黔东南下江群岩相组合、物源特征及其构造意义

发布时间：2018-07-13 12:42

【摘要】：华南板块由扬子地块和华夏地块拼贴而成,二者的拼贴作用在扬子东南缘形成了一条呈弧形展布的江南造山带,该造山带形成和演化过程长期受到地学界的关注。下江群分布于江南造山带西段的湘黔桂地区,角度不整合覆于梵净山群之上,平行不整合于南华系之下,为一套巨厚的浅变质陆源碎屑岩夹火山碎屑岩组合,对于该套地层的时代、盆地类型及构造背景等问题一直以来存在很多争议,对其进行详细研究能有效探索江南造山带西段的演化过程。本文以黔东南及其邻区下江群为研究对象,在详细野外地质调查基础之上,应用沉积学、地球化学和同位素年代学等分析手段,对其进行沉积相和物源属性综合研究,并结合区域研究成果系统分析下江群的盆地性质及构造背景。通过相标志的识别和岩性横向、垂向上的变化特征研究,发现研究区下江群为一套同期异相的沉积产物,梵净山和芷江地区为滨岸-浅海陆棚环境,黔东南黎平-天柱地区为大陆斜坡环境,反映出盆地北高南低的古地理格局。镜下鉴定显示下江群砂岩岩石类型为岩屑长石砂岩和长石岩屑砂岩,碎屑成分主要为石英、长石、长英质火山岩屑及少量沉积岩屑;稀土和微量元素特征表明下江群源岩为长英质岩浆岩;Dickinson物源判别图解和微量元素投图结果表明下江群物源区构造背景主要为岩浆弧,底部层位物源区构造背景为再旋回造山带。这些特点暗示下江群物源可能来自岩浆弧的中酸性火山岩。梵净山地区清水江组底部凝灰岩测年结果为777±9Ma,下江群碎屑锆石的年龄谱显示其年龄值主要集中在810~760Ma,具有794Ma主峰值和2016Ma弱峰值;810~760Ma的锆石自形程度好、发育振荡环带且Th/U比值大于0.4,具有岩浆成因的特征。通过与扬子周缘和华夏地块的锆石年龄谱对比,发现下江群和扬子北缘具有相似的年龄谱,结合向南的古流向及砂岩岩屑特征,认为下江群的物源区应为下伏梵净山群和扬子北缘的中酸性火山岩。岩石组合、沉积相带展布和物源特征等表明下江群沉积盆地类型为楔顶盆地,其演化经历了缓慢沉降期(甲路组-乌叶组;横路冲组-通塔湾组)、初始裂陷期(番召组-清水江组;五强溪组-多益塘组)和强烈裂陷期(平略组-隆里组;百合垅组-牛牯坪组)三个阶段的充填过程。扬子与华夏地块俯冲造山作用可能一直持续到早古生代晚期。
[Abstract]:The South China plate is composed of Yangtze block and Huaxia block. The collage between them formed an arc-shaped Jiangnan orogenic belt on the southeast margin of Yangtze River. The formation and evolution of the orogenic belt has been paid close attention to by geoscientists for a long time. The Lower Yangtze Group is distributed in Hunan, Guizhou and Guangxi areas in the western section of the Jiangnan orogenic belt, with angle unconformity overlying the Fanjingshan Group and parallel unconformity under the South China series, which is a set of extremely thick continental clastic rocks with volcanic clastic rocks. There have been many controversies about the age of the strata, basin type and tectonic background, and detailed study of them can effectively explore the evolution process of the western section of the Jiangnan orogenic belt. Based on the detailed field geological survey, the sedimentary facies and provenance properties of the Lower Yangtze Group in the southeast of Guizhou and its adjacent areas are studied by means of sedimentology, geochemistry and isotopic chronology. The basin nature and tectonic setting of Xiajiang Group are systematically analyzed based on the regional research results. Based on the identification of facies markers and the study of lateral and vertical variation of lithology, it is found that the Xiajiang Group in the study area is a set of contemporaneous and heterogeneous sedimentary products, and the Fangjingshan and Zhijiang areas are shore-shallow-sea shelf environments. The Liping-Tianzhu area in the southeast of Guizhou is a continental slope environment, which reflects the paleogeographic pattern in the north, south and south of the basin. The results show that the rock types of the sandstone are lithic feldspathic sandstone and feldspathic sandstone, and the main clastic components are quartz, feldspar, felsic volcanic rock and a little sedimentary lithology. The characteristics of rare earth and trace elements show that the source rocks of the Lower Yangtze Group are derived from the Dickinson source of felsic magmatic rocks. The results show that the tectonic setting of the source area of the Lower Yangtze Group is mainly a magmatic arc, and the tectonic setting of the source area of the bottom layer is a re-cycle orogenic belt. These characteristics suggest that the provenance of the Lower Yangtze Group may come from the intermediate-acid volcanic rocks of magmatic arcs. The dating of tuff at the bottom of Qingshuijiang formation in Fanjingshan area is 777 卤9 Ma.The age spectrum of detrital zircon from the Lower Yangtze Group shows that the age range is mainly 810 ~ 760 Ma, with a main peak value of 794 Ma and a weak peak value of 2016 Ma at 810 ~ 760 Ma. The oscillating ring zone and the ratio of Th- / U > 0.4 have the characteristics of magmatic genesis. By comparing the zircon age spectra with those of the Yangtze and Huaxia blocks, it is found that the lower Yangtze group and the northern Yangtze block have similar age spectra, combined with the southward paleodirection and sandstone lithic characteristics. It is considered that the source region of the Lower Yangtze Group should be the intermediate-acid volcanic rocks of the Lower Flavand Jingshan Group and the northern margin of Yangtze River. The rock assemblages, distribution of sedimentary facies and provenance characteristics indicate that the sedimentary basins of the Lower Yangtze Group are wedge-top basins, and their evolution has undergone a slow subsidence period (Jialu formation-Wuye formation); The filling process of three stages: Hengluchong formation-Tongtawan formation, initial rifting stage (Fanzhao formation-Qingshuijiang formation; Wuqiangxi formation-DuoYitang formation) and strong rifting stage (Pingliao formation-Longli formation; Lilong formation-Niu-Ping formation). The subduction orogeny of Yangtze and Cathaysian massif probably lasted until the late early Paleozoic.
【学位授予单位】：中国地质大学(北京)
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P542.2

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