摘要
该文运用多种沉积地球化学指标结合地质事实,对埃迪卡拉纪晚期沉积于皖南蓝田地区的皮园村组硅质岩进行了综合研究,揭示了皮园村组硅质岩具有海相沉积硅质岩典型的微晶石英结构和较高的SiO2含量,同时具有Fe/Ti>20,Al/(Al+Fe)<0.4和Eu正异常的地球化学特征。根据Al-Fe-Mn图解认为,硅质岩含有来自海底热液流体的组分,而接近现代开阔洋海水的Y/Ho比值和海水型稀土配分特征则显示沉积水体主要为海水。硅、氧同位素在地层序列上无趋势性变化和较高δ30Si值指示硅质岩是由海水中的溶解态硅化学沉积形成,而不是由海底热液直接沉淀形成。氧同位素温度计也表明硅质岩形成于当时常温的海水环境。上升流使得混合了海底热液而富硅的底层海水上涌,运移至离喷口较远的相对偏酸性海域,导致局部海水硅过饱和而发生二氧化硅沉淀。皮园村组产出大量藻纹层和微体化石,并在顶部见有保存完好的埃迪卡拉纪微体化石Palaeopascichnus jiumenensis,表明本海域生产力旺盛,生物和有机质降解作用产生的酸类物质可能降低了海水中溶解硅的溶解度,间接促进了硅质岩的沉积。
To determine the origin of silica in terminal Ediacaran chert, we have undertaken a variety of sedimentary geochemical proxies, in conjunction with geological features, on well-characterised Piyuancun chert at Lantian section from southern Anhui province. The Piyuancun cherts are typical marine siliceous deposits with microcrystalline structure and high content of SiO2. Silica constituting the chert is partially derived from submarine hydrothermal fluid, indicated by geochemical ratios of Fe/Ti 20 and A1/（AI＋Fe） 〈 0.4, as well as position in A1-Fe-Mn triangular diagram. However, Y/Ho ratios close to seawater from modern open sea and seawater-like rare earth element distribution patterns demonstrate that the sedimentary water body of Piyuancun chert primarily sourced from seawater. The isotope compositions of silicon and oxygen illustrate no variation trend stratigraphically whilst relatively high δ30Si values indicate that Piyuancun cherts were formed by chemical precipitation of dissolved silica from seawater rather than direct precipitation from hydrothermal fluid. Moreover, estimated values from oxygen isotopic geothermometer are also consistent with normal coeval seawater temperature. Bottom seawater was first mixed with silicarich submarine hydrothermal fluid and subsequently driven by upwelling to relatively acidic waters far away from hydrothermal activity center. As a result, silica within the regional seawater became over-saturated and thus precipitated. Extensively existence of microfossils and algae mats including Palaeopascichnus fiumenensis in Piyuancun chert indicates the thriving ocean paleo- productivity. Acidic substance produced through degradation of organic matter and biodegradation process would have, to some extent, dropped the solubility of dissolved silica in seawater and thus promoted the deposition of Piyuancun chert.
出处
《高校地质学报》
CAS
CSCD
北大核心
2013年第4期620-633,共14页
Geological Journal of China Universities
基金
国家自然科学基金青年科学基金项目(41102018)
国家重点基础研究发展计划973项目(2013CB835000)联合资助
关键词
硅质岩
地球化学
皮园村组
埃迪卡拉系
皖南
chert
geochemistry
piyuancun formation
ediacaran
southern Anhui province
