【作者】 密文天；

【导师】 林丽；

【作者基本信息】 成都理工大学 ， 古生物学与地层学， 2010， 博士

【摘要】 新元古代末-寒武纪的地史转折期是第一个全球性海相磷块岩大规模沉积的时期。在中国扬子地台上,震旦纪陡山沱期也出现了大规模成磷现象。陡山沱期广泛海侵形成的碳酸盐岩-磷块岩-黑色页岩的沉积组合广泛分布于扬子地台。地史时期海相磷块岩的沉积问题是目前地学领域的研究难点之一,存在诸多争议。陡山沱期的成磷事件、生物进化事件都与冰期后的全球性环境改变有很大关系。如Rodinia超大陆的解体、“雪球”事件结束后古海洋生态系统的重建都对成磷事件及生物进化与生命创新事件(略早于典型的Ediacaran生物群出现)产生不可低估的影响。而磷的生物聚集作用是磷富集的重要方式,其大规模埋藏记录可以反映古海洋变化的重要信息,也可以反映出生物的成磷作用。同时,生物的新陈代谢、光合作用等生命活动都离不开磷的参与,磷对生命的演化有重要作用。越来越多的研究成果表明海相磷块岩与生物作用存在密切联系,这也反映了环境与生物具有协同演化的关系。受国家自然科学基金(编号:40743016)和教育部“高等学校博士学科点专项科研基金”(编号:20070616014)共同资助,在综合国内外的磷块岩成因、陡山沱期古生物群研究进展的基础上,以震旦纪陡山沱期形成的代表性磷矿——贵州瓮安磷矿与湖北宜昌磷矿为研究对象,从上述地区的岩相学、古生物学、沉积学、元素地球化学、同位素地球化学及有机地球化学等方面,探讨了上扬子区陡山沱期磷矿的形成及其中的生物成矿作用,并对成磷事件与生物演化的关系进行了研究。贵州瓮安磷矿代表的碳酸盐岩型含磷岩系沉积体系与湖北宜昌磷矿代表的泥页岩-碳酸盐岩型含磷岩系沉积体系之间缺乏物质组合、沉积环境及磷块岩成因对比,有机地球化学研究比较匮乏,同时两地又分别是瓮安生物群与庙河生物群的主要化石产地,因此,以它们为研究对象探讨了上扬子区新元古代陡山沱期磷块岩的形成与海平面变化、生物成矿作用之间的关系,取得了如下成果:(1)对瓮安生物群化石出现层位的磷块岩进行有机地球化学分析,烃类有机组分包括正烷烃、萜类、甾类和类异戊二烯。分析图谱结果显示正烷烃碳数分布范围很宽;OEP值接近1,Pr/Ph植烷略显优势;规则甾烷呈不对称的“V”字形分布,C27占优势可能是由大量的带刺疑源类导致的。正烷烃的碳同位素值随碳数增加变化范围较窄,反映陡山沱组磷块岩中有机质的母源物质输入并不复杂的特征。磷块岩样本中测试出的生物标志物及其组成特征指示了瓮安陡山沱组磷块岩中的有机物来源于多细胞藻类和细菌。(2)将白果园矿区的陡山沱组地层划分为多个不同的三级层序。通过宜昌、瓮安两个磷矿成磷环境对比发现,能量较高并具有丰富藻类的潮下带为成磷的优势区。冰期时的物理风化和陡山沱期化学风化作用将地表含磷物输入海洋,在高的生物生产率等因素共同作用下,为生物有机质的成矿作用提供了机遇。陡山沱期的成磷作用与海侵作用关系密切,海平面的持续波动为磷块岩的形成提供了物源与动力条件。海侵对磷矿形成十分有利,高品位的含磷层对应于最大海侵阶段。(3)大冰期过后生物生产率迅速恢复,为生物有机质的成矿作用提供了条件。宜昌白果园磷矿有机碳含量介于盆地型磷块岩与台地型磷块岩之间,显示了泥页岩-碳酸盐岩型含磷岩系Corg含量的“过渡性”特点;由于其含磷岩系形成的环境为半局限性滞留洼地,白果园磷块岩是弱还原-弱氧化环境的沉积产物。与邻区贵州瓮安的台地型磷块岩进行对比,台地型磷块岩由于较为氧化的环境而具有低Corg和高P2O5特点。Corg和P2O5含量的指标说明生物有机质对磷元素的富集有明显作用。(4)白果园整个含磷岩系的磷质来源为火山喷发带来的深部的磷质输入可能性很小,以冰期时的物理风化和陡山沱期化学风化作用将地表含磷物输入海洋为主;白果园下部磷块岩Ceanom指标反映了较还原的环境,说明南沱冰期过后的最早期,古海洋条件仍较为还原,古海洋水体并未完全被氧化。(5)同位素数据表明,在陡山沱早期,生物生产率逐渐提高;同时暗示了上升洋流携带的富磷底水成为磷质的来源。随后,生物生产率进一步提高,导致δ13C值变化。磷质海水的运移为陡山沱期生物群的繁盛提供了营养物质,而生物的繁盛又对磷质的进一步聚集及沉淀提供了条件。对瓮安生物群开始出现层位分析,自下而上,δ13Corg值的逐渐减少,与该期海平面变化相一致,显示了明显的升降旋回特征;δ13C值的持续变化,反映了多种条件的影响导致生物生产率的起伏波动。(6)对瓮安磷块岩的某些微量元素特征比值进行分析,获得的证据并不支持磷块岩热水沉积成因;瓮安磷矿大塘、穿岩洞矿段的下矿层ΣREE比上矿层ΣREE高出很多,瓮安磷块岩轻、重稀土分馏程度不如鄂西地区。更多还原

【Abstract】 Sedimentary of marine phosphorites between the late Neoproterozoic and Cambrian was the first large-scale global phosphogenesis event.Phosphogenesis also emerged on the Yangtze platform in Sinian Doushantuo period. The broad transgressive activity in Doushantuo period formed carbonatite -phosphorite- black shale deposition fabric widely distributed on the Yangtze platform.The problem of marine phosphorites’deposition is one of the difficulties in the geological research field.There are many controversies. The phosphogenesis and biological evolution events were associated with changes of global environment. For example,the disassembly of the Rodinia supercontinent, the late Neoproterozoic glacial events’ closing and the reconstruction of new ecosystem in ancient marine,which influenced the phosphogenesis and biological evolution and innovation of life.Biological congregation of phosphorus is an important way to phosphorus enrichment.Its large-scale burial records could reflect the important information about changes in the ancient ocean.At the same time, biological metabolism, photosynthesis and other activities of life are inseparable from the participation of phosphorus, phosphorus plays an important role on the evolution of life.More and more research shows that marine phosphorites exist in close contact with the biological effect, which also reflects the co-evolution of the relationship between environment and biology.The subject is supported by Natural Science Foundation(N0. 40743016) and Ministry of Education university special scientific research fund for doctor(N0.20070616014).Based on the advances in the study of genesis of phosphorite and biota of Doushantuo period, analyses of petrography, paleontology, sedimentology,elemental-chemistry, isotopic geochemistry and organic geochemistry are conducted on representative phosphorite of Doushantuo Formation-Weng’an phosphorite in Guizhou and Yichang phosphorite in Hubei. The purpose of this study is to explore Doushantuo phosphorite’s formation in the upper Yangtze region and its biomineralization and the relationship between phosphogenesis and biological evolution.There is less contrast material fabric-sedimentary environment-genesis of phosphorite between the carbonatite phosphatic series in Weng’an and the shale-carbonatite phosphatic series in Yichang.The study of organic geochemistry is deficient too.At the same time, Weng’an biota and Miaohe biota were discovered in Guizhou and Hubei.Therefore, we probed into the relationship between Doushantuo phosphorite’s formation and sea-level changes and the bio-mineralization in the upper Yangtze region, yielded the following results:(1)An organic geochemical analysis of the phosphorite samples coming from the stratigraphic position of Weng’an biota has been carried out, and the hydrocarbons fractions contain n-alkanes,terpanes, steranes and isoprenoids. The results show that the distribution pattern of n-alkanes has a wide range of carbon atom numbers.The OEP near 1.The Pr/Ph ratio indicating phytane dominance.The sterane content of C27>C29>C28 shows asymmetrical“V”shape.The dominance of C27 may be caused by acritarchs. The curve of the carbon isotope compositions of individual n-alkanes of the phosphorite has a narrow variation range.Carbon isotope compositions of individual n-alkanes show that the organic material of the phosphorite from the Doushantuo Formation originates is singleness.The biomarkers characteristics indicate that the main sources of the organic matters are algaes and bacteria.(2)The Doushantuo Formation of Baiguoyuan could be divided into some third-order sequences. Compared Yichang and Weng’an phosphorite’s depositional environment, we could conclude that subtidal zone was favorable area for phosphogenesis,which had high energy and abundant algaes.The physical weathering during glacial epoch and chemical weathering during the Doushantuo period produced the phosphoric material,which was input into ocean,with high biological productivity,providing opportunity for biomineralization.Phosphogenesis and transgression have the closed relationship.Intermittent fluctuations of sea level provided the source and power conditions for phosphorite’s deposition. The transgression was very favour of phosphorite’sformation,high grade phosphoria formation corresponds to the maximum transgression.(3)After the great glaciation period,the biological productivity recovered rapidly, providing condition for biomineralization. The Yichang phosphorite’s organic carbon content is between basin phosphorite and platform phosphorite’s content, intergrade is characteristic of its.Because the environment was retention basin of semi-limitations, the phosphorites of Baiguoyuan were resulted from weak reduction-weak oxidation environment.Compared with platform phosphorite of Weng’an,platform phosphorite has the characteristic of low organic carbon content and high P2O5 content as a result of more oxidative environment.Carbon content and high P2O5 content indicated that organic material played a significant role in phosphorus element’s enrichment. (4)The source of Baiguoyuan phosphatic series’ was discussed. The deep phosphorus’ input caused by volcanic eruptions was unlikely.The physical weathering during glacial epoch and chemical weathering during the Doushantuo Period produced the phosphoric material,which was input into ocean. The Baiguoyuan lower phosphate rock Ceanom indicators reflect a relatively reducing environment,after the Nantuo ice age,the conditions of ancient sea was a still relatively reducing environment.Ancient ocean water was not completely oxidized.(5)The isotope data indicate that the biological productivity was gradually increased in the early stages of the Doushantuo period.It implied that upwelling of phosphorus-rich bottom water brought the phosphorus source.And then,biological productivity’s increase causedδ13C value’s changes. The migration of phosphorus water provided nutrients for the Doushantuo biota’s flourished, and biota’s flourished provided the condition for phosphorus’s further aggregation and precipitation. Began the analysis from Weng’an Biota’s occurrence layer, the value ofδ13Corg gradually diminished,and the sea-level changes unanimously,showing the obvious characteristics of fluctuating cycles.With theδ13C value’s shocking, the impact of various conditions caused the large fluctuations of biological productivity.(6)With the analysising of Weng’an phosphate rock’s characteristic ratios of trace elements,the evidence did not support the phosphate rock’s hydrothermal sedimentation. Datang and Chuanyandong’s lower seam ofΣREE was much higher than the upper seam;The LREE/HREE of Weng’an phosphorite was less than the western of Hubei.更多还原