)。在扬子地块北缘、西缘及雪峰山地区的下江群地层与上覆南华系地层角度不整合关系,延至研究区已演变为微角度不整合→平行不整合→整合接触(北西向南东方向),反映雪峰造山运动的核心区域不在扬子地块南东缘的黔东南地区。雪峰运动使得广西龙胜一带的火山弧消亡。(9)建立了江南造山带西段下江期的俯冲(弧-陆)—伸展动力学模式。(a)870-830Ma时期：江南造山带西段处于沟—弧—盆体系,沉积了梵净山/四堡群的一套具复理石建造的碎屑岩。(b)830～815 Ma时期：由于扬子地块与华夏地块之间的洋—陆汇聚导致四堡火山弧的消失,发生强烈的挤压造山作用(武陵运动),江南褶皱带雏形最终形成。晚期发生造山后的垮塌。(c)815～770 Ma时期：江南造山带西段持续发生挤压缩短,广西龙胜一带岛弧环境逐渐发育并最终成熟。黔东南至桂北地区处于弧后伸展环境,沉积了下江群下部的甲路—番召组地层。(d)770-745 Ma时期：扬子地块周缘发生强烈俯冲汇聚——弧-陆造山作用(雪峰运动)。但其汇聚强度存在差异,在扬子地块西缘与北部规模大、活动强烈；而扬子东南缘江南造山带西段规模较小。(e)745～720 Ma时期：经雪峰运动以后,华夏地块与扬子地块可能已经完全拼贴一体。
[Abstract]:The lower river group is a set of pyroclastic rock assemblage of the Precambrian shallow metamorphic terrigenous clastic rocks distributed in the western part of the Jiangnan orogenic belt (Qiandongnan area). Its stratigraphic age and division, especially the regional stratigraphic contrast has not been well solved. In addition, the sedimentary evolution of the lower river group, the nature of the basin and the mechanism of the basin dynamics are long term. In this paper, we mainly use the research ideas and methods of the sedimentary geology of the orogenic belt to study the sedimentary geology, the whole rock geochemistry of the detrital rock, the sandstone debris composition, the trace elements of the detrital zircon and U-Pb chronology in the Qiandongnan area: (1) the lower river group is a set of complex sets. The lithic rocks are characterized by terrigenous clastic rocks. The lithology is mainly composed of silty slate, residual siltstone, residual powder - fine sandstone, tuffaceous slate, tuffaceous siltstone and tuff. The sedimentary structures have horizontal bedding, parallel bedding, granular bedding, interlaced bedding, homogeneous bedding, pulse and lenticular Stratification and so on. The sedimentary and turbidity current deposits are deposited in the coastal - shallow sea - semi deep sea. (2) using LA-ICP MS dating technique, the clastic rocks at the top of the four Fort Group and the top of the first section of the first section of the Wu Ye formation in the lower Jiangnan group of the Jiangnan orogenic belt, and the bottom and the middle of the Qingshui River group, are measured. The weighted average age of the minimum age group of zircon in the middle and upper part of the middle and lower part of the Luli group is 819.8 + 6.4 Ma and 779.5 + 4.7 Ma respectively, 764 + 6.3 Ma and (756.8 + 7.6 Ma, 756 + 13 Ma), 733.9 + 8.8 Ma and 725 + 764 Ma. under these age data constraints. The limit is about 815~720 Ma., and the time limit for the first section of the Jia Road Group is about 815~805 Ma, the two section of the two section of the Jia Road Group is about 805~800 Ma, the time limit for the first section of the Wu leaf group is about 800~780 Ma, the sedimentary time of the second to the summon group in the Wu Ye formation is about 780~770 Ma, and the sedimentation time of the Qingshui River group is about 770~745 Ma; The sedimentary time is about 745~720 Ma. combined with the age of the stratigraphic age reported in the Jiangnan orogenic belt or the magmatic age which has a distinct relationship with the strata. The stratigraphic division and comparison of the lower river group are carried out. The age data shows that the Jiangnan group belongs to the Neoproterozoic, which shows the tectonic background and attributes of the basement of the Jiangnan orogenic belt and the greenway. (3) through the analysis of the sedimentary facies and sedimentary environment of the lower river group, it is believed that the sedimentary evolution of the lower river period has experienced the following stages: (1) the stage of denudation and leveling and filling (a section of the Jia Lu Group); (2) the initial extension stage (back arc extension) of the basin (two sections of the Jia Lu formation); (3) the continuous extension of the basin (back arc extension) During the period of the Zhao group); (4) the basin difference uplift stage (Qingshui River Group); 5. Basin atrophy and rapid extinction phase (period of the plateau to the Longli formation). (4) the sandstone debris particle composition of the lower river group shows that the overall separation grinding circle of the sandstone is poor - medium, and a few separation grinding circles are better, with more debris and feldspar particles and tuff. Hetero base, partial silicification, sericite and argillaceous metamorphism, and other accessory minerals such as mica and zircon. The weathering alteration of the feldspar is obvious, the volcanic ash has altered microcrystalline and felsite structure. The main components of quartz (7.7% to 89.3%), single crystal quartz (5.5% to 78.9%), polycrystalline quartz (2.1%-38.6%), total feldspar (2.1% to 34.3%), and plagioclase (2.) 1% ~ 27%), basic feldspar (1.1%-36.8%), total rock mass (4.9% ~ 78.6%), sedimentary debris + metamorphic cuttings (4.4% ~ 73.1%) and volcanic debris (0% to 65.9%).Q-F-L, Q-M-Lt and Qp-Lv-Ls diagrams show that the source of the lower river group sandstone is more complex, mainly from the source of the re gyratory orogenic belt and the source of magma arc. It is indicated that the source area is mainly related to the arc. There is an obvious arc orogenic belt in the diagrammatic orogenic belt. It shows that there is a magma arc in the west section of the Jiangnan orogenic belt during the period of the lower Jiangnan group. The island arc is located in the Longsheng area of Guangxi, and the lower river group is in the sedimentary environment of the back arc basin. (5) the content and ratio of the main elements and the trace elements of the rare earth elements and the content of the rare earth elements in the whole rock geochemistry study. The ratio, rare-earth element distribution pattern, principal quantity, trace element discriminant diagram and so on, multiple parameter identification results show that the detrital sediments of the lower river group are formed on the active continental margin environment. (6) the author divides the data of all years of age of the lower river group and the four Fort group to 723Ma-745 Ma, 745 Ma-775 Ma, 775 Ma-815 M. A, 815 Ma-825 Ma, 825 Ma-875 Ma and 875Ma and other 6 age groups, determine the trace elements of zircon. The results of the six groups are basically similar. In the Th/U-Nb/Hf and Th/Nb-Hf/Th diagrams, almost all fall into the magma arc / orogenic environment, but far away from the inner / non orogenic belt. In Hf-Th/Yb, Hf-U/Yb, Y-Th/Yb, and Y-U/Yb diagrams. The zircon regions that fall into the continental granitic rocks include about 1/2 data points falling into the continental crust of zircon and the mixed region of the oceanic zircon and the zircon region of the oceanic crust. It is believed that the Neoproterozoic zircons in the western section of the southern orogenic belt may be formed in the island arc subduction environment, not the internal environment, and the island arc subduction environment at least holds. To the New Proterozoic era, the lower Jiang Group (760Ma +). (7) the Shuangqiao mountain group in the eastern section of the southeast of Yangtze block (Jiangnan orogenic belt), the detrital zircon of the van Jing mountain group, the Leng Jia Xi group and the four Fort Group in the western part of the Yangtze block, with similar age peaks. The lower river group, the Dan Zhou group, the Banxi group and the east segment of the lower Yangtze stage of the lower Yangtze River in the western section of the southeast margin of the Yangtze block. The peak values of the age spectrum are also similar, and the difference lies in the differences in the two older peaks, indicating that their source areas are generally similar, but there are some differences. The peak value of the four Fort Group and its equivalent level and the lower river group and its corresponding clastic zirconium peaks shows that the peak value of the 1600 Ma and 1950 Ma ages of the four Fort Group is more significant. It is obvious that the peak value of 2500 Ma and 2000 Ma in the lower river group is more obvious, reflecting the change of the source area or the variation of the erosion layer in the source area. The two are obviously different from the peak age spectrum of magmatic rocks in the magmatic rocks and the Yangtze massif, which are similar to the annual peak of the annual age spectrum of the magmatic rocks on the margin of the Yangtze block, which reveals that the main source area is the Yangtze. The presence of a large number of sedimentary clastic zircons shows that the occurrence of a large number of sedimentary clastic zircons reflects the active volcanic materials. The source information from the interior of the Yangtze block is weak and does not come from the source information of the Huaxia massif, suggesting that the Neoproterozoic four Fort period and the lower river group in the Neoproterozoic of the Huaxia and Yangzi massif have not yet been completely collaged, and the deep-sea (trench) facies exists between the two. The west section of the Jiangnan orogenic belt may be in the "gully arc basin" system. (8) the Qingshui River group is the key period of the transformation of the source of the next Jiang River. There is almost no old zircon age, suggesting that the early provenance of the lower river group may mainly come from the four Fort Group, the sedimentary area and the basement of the Yangtze block on the margin of the Yangtze block, and the late provenance is transformed into the early sediments of the lower river group and less from the source of the four Fort period. In addition, the age of the sedimentary tuff in the Qingshui River group is 770~745 Ma and the region. The age of occipital basalt and basic magmatic rock (760 Ma) in Longsheng, Hunan and Guzhang, Hunan, is consistent with the error range, suggesting that the source area of the Qingshui River group may come from Guangxi Longsheng and Hunan Guzhang. The northern margin of the Yangtze block, the western margin and the 770~745 Ma of the Xuefeng mountain area are obviously active, but they are unable to be separated by the Yangtze block. In the northern margin of the Yangtze block, the lower river group in the western margin and the Xuefeng mountain area is unconformable with the angle of overlying the South China system, which has evolved into the micro angle unconformity, parallel unconformity, and integrated contact (North West to the east direction) in the northern margin of the Yangtze block. The core area reflecting the snow peak orogeny is not in the Qiandongnan area of the South East edge of the Yangtze block. The snow peak movement has led to the extinction of the volcanic arcs in Longsheng, Guangxi. (9) the subduction (arc land) extension dynamic model of the lower Yangtze River in the west of the Jiangnan orogenic belt was established. (a) 870-830Ma period: the western section of the Jiangnan orogenic belt is in a ditch arc basin system and sinks. A set of clastic rocks built in the van Jing Shan / four Fort Group. (b) 830~815 Ma period: since the oceanic and terrestrial convergence between the Yangzi block and the Huaxia block led to the disappearance of the four Fort volcanic arc, the strong extrusion orogeny (Wuling movement), the embryonic form of the Jiangnan fold belt finally formed. The late orogenic collapse. (c) 815 ~ 770 Ma period: the western section of the Jiangnan orogenic belt continued to be compressed and shortened, and the island arc environment in Longsheng, Guangxi was gradually developed and matured. The region of Qiandongnan to North Guangxi was in the post arc extension environment and deposited the formation of the Jia Lu and Sumi formation in the lower part of the lower river group. (d) the period of the strong subduction and convergence in the periphery of the Yang Zi block - arc land Orogeny (snow peak movement), but its convergence intensity is different, in the west margin of the Yangtze block and in the north, the activity is strong, while the west section of the southern orogenic belt on the southeast margin of the Yangtze is small. (E) 745~720 Ma period: after the snow peak movement, the Huaxia block and the Yangtze block can be completely collaged.
【学位授予单位】：中国地质大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：P535;P534.1

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