【作者】 赵彦彦；

【导师】 郑永飞；

【作者基本信息】 中国科学技术大学 ， 地球化学， 2009， 博士

【摘要】 新元古代晚期气候剧烈变化,冰川事件广泛发育。冰期沉积物之上往往被碳酸盐岩直接覆盖,并且这些碳酸盐岩具有特殊的沉积结构、元素和同位素地球化学组成,因此它们已成为近年来国际地球科学研究的前沿和热点之一。中国华南地区发育完整的新元古代震旦系（Ediacaran system）地层,是研究“雪球地球事件”之后古环境变化、古气候变迁、沉积水体组成和生物演化等科学问题最理想的天然实验室。安徽南部震旦系雷公坞组对应于Marinoan冰期沉积,与华南其它地区的南沱组冰碛岩可以对比。蓝田组碳酸盐岩直接不整合地覆盖在雷公坞组冰碛岩之上,其生物标志、化学地层和年代学证据表明它与华南其它地区陡山沱组可进行对比。蓝田组发育上下两段碳酸盐岩地层,其中下段碳酸盐岩属于Marinoan冰期之后的帽碳酸盐岩,可与华南陡山沱组下部碳酸盐岩进行对比;而上段碳酸盐岩直接覆盖在黑色页岩之上,可与陡山沱组上部碳酸盐岩进行对比。由于蓝田组碳酸盐岩与华南其它地区同时期地层既具有相似性又存在一定的差别,因此对其形成环境的研究可为“雪球地球事件”之后的古沉积环境和古气候变迁提供重要线索。本学位论文采用多种地球化学分析手段,包括主量元素和微量元素分析以及C、O和Sr同位素分析,对皖南石盂和皮园村两个地点的震旦系蓝田组上下两段碳酸盐岩进行了系统的剖面采集及其对应的地球化学研究。由于这些碳酸盐岩中还发育细小的方解石脉体,因此对脉体的元素和同位素组成采用显微取样方法进行了研究。所获得的结果为理解蓝田组碳酸盐岩的沉积环境及其与“雪球地球事件”之间的关系提供了地球化学制约。蓝田组碳酸盐岩并不是由纯碳酸盐矿物组成,其中还含有少量非碳酸盐岩物质,如陆源硅质碎屑、FeMn氧化物、硫化物等。由于这些组分的物理化学性质不同,在化学沉积和成岩蚀变以及分析过程中,不同组分间会相互影响,因此本文应用不同强度的酸对碳酸盐岩进行了分步溶解。结果表明,0.5M醋酸溶解的物质（溶液Ⅰ）主要是方解石,其微量元素和锶同位素组成没有受到非碳酸盐岩物质及蚀变作用的影响,能够代表碳酸盐沉积时的地球化学特征。上下两段碳酸盐岩的溶液Ⅰ都表现为高锶含量、高⁸⁷Sr/⁸⁶Sr比值、LREE弱亏损、弱La正异常、弱Gd和Ce以及Er负异常、低Y/Ho比,而下段碳酸盐岩还有明显Eu正异常。这些特征说明,蓝田组上下两段碳酸盐岩具有相似的沉积环境,但是与海相碳酸盐岩的地球化学特征存在明显差别,因此是从淡水为主的环境中沉淀的,其中Ce负异常说明沉积水体呈氧化状态。大部分样品中3.4M醋酸溶解物质（溶液Ⅱ）的微量元素组成明显不同于溶液Ⅰ已经不同程度地受到硅酸盐矿物的污染,不能用来指示沉积环境。残渣中元素组成说明,上段和下段碳酸盐岩的硅酸盐物质来源于不同的陆源区。蓝田组上下两段碳酸盐岩中纯碳酸盐部分由不同矿物组成。为了更好识别这些矿物是否保存原始地球化学特征,本文应用温差法对碳酸盐岩碳氧同位素进行分析。通过共生白云石和方解石的热力学分馏和水岩交换模型,计算得到皮园村剖面上段碳酸盐岩的初始δ¹⁸O值高于-11.9‰（PDB）,初始δ¹³C值为-10.5--9.5‰;石盂剖面上段碳酸盐岩的初始δ¹⁸O值为-25.6--18.6‰（PDB）,δ¹³C值为-11.7--7.9‰;石盂剖面下段碳酸盐岩的初始δ¹⁸O值为-12.8--10.9‰（PDB）,初始δ¹³C值为-5.3--3.5‰。石盂剖面上段碳酸盐岩中低δ¹⁸O值与现代极地沉积碳酸盐岩的δ¹⁸O相近,要求沉积水体的δ¹⁸O值低达-30--18‰（SMOW）。这说明,该段碳酸盐岩可能在大陆边缘盆地中沉淀的,沉积水体中有大量低δ¹⁸O值淡水的加入。δ¹⁸O值相对较高的皮园村剖面上段碳酸盐岩和石盂剖面下段碳酸盐岩可能是在δ¹⁸O值与海水相近的水体中沉淀的。由于上段碳酸盐岩中低的δ¹³C值与低δ¹⁸O值共生,因此低δ¹³C值可能与陆源有机质的分解有关,而下段碳酸盐岩中δ¹³C值可能与甲烷渗漏有关。皮园村剖面上段碳酸盐岩中围岩的稀土配分型式以及Sr同位素组成表明,有大量陆源物质加入沉积水体中。其相对较高的δ¹⁸O值说明,沉积水体氧同位素组成可能与新元古代海水相近,因此其沉积时可能有δ¹⁸O值与当时海水相近的海洋冰川融水加入;并且水体携带有陆源物质,结果使得沉积盆地中水体δ¹⁸O值与海水相近,但是微量元素组成却受控于陆源物质。石盂剖面上段碳酸盐岩具有与皮园村剖面相似的稀土配分特征和Sr同位素组成,但是具有相对较低的δ¹⁸O值,显示有大量低δ¹⁸O值大陆冰川融水携带了陆源物质加入沉积水体中。由于上段碳酸盐岩是在Ediacaran生物群出现之前、Gaskiers冰期结束之后沉淀的,因此海洋冰川和大陆冰川可能都属于Gaksiers冰期产物。两个剖面在主微量元素和同位素组成之间的差异说明,碳酸盐岩是从不同氧同位素组成的沉积水体中快速沉淀的,因此当时海水与冰川融水之间没有达到氧同位素均一化。由此可以推测,当时皖南地区属于大陆边缘盆地,皮园村沉积盆地位于大洋一侧,沉积水体与大洋之间有充分交换;而石盂沉积盆地位于大陆一侧,与大洋之间缺乏连通。石盂剖面下段碳酸盐岩中围岩的稀土配分型式和Sr同位素组成表明有大量陆源物质的加入,其δ¹⁸O值说明沉积水体氧同位素组成与新元古代海水相近。由于蓝田组下段碳酸盐岩是在全球性Marinoan冰期结束之后形成,其沉积时可能有δ¹⁸O值与当时海水相近的海洋冰川融水加入。蓝田组上下两段碳酸盐岩中围岩和脉体的岩相学特征、元素组成和碳氧同位素特征显示,每一部分方解石脉的δ¹³C值与各自微晶围岩的δ¹³C值相近,指示成岩流体的碳主要来自于早期沉积的微晶围岩。从蓝田组上下两段碳酸盐岩中方解石脉的碳氧同位素和元素组成来看,成岩流体大部分都属于外部流体,但皮园村剖面上段碳酸盐岩中的一期成岩流体属于内部流体。三段碳酸盐岩中的一类方解石脉的δ¹⁸O值比较低（-28.6--18.1‰）,指示这类成岩流体来源于大陆冰川融水,但它们在各段岩石中具有不同的稀土配分型式,这说明了这些成岩流体受到不同陆源物质的影响。这类低δ¹⁸O值的成岩流体在皮园村和石盂地区广泛分布。皮园村剖面的蓝田组上段碳酸盐岩中另一类方解石脉的δ¹⁸O值比上一类的高,为-17.2--11.3‰,与围岩的相近,这指示了这类成岩流体属于内部流体,与沉积流体的组成相近。但脉体与围岩具有不同的稀土配分型式,说明了它们是受不同陆源区的物质控制的。皖南地区新元古代Marinoan冰期后沉积碳酸盐岩地层与华南及世界上其它地区地层可以对比,因此本文研究结果有助于理解Ediacaran时期大陆边缘沉积盆地的古环境、古气候、生物产率和水体组成等相关科学问题。尤其是对碳酸盐岩中细脉和围岩进行微区碳氧同位素和微量元素研究,为我们认识成岩/蚀变流体的地球化学特征及其源区性质打开了一扇新的窗户。由于这些碳酸盐岩的同位素和元素组成变化与生物地球化学循环改变密切相关,是沉积水体环境变化的指示剂,因此本文在解译这些变化的基础上提出了可能的成因模型,以期对于认识世界上其它地区同时代沉积环境及其生命辐射效应的都有重要意义。更多还原

【Abstract】 The Late Neoproterozoic Eon was accompanied by drastic climate perturbations with serious glaciations. At least two glacial events （Sturtian and Marinan iceages） are well recorded in sediment rocks, which are usually covered by cap carbonate with no gap between them. In particular, the carbonate subsequent to the Marinoan glaciation exhibits unique sedimentary structures, major and trace elemental compositions and stable isotopes ratios, and has become a focus of much studies. The rocks of Sinian system （Ediacaran system） in the late Neoproterozic outcrop at many localities across the Yangtze Platform and provide good opportunities for the investigations of paleoenvironments, paleoclimate and depositional waters and biological evolution after the "snowball Earth event".The Leigongwu diamictite in southern Anhui belongs to deposition during the Marinoan glaciation, corresponding to the Nantuo diamictite on other profiles in South China. The Lantian Formation sharply and disconformably overlies the Leigongwu diamictite. Biostratigraphic, chemostratigraphic and geochronological studies have demonstrated that the Lantian Formation is equivalent to the Doushantuo Formation elsewhere in South China. Two distinct carbonate units occur in the Lantian Formation in southern Anhui. The carbonate in the lower unit directly follows the Leigongwu Formation and corresponds to the Marinoan cap carbonate. The carbonate in the upper unit of the Lantian Formation, conformably overlying the black shale, compared well with the top carbonate of the Doushantuo Formation elsewhere in South China. Because of the similarities and differences between the carbonates in the Lantian Formation and in equivalent sequences in other sections, the study of the Lantian Formation can provide a good opportunity to decipher paleoenvironmental and paleoclimatic evolution.In this thesis, a systematic study of major and trace elements as well as C-O-Sr isotopes in the carbonate of the Lantian Fornation was carried out. Veinlets are found in the carbonate, so that their elemental and isotopic compositions were analyzed together with their wallrock. The results provide geochemical constraints on the relationship between the depositional environments and the snowball Earth event.The upper and lower carbonate units in the Lantian Formation are impure and composed of various components. These components include main carbonate minerals, such as aragonite, calcite and dolomite, as well as non-carbonate materials such as siliciclastics, oxides and sulfides. Because these components have a variety of physical and chemical characteristics, they suggest possible interaction during sedimentary and diagenetic processes as well as laboratory analyses. Thus, we attempted to separate the different components by stepwise chemical leaching for the element concentrations and Sr isotopic compositions. The results show that the carbonate component dissolved in 0.5 M acetic acid （Dissolution I） is principally composed of calcite and almost free from non-carbonate contamination. Thus its elemental and isotopic compositions can be used to unravel the geochemical feature of the water from which the carbonate precipitated. The Sr concentrations and isotope ratios, and REE+Y concentrations and patterns of Dissolution I are similar to each other in the two units. They exhibit high Sr concentrations and high ⁸⁷Sr/⁸⁶Sr ratios, depletion of LREE relative to MREE and HREE, weak positive La anomalies, subtle negative Ce, Gd and Er anomalies, and near-chondritic Y/Ho ratios. There are obvious positive Eu anomalies in the lower unit of carbonate. These indicate that the two carbonate units were precipitated from the similar nature of basin water, which lack of trace element composition typical of normal marine carbonate but similar to those in freshwater carbonate. The negative Ce anomalies in the carbonate of the two units suggest oxygenation of the precipitation water.The selective acid extraction method was used to liberate CO₂ from calcite and dolomite in the Lantian carbonates. The results enable us to distinguish between primary and secondary carbonates for the purpose of constraining their origin. In view of trace element compositions and water-rock interaction modeling for coexisting dolomite and calcite, the primaryδ¹³C andδ¹⁸O values for the upper unit at Shiyu are -25.6 to -18.6‰and -11.8 to -7.9‰in PDB, respectively; and those at Piyuancun are higher than -11.9‰and -10.5 to -9.5‰, respectively. In this regard, the primary sedimentaryδ¹³C andδ¹⁸O values for the lower unit at Shiyu are -5.3 to -3.5‰and -12.8 to -10.9‰（in PDB）, respectively. The very lowδ¹⁸O values of-25.6 to -18.6‰（PDB） for the upper carbonate unit at Shiyu are comparable to those for modern Arctic carbonates, corresponding to waterδ¹⁸O values as negative to -30 to -18‰in SMOW. This is interpreted as precipitation of carbonate from freshwater-derived fluid in a continental marginal basin. In contrast, the carbonate with less lowδ¹⁸O values for the upper unit at Piyuancun and lower unit at Shiyu would precipitate from seawater-like fluid. Our interpretations are strengthened by terrigenous REE+Y patterns and high ⁸⁷Sr/⁸⁶Sr ratios for the both carbonate units of the Lantian Formation. Since the negative carbonateδ¹³C excursion from -11.7 to -7.8‰in the upper unit is also associated with the extreme ¹⁸O-depletion, the terrestrial dissolved organic carbon reservoirs is considered as a basic cause for the lowδ¹³C values. The C, O and Sr isotopic ratios and elemental concentrations of carbonate in the upper and lower units of the Lantian Formation indicate that the precipitating water are composed not only of the seawater but also of the meltwater from glaciation. As for the upper carbonate unit at Piyuancun, the REE+Y patterns and Sr isotopic compositions indicate that a significant amount terrigenous materials pouring into the depositional basin. The relatively high carbonateδ¹⁸O values imply that the O isotopic ratios of water are similar to those of Neoproterozoic seawater. This indicates that the precipitating water has the similar O isotopic compositions to the seawater, but terrigenous REE+Y patterns and high ⁸⁷Sr/⁸⁶Sr ratios. As for the carbonate in the upper unit at Shiyu, the precipitating water is characterized as extremely lowδ¹⁸O values, terrigenous REE+Y patterns and high ⁸⁷Sr/⁸⁶Sr ratios. Since the upper unit predates the appearance of Ediacaran biota but postdates the Gaskiers iceage, the freshwater would probably originate from continental deglacial meltwater in the post-Gaskiers phase. The large differences in the concentrations of the trace elements and stable isotopes between the carbonate in the upper unit and the equivalent Neoproterozoic carbonates suggest that the precipitation rate of the upper carbonate would be high without the homogeneity between glacial meltwater and seawater. Thus, the continental marginal basin precipitating the Lantian carbonates may be one of rift basins between the Cathaysia and Yangtze Blocks with limited channel to the open ocean. Piyuancun was located at distal section whereas Shiyu was located at proximal section and thus was not connected with the open ocean. As for the lower carbonate unit, the REE+Y patterns and Sr isotopic compositions are similar to those infreshwater carbonate although theirδ¹⁸O values are in the same range with those of Neoproterozoic marine carbonate in the equivalent carbonate elsewhere in the world. The carbonate in the lower unit deposited after the global Marinoan glaciation. The seawater-like fluid would be derived from global deglacial meltwater in the post-Marinoan phase and carry the terrigenous materials poured into the sedimentary basin. Thus the precipitating water has the similarδ¹⁸O values to the seawater and terrigenous REE+Y patterns and high ⁸⁷Sr/⁸⁶Sr ratios.The differences in texture, elemental and isotopic compositions between wallrock and veinlet in the both lower and upper carbonate units indicates that the wallrock did not experience significant modification by diagenetic fluid. Theδ¹³C values for the calcite veinlet are in the same range with the values for the micritic wallrock in both the upper and lower carbonate unit of the Lantian Formation, indicating that the carbon sources for the veinlets are probably the micrite carbonate in the wallrock. Theδ¹⁸O values and REE+Y patterns are different between the calcite veinlets and micritic wallrock in most carbonate, which imples that the diagenetic fluids are external, except that other veinlets of the upper carbonate at Piyuancun is internal fluid. The veinlets of one group in not only the upper unit at Shiyu and Piyuancun and the lower unit at Shiyu have lowerδ¹⁸O values of-28.6 to -18.1‰which were from the continental glacial meltwater, but different REE+Y patterns which were controlled by the different terregenous weathering materials. Such veinlets occur at both Shiyu and Piyuancun. The veinlets in the other group occur in the upper unit at Piyuancun, which have relatively higherδ¹⁸O values of-17.2 to -11.3‰similar to the wallrocks. This indicates that the diagenetic fluids are internal and have the similar source to the precipitation fluids. But the REE+Y patterns are different in veinlets and wallrock, which implies that they influenced by the different terregenous weathering matterials.Correlating the chemical stratigraphy in southern Anhui with that elsewhere in South China and in the world can help us to understand the palaeoenvironment, palaeoclimate, atmospheric oxygen content, organic productivity and fluid compositions. Moreover, the elemental and isotopic compositions of the sedimentary carbonate rocks are closely associated with the circulation of geochemistry and biochemistry and thus can indicate the environmental evolution. In particular, the study of in-situ microanalyses of C-0 isotopes and trace elements in veinlet and its wallrock provides a new insight into the geochemistry of diageneic fluid, enabling us to trace its source. As a consequence, we have constructed a model for carbonate formation based on the change of elemental and isotopic compositions. Thus, our study can help to understand the palaeoenvironmental evolution during the Ediacaran phase, which is important to understanding of the radiation of the Ediacaran biota.更多还原