华南陡山沱组磷块岩中微米级自形钠长石的发现及其可能成因  

Discovery and possible genesis of micron-sized euhedral albites in the Ediacaran Doushantuo phosphorites,South China

作　　者：李凯月 佘振兵[2] 

机构地区：[1]中国地质大学生物地质与环境地质国家重点实验室,湖北武汉430074 [2]中国地质大学(武汉)地球科学学院,湖北武汉430074

出　　处：《地学前缘》2017年第1期308-320,共13页Earth Science Frontiers

基　　金：国家自然科学基金项目(41272038)

摘　　要：文中报道了在华南新元古代震旦纪陡山沱组磷块岩中首次发现的大量微米级自形长石。岩相学观察显示这些长石为自形-半自形板-柱状,呈团聚状或分散状产于砂级胶磷矿颗粒中,粒径大多在10~30μm,光学显微镜下无法观察到双晶。扫描电镜能谱(EDS)半定量分析表明这些自形长石为钠长石。电子探针(EPMA)分析显示所测样品钠长石端员(Ab)平均含量为97.96%,与典型自生钠长石成分一致。钠长石所包裹有机质的拉曼光谱特征指示其经历的最高温度不超过350℃,排除了其来源于高温的岩浆岩或高级变质岩的可能性。以上资料表明这些微米级自形长石为典型的自生成因高纯钠长石,可能形成于磷块岩的早期成岩阶段。自生钠长石与丝状化石常相伴出现,与前人通过嗜碱菌还原黏土矿物实验所获得微米级长石及微生物的产出关系相似;钠长石核部可见富含有机质,表明后者可能促进了矿物的生长。文中提出磷块岩中自生钠长石的成因模式:钠长石的Al和Si来自于黏土矿物的分解,该过程可能受嗜碱微生物介导;Al和Si与来源于海水或碳氟磷灰石晶格中的Na^+结合,在低温条件下结晶形成高纯度钠长石。微生物及其相关有机分子可能为钠长石结晶提供了成核点。In this paper we report the first discovery of abundant micron-sized euhedral feldspars in phosphorite from the Ediacaran Doushantuo Formation in Yichang,South China.The feldspars are euhedral-subhedral,platy minerals and occurred in sand-sized granule composed of cryptocrystalline apatite.Grain sizes of the feldspars are mostly 10-30μm and no twinning is observed under optical microscope.Scanning electron microscope（SEM）observations and energy dispersive spectrometer（EDS）analyses confirmed that these feldspars are albites.Electron probe micro analysis（EPMA）data yielded very high albites end members（Ab97.96% on average）,consistent with that of typical authigenic albites.Raman microspectroscopy of carbonaceous nanoscopic inclusions within the albites showed that the minerals crystallized under low temperature conditions（350 ℃）,thus excluding their derivation from igneous or high-grade metamorphic rocks.Collectively,the data suggest that these euhedral feldspars are authigenic albites with high chemical purity,which formed in the early diagenetic stage during the formation of the phosphorites.The close association of the albites and filamentous microfossils point to a biological origin of the minerals,which has been documented by previous experimental studies that showed the active role of alkaliphilic bacterium in the growth of authigenic feldspar.Enrichment of organic particle in the core or albites is also consistent with the role of organic matter in mineralization.A mode is proposed to explain the genesis of authigenic albites in the phosphorites：decomposition of clay minerals might have provided Al and Si which,when combined with Na~＋from seawater or carbonate fluorapatite（CFA）,led to the precipitation of authigenic albite.Microorganisms and associated organic molecules probably have served as nuclei for the growth of albites.

关 键 词：自生钠长石 陡山沱组 磷块岩 微生物 

分 类 号：P578.968[天文地球—矿物学] P588.244[天文地球—地质学]