CAJ | 학술논문

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。
해문운용다충침적지구화학지표결합지질사실，대애적잡랍기만기침적우환남람전지구적피완촌조규질암진행료종합연구，게시료피완촌조규질암구유해상침적규질암전형적미정석영결구화교고적SiO2함량，동시구유Fe/Ti＞20, Al/(Al+Fe)＜0.4화Eu정이상적지구화학특정。근거Al-Fe-Mn도해인위，규질암함유래자해저열액류체적조분，이접근현대개활양해수적Y/Ho비치화해수형희토배분특정칙현시침적수체주요위해수。규、양동위소재지층서렬상무추세성변화화교고δ30Si치지시규질암시유해수중적용해태규화학침적형성，이불시유해저열액직접침정형성。양동위소온도계야표명규질암형성우당시상온적해수배경。상승류사득혼합료해저열액이부규적저층해수상용，운이지리분구교원적상대편산성해역，도치국부해수규과포화이발생이양화규침정。피완촌조산출대량조문층화미체화석，병재정부견유보존완호적애적잡랍기미체화석Palaeopascichnus jiumenensis，표명본해역생산력왕성，생물화유궤질강해작용산생적산류물질가능강저료해수중용해규적용해도，간접촉진료규질암적침적。
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 Al/(Al+Fe)<0.4, as well as position in Al-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 silica-rich 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 jiumenensis 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.