华南陡山沱组盖帽碳酸盐岩中席状裂隙构造的区域差异性研究：对大规模冰川均衡调整的记录

The Marinoan glaciation stands out in Earth history due to the large-scale ice cover. The abrupt termination of the Marinoan glaciation at 635 Ma caused rapid marine environment oscillations and obvious glacial isostatic adjustment (GIA). Sheet-crack, Which is most typical feature of Marinoan cap carbonate, is the best geological record of large-scale GIA after the termination of Marinoan glaciation. Our previous studies on the fluid inclusions of sheet-crack cements from different sedimentary facies fractures in South China suggest that these cements indicate a ubiquitous hydrothermal episode, which caused by the brine in fault basins that drove from geothermal gradient anomalies induced by magmatic action triggered by GIA. From the platform facise to deep-water slope facise, the hydrothermal fluid has a tendency of decreasing temperature and increasing salinity. However, this is still a lack of detailed spatial variation of the hydrothermal fluid temperature and salinity, and precise time of the hydrothermal activity. Hence, this project will focus on the sheet-crack cements from different sediment environment in South China. Based on systematically analysis of fluid inclusions, trace elements and C-O-Sr isotopes, the regional evolution rule of hydrothermal fluid from platform to basin would be clarified. Moreover, the precise age of hydrothermal activity will be determined by calcite geochronology from LA-ICP-MS U-Pb analysis. The project has important theoretical significance for understanding about the genesis of sheet-crack, and the detailed changes in the marine environment after the Marinoan deglaciation.

Marinoan冰期是地球历史上的大规模冰川事件，635Ma左右冰川快速消融对应同期海洋环境急剧变化和大规模均衡调整作用（GIA），席状裂隙构造是Marinoan盖帽碳酸盐岩中最典型特征组构，是研究冰消后大规模GIA的最佳地质记录。申请者对华南不同沉积相席状裂隙构造胶结物包裹体的初步研究表明，胶结物代表了GIA有关的岩浆作用诱发地温梯度异常进而驱动盆地卤水活动的热液事件，从古陆向深水相，热液流体具有温度逐渐降低而盐度逐渐增高的趋势。然而对热液流体温度和盐度空间精细变化及热液活动发生的精确时限还有待厘定。本项目拟以华南不同沉积环境盖帽中席状裂隙构造为研究重点，通过系统流体包裹体、微量元素及C-O-Sr同位素研究，阐明热液流体从台地到盆地相的区域演化规律；采用方解石原位U-Pb定年厘定热液活动的精确年龄。项目对于完善盖帽中席状裂隙构造的成因及精细刻画冰消后海洋环境变化过程都具有重要的理论意义。

635Ma左右Marinoan冰期冰川快速消融对应同期海洋环境急剧变化和大规模均衡调整作用（GIA），席状裂隙构造是Marinoan盖帽碳酸盐岩中最典型特征组构，是研究冰消后大规模GIA的最佳地质记录。本项目对华南扬子区不同沉积相典型盖帽碳酸盐岩剖面中的席状裂隙构造开展了系统的岩石学和地球化学分析，建立盖帽中席状裂隙构造的成因模型，进而探讨了冰川消融后盖帽沉淀的海洋环境。获得以下几点认识：.1.从台地相、上斜坡相到下斜坡相，席状裂隙胶结物具有统一的矿物共生序列，这些胶结物通常开始于等厚白云石±重晶石±黄铁矿，然后是等厚纤维方解石，硅质相（包括等厚放射状玉髓和结晶石英）±黄铁矿，最后是粗晶方解石或块状白云石±重晶石±黄铁矿±闪锌矿。.2.首次发现被硅化后保留在席状裂隙中的古洞穴沉积物，包括石钟乳、石笋、石枝、月奶石、葡萄状体和结壳。表明冰消后均衡反弹作用导致盖帽白云岩的抬升暴露并发生喀斯特作用，可以大致推算出盖帽暴露的时间约1 Ka～3.5 Ka。古洞穴沉积物的发现可推测埃迪卡拉纪此次喀斯特事件中土壤微生物生态系统已经建立。.3.运用流体包裹体的方法对台地相、上斜坡相和下斜坡相的盖帽席状裂隙构造胶结物进行分析，显示台地相的包裹体盐度较低温度高，过渡到斜坡相深水相盐度出现低盐度和高盐度混合以及相对较低温度的现象。我们认为这是由于冰消后均衡反弹作用引起的地热异常在台地相高于深水盆地相所造成的。.4.建立了席状裂隙构造成因模式：Marinoan冰期结束后大规模均衡反弹（陆壳的垂直上升）强烈，使得沉积未完全固结的盖帽碳酸盐岩中普遍存在大量软变形构造；同时滨海、浅海地壳短暂快速抬升暴露，喀斯特作用使盖帽碳酸盐岩中软变形构造进一步溶蚀为高渗透性地层；大规模均衡反弹可能诱发深部地壳的减压部分熔融，地热异常沿深大断裂驱动断陷盆地的卤水，选择早期高渗透性地层建立热液循环，最终充填沉淀形成席状裂隙构造。