扬子地块莲沱组沉积环境及其时代归属研究

There has been much debate on the stratigraphic division and regional correlations of the Liantuo Formation, which is a key element in establishing the Nanhuan system. Moreover, the Liantuo Formation is the first sedimentary cover in the Gorges area of the Yangtze Block after the Jingning orogeny. The geochronology and paleoenvironment of the Liantuo Formation are still highly controversial. This foundation detail the geochronology, paleoenvironment, and tectonic setting of Nanhuan deposits of the Neoproterozoic basin in South China. The Nanhua rift basin is the largest Neoproterozoic basin in the Yangtze Block, and it has some of the best-preserved Neoproterozoic (850-635 Ma) sedimentary and volcanic records. Such a well-preserved, continuous Cryogenian stratigraphic record has provided vital information regarding the Precambrian crustal evolution of the South China Block (SCB), the assembly and breakup of the Rodinia supercontinent, and Neoproterozoic "snowball Earth" events. An integrated approach of facies analysis, paleogeography, and geochronology provide new insight into the sedimentology and paleogeography in sedimentary basin during Liantuo Period. This foundation aims to confirm the stratigraphic framework and discuss onset time of the Sturtian glaciation, thereby promoting a better understanding of the Nanhuan System tectonic development of South China.

扬子地块莲沱组地层的划分和对比，一直是我国的南华系划分、对比问题中的关键难题之一，历存分歧。由于某些地区地层序列不完整，而且过分依赖岩石地层单位对比，造成了种种误解，由此也带来一系列认识上的分歧，典型的就是关于板溪群与莲沱组的地层对比、其时代归属及莲沱组沉积环境问题。扬子地块南华盆地是中国南方最大的新元古代沉积盆地，它同时也是成冰纪（南华纪）地层（850 - 635 Ma）和新元古代火山活动记录保存最完好的沉积盆地。扬子地块成冰纪地层分布广泛且保存完整，为华南前寒武纪地壳演化，Rodinia超大陆的拼合和裂解以及新元古雪球事件研究提供了重要的地质信息。本基金通过对中扬子地块莲沱组及相当地层典型剖面的沉积充填序列，沉积相展布规律研究，拟建立莲沱组岩石地层和年代地层格架。特别是"莲沱晚期"与南华冰期地层的转换关系，结合同位素年代学，对Sturtian冰期的启动时限开展对比研究。

基金通过对中扬子地块莲沱组及相当地层典型剖面的沉积充填序列，沉积相展布规律研究，拟建立莲沱组岩石地层和年代地层格架。特别是"莲沱晚期"与南华冰期地层的转换关系，结合同位素年代学，对Sturtian冰期的启动时限开展对比研究.(1)明确了扬子陆块莲沱组—休宁组的时代归属及其沉积大地构造属性.通过对上扬子地块莲沱组沉积古环境分析和锆石U–Pb精确定年。认为莲沱组沉积时限为790Ma–730 Ma，相当于板溪群中上部的五强溪组，是华南新元古代裂谷盆地沉积充填序列的第Ⅱ旋回。由于地壳抬升而又沉降，海水从北向南逐渐超覆，上扬子地块从南向北展布了莲沱组陆相－海陆过渡相－海相不同的沉积单元，并对识别出来的六种岩相组合的沉积特征观察研究；莲沱组顶部凝灰岩LA-ICP-MS Ｕ–Pb年龄为736±5.8 Ma，制约了扬子地块Sturtian冰期的启动年龄　≤730 Ma。为南华系研究提供了精确的年代学证据，对认识中国南方构造演化具有重要意义。.(2)识别了扬子地块莲沱组沉积特征，编制扬子陆块新元古代板溪群晚期（莲沱组沉积期）的构造岩相古地理图,在构造模型、充填样式、沉积模式、古地理演化及古大陆重建等方面，取得了一系列全新的认识。.3）限定了神农架群石槽河组沉积年龄，重新审视了扬子北缘的构造演化.获得神农架群石槽河组凝灰岩锆石精确的U-Pb同位素年龄1180Ma,且石槽河组钙碱性玄武质凝灰岩地球化学特征相似于现代陆内裂谷火山岩。提出石槽河组形成于1180Ma被动大陆边缘裂谷环境，而神农架群顶部郑家亚组与弧相关的火山岩时代1103Ma，扬子北缘大地构造环境由在1103Ma由陆内裂谷向岛弧构造环境转变。