Petrology and geochemistry of chert on the marginal zone of Yangtze Platform,western Hunan,South China,during the Ediacaran–Cambrian transition

The Ediacaran–Cambrian transition was one of the most critical intervals in Earth history. During this interval, widespread chert was precipitated, commonly as a stratal wedge in carbonates, along the southern margin of the Yangtze Platform, South China. The chert wedge passes into a full chert succession further basinward to the south‐east. Four lithotypes of chert are identified across the marginal zone in western Hunan: mounded, vein, brecciated and bedded chert. The mounded chert is characterized by irregular to digitiform internal fabrics, generally with abundant original vesicles and/or channels that mostly are lined by botryoidal chalcedony cements with minor quartz and barite crystals. The host chert (or matrix) of these mounds is dominated by amorphous cryptocrystalline silica, commonly disseminated with pyrite. The vein chert, with minor quartz locally, generally cross‐cuts the overlying dolostone and chert horizons and terminates under the mounded and/or bedded chert bodies. The brecciated chert commonly occurs as splayed ‘intrusions’ or funnel‐shaped wedges and cross‐cuts the topmost dolostones. The bedded chert, the most common type, generally is thin to medium‐bedded and laminated locally; it is composed of amorphous silica with minor amounts of black lumps. Microthermometry of fluid inclusions from vein and void‐lining minerals (mainly quartz and barite) revealed homogenization temperatures from 120 to 180°C for the trapped primary fluids. Compositionally, these chert deposits generally are pure, with SiO₂ > 92 wt%, and only minor Fe₂O₃ and Al₂O₃ contents, most of which show positive Europium anomalies in rare earth element patterns, especially for the mounded chert. All these data suggest that the marginal zone chert deposits resulted from a low‐temperature, silica‐rich hydrothermal system, in which the mounded chert was precipitated around the releasing vents, i.e. as silica chimneys. The vein and splayed brecciated chert, however, was formed along the syndepositional fault/fracture conduits that linked downward, while the bedded chert was precipitated in the quieter water column from the fallout of hydrothermal plumes onto the sea floor. These petrological and geochemical data provide compelling evidence and a new clue to the understanding of the extensive silica precipitation; rapid tectono‐depositional and oceanic changes during the Ediacaran–Cambrian transition in South China.     

塔西北地区早寒武世玉尔吐斯组热液作用及沉积模式

埃迪卡拉纪-寒武纪之交全球沉积了一套黑色泥页岩和硅质岩组合,塔里木盆地早寒武世玉尔吐斯组即包含这样一套组合,并被认为是塔里木古生界主力烃源岩之一,然而目前对这套岩石组合的成因及沉积过程的研究还相对薄弱。在野外研究的基础上,将玉尔吐斯组分为5段,分为两个海进-海退旋回,认为其总体沉积于缓坡背景之上。对第一段硅质岩进行主量元素及稀土元素分析,结果显示其含有较低的Al/(Al+Fe+Mn)和较高的Fe/Ti值(均值分别为0.29和108.24)。在Al-Fe-Mn和Fe/Ti-Al(Al+Fe+Mn)图解上,多数样品落入热液作用范围,表明硅质岩主要受热液作用控制。硅质岩REE配分模式显示其不具有Eu的正异常和LREE富集的特征,这显示了海水对热液的稀释作用。靠近热液盆内口的库勒和什艾日克剖面发育较厚的硅质岩和一系列滑移变形构造。早寒武世时期,南天山洋已经打开,硅质热液广泛发育于整个塔北的深水和浅水区,这些迹象表明该区早寒武世仍然有很强的伸展构造活动。     

Hydrothermal venting activities in the Early Cambrian,South China: Petrological,geochronological and stable isotopic constraints

Multi-episodic (three episodes at least) hydrothermal silica chimneys are identified in the black chert successions (up to ～ 100 m thick locally), overlain by thick black shales (up to a few 100 m thick), in the Ediacaran–Cambrian boundary successions along the southern marginal zone of the Yangtze Platform, South China. Their occurrences are constrained within the lowermost Cambrian by the SHRIMP U–Pb zircon dating and stable isotopic chemostratigaphic data, and are stratigraphically coincident with the negative isotopic excursions of organic carbon (～ 10‰ VPDB in magnitude) and sulfide sulfur (between 0 and 10‰ CDT), i.e., in bases of Nemakit–Daldynian and Tommotian stages in the Lower Cambrian. In this case, a causal link could have existed between which the hydrothermal venting could have released huge amounts of reduced silica-rich hydrothermal fluids with abundant ¹³C-depleted greenhouse gases (methane) and volcanic-originated H₂S into the ocean and/or atmosphere, resulting in an extreme warm climate and oceanic anoxia, oceanic chemical perturbation, and subsequent massive precipitation of silica (chert), and large-magnitude negative isotopic excursions of organic carbon and sulfide sulfur. This scenario provides a unique chance to explore the causes of apparent tectono-depositional, oceanic and geochemical perturbations during this critical interval in South China and elsewhere.     

Submarine hydrothermal contribution for the extreme element accumulation during the early Cambrian,South China

Throughout the geological history of the Earth, submarine hydrothermal activity has played an important role in seawater chemistry, biological evolution and enrichment of metals in the Earth crust. However, the prospect of hydrothermal activity for extreme element accumulation during the early Cambrian, a key geological period, in South China has not been well-constrained. This study reports geochemical (e.g. REE and Sr isotope) investigations of a coarse-grained limestone layer and associated calcite veins in Zunyi and Nayong areas, Guizhou Province, to constrain the hydrothermal activity and evaluate the significance of hydrothermal contribution to extreme element accumulation during the early Cambrian, South China. Our results reveal positive Eu anomalies and higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.7083–0.7150) for carbonate samples than those of early Cambrian seawater, indicating the presence of hydrothermal processes. Combined with constraints from the spatial relationships and coincidence with adjacent mineralization, these hydrothermal processes provide the most probable contribution for polymetallic Ni–Mo–PGE mineralization. Furthermore, there are abundant hydrothermal dolomite and barite-calcite veins in the dolostone of the Dengying Formation, indicating the occurrence of a variety of hydrothermal fluids. Overall, multi-stage hydrothermal pulses with different fluid compositions spanned the Ediacaran–Cambrian transition in South China. In particular, these hydrothermal fluids with positive Eu anomalies and enriched radiogenic Sr, originating from Proterozoic mafic/ultramafic rocks, may have flowed through the underlying Precambrian silicate clastic rocks (e.g., Xiajiang, Banxi and Lengjiaxi Groups) and may have been crucial for the marine environment, biological diversity and extreme element accumulation during the early Cambrian, South China.     

湘西地区前寒武纪-寒武纪转折期碳酸盐-硅泥质沉积体系的截然转换:地层-沉积样式,形成机理及意义

前寒武纪一寒武纪转折期是地球历史演化的重要阶段之一,不仅纪录了后生动物的产生、灭绝以及加速分异的过程,同时也伴随着海洋地球化学明显的变化、长期全球性海洋缺氧等.为了更好地认识这一重要时期地球表层环境演变及动力机制,我们对湘西地区台地一盆地转换带不同沉积类型及空间变化进行了重点解剖,发现寒武纪最早期该地区在台一盆转换带上...     

Activity of Silica-Rich Hydrothermal Fluid and Its Impact on Deep Dolomite Reservoirs in the Sichuan Basin, Southern China

Well-developed dissolution pores occur in the dolomites of the Sinian Dengying Formation, which is an important oil and gas reservoir layer in the Sichuan Basin and adjacent areas in southern China. The pores are often filled with quartz, and some dolomites have been metasomatically altered to siliceous chert. Few studies have documented the characteristics, source or origin of silica-rich fluids and their effects on the dolomite reservoir. The peak homogenisation temperatures(T_h) of fluid inclusions in pore-filling quartz are between 150℃ and 190℃, with an average of 173.7℃. Gases in the inclusions are mainly composed of CO_2, CH_4 and N_2. Compared with host dolomite, pore-filling quartz and metasomatic chert contain higher amounts of Cr, Co, Mo, W and Fe, with average concentrations of 461.58, 3.99, 5.05, 31.43 and 6666.83 ppm in quartz and 308.98, 0.99, 1.04, 13.81 and 4703.50 ppm in chert, respectively. Strontium levels are lower than that in the host dolomite, with average concentrations in quartz and chert of 4.81 and 11.06 ppm, respectively. Rare earth element compositions in quartz and chert display positive Eu anomalies with a maximum δEu of 5.72. The δD_(SMOW) values of hydrogen isotopes in water from quartz inclusions vary from-85.1‰ to-53.1‰ with an average of-64.3‰, whereas the δ~(18)O_(SMOW) values range from 7.2‰ to 8.5‰ with an average of 8.2‰. The average ~(87)Sr/~(86)Sr ratios in quartz and chert are 0.711586 and 0.709917, respectively, which are higher than that in the host dolomite. The fluid inclusions, elemental and isotopic compositions demonstrate that the formation of quartz and chert was related to silica-rich hydrothermal fluid and that the fluid was the deep circulation of meteoric water along basement faults. Interactions with silica-rich hydrothermal fluids resulted in densification of dolomite reservoirs in the Dengying Formation through quartz precipitation and siliceous metasomatism. However, it increased the resistance of the host dolomite to compaction, improving the ability to maintain reservoir spaces during deep burial. Evidence for silica-rich hydrothermal activity is common in the Yangtze Platform and Tarim Basin and its influence on deep dolomite reservoirs should be thoroughly considered.     

浙西底本剖面皮园村组硅质岩元素地球化学特征及成因分析

埃迪卡拉—寒武纪（Ediacaran-Cambrian, E-C）过渡时期在下扬子地区沉积了一套层状黑色硅质岩,在浙西底本剖面称为皮园村组,大致对应于上扬子地区的留茶坡组或老堡组。该套硅质岩普遍具有高有机碳含量和强烈富集铀（U）、钒（V）、钼（Mo）等氧化还原敏感元素的特征,与早寒武世沉积的黑色泥岩共称为黑色岩系,其成因存在争议。文章对浙西地区皮园村组硅质岩开展了系统的主量、微量和稀土元素地球化学研究,探讨了其物质来源和成因。研究表明：（1）如此大范围的深水沉积的硅质岩仅靠热液提供硅质来源是很难形成的,海底热液对其影响很小;（2）埃迪卡拉纪末期生物降解产物对硅质岩的形成具有重要作用,但因为缺乏可靠的古生物证据,目前还不能判定构成硅质岩的物质是否主要来源于生物体,仅能推断生物活动对硅质岩的形成有一定的影响;（3）皮园村组硅质岩主要是化学沉积的产物,二氧化硅主要来源于大陆硅酸盐矿物的风化。     

The significance of cherts as markers of Ocean Plate Stratigraphy and paleoenvironmental conditions: New insights from the Neoproterozoic–Cambrian Blovice accretionary wedge,Bohemian Massif

The Ediacaran to early Cambrian Blovice accretionary complex, Bohemian Massif, hosts abundant chert bodies that formed on an oceanic plate and were involved in subduction beneath the northern margin of Gondwana. Field relationships of cherts to their host, their microstructure and elemental as well as isotopic compositions revealed diverse processes of chert petrogenesis reflecting depositional environment and position on the oceanic plate. The deep-water cherts formed through a hydrothermal precipitation of silica-rich gels on outer trench swell of the subducted slab with none or only minor addition of terrigenous material. On the contrary, the shallow-water cherts formed in lagoons on seamount slopes, and at least some of them represent a product of hydrothermal replacement of former carbonate and/or evaporite precursors. For both chert types, the hydrothermal fluids were of low temperature and continuous pervasive hydrothermal alteration of oceanic crust, together with an elevated Si content in Neoproterozoic seawater, served as the major source of silica. On the other hand, minor carbon enrichment in chert is mostly linked to variable incorporation of organic matter that was deposited on the seafloor. Rare earth element (REE) systematics of the cherts indicate predominantly oxygenated environment for the shallow-water cherts whereas the deep-water cherts were deposited in diverse redox conditions, depending on their distance from hydrothermal vent. Using these data, we demonstrate that the cherts once formed a part of Ocean Plate Stratigraphy (OPS) now dismembered and mixed with terrigenous siliciclastic material to form OPS mélanges. Combining our data with those from the existing literature, we show that cherts can serve as significant markers of OPS since the Archean, recording a complex interplay between seafloor-related volcanic (production of MORB- and OIB-like magmas) and sedimentary processes, hydrothermal activity at mid-ocean ridges and seamount chains as well as at outer slopes of subducting slabs. However, the cherts also exhibit a secular change in composition and petrogenesis most profoundly affected by an overturn in seawater silica cycle across the Precambrian–Phanerozoic boundary.     

Volcanic degassing,hydrothermal circulation and the flourishing of early life on Earth: A review of the evidence from c. 3490-3240 Ma rocks of the Pilbara Supergroup,Pilbara Craton,Western Australia

New data gathered during mapping of c. 3490–3240 Ma rocks of the Pilbara Supergroup in the Pilbara Craton show that most bedded chert units originated as epiclastic and evaporative sedimentary rocks that were silicified by repeated pulses of hydrothermal fluids that circulated through the footwall basalts during hiatuses in volcanism. For most cherts, fossil hydrothermal fluid pathways are preserved as silica ± barite ± Fe-bearing veins that cut through the footwall and up to the level of individual bedded chert units, but not above, indicating the contemporaneity of hydrothermal silica veining and bedded chert deposition at the end of volcanic eruptive events. Silica ± barite ± Fe-bearing vein swarms are accompanied by extensive hydrothermal alteration of the footwall to the bedded chert units, and occurred under alternating high-sulphidation and low-sulphidation conditions. These veins provided pathways to the surface for elements leached from the footwall (e.g., Si, Ba, Fe) and volcanogenic emissions from underlying felsic magma chambers (e.g., CO₂, H₂S/HS⁻, SO₂).Stratigraphic evidence of shallowing upward and subsequent deepening associated with the deposition of Warrawoona Group cherts is interpreted to relate to the emplacement of subvolcanic laccoliths and subsequent eruption and/or degassing of these magmas. Heat from these intrusions drove episodes of hydrothermal circulation. Listric normal faulting during caldera collapse produced basins with restricted circulation of seawater. Eruption of volcanogenic emissions into these restricted basins formed brine pools with concentration of the volcanogenic components, thereby providing habitats suitable for early life forms.Fossil stromatolites from two distinct stratigraphic units in the North Pole Dome grew in shallow water conditions, but in two very different geological settings with different morphologies. Stratiform and domical stromatolites in the stratigraphically lower, c. 3490 Ma, Dresser Formation of the Warrawoona Group are intimately associated with barite and chert precipitates from hydrothermal vents, suggesting that component microbes may have been chemoautotrophic hyperthermophiles. Evidence of shallow water to periodically exposed conditions, active growth faulting and soft sediment deformation indicates that the volcanogenic emissions were erupted into a shallow water, tectonically active caldera and concentrated therein to produce an extreme habitat for early life.Widespread conical and pseudocolumnar stromatolites in the c. 3400 Ma, Strelley Pool Chert at the base of the unconformably overlying Kelly Group occur in shallow marine platform carbonates. Silicification was the result of later hydrothermal circulation driven by heat from the overlying, newly erupted Euro Basalt. The markedly different morphology and geological setting of these only slightly younger stromatolites, compared with the Dresser Formation, suggests a diversity of microbial life on early Earth.The biogenicity of putative microfossils from this and younger hydrothermal silica veins in the Warrawoona Group remains controversial and requires further detailed study.     

广西来宾蓬莱滩剖面中上二叠统硅质岩的地球化学特征及沉积背景

广西来宾地区中上二叠统广泛发育硅质岩,蓬莱滩剖面和铁桥剖面是本地区中上二叠统出露最好的剖面.本文对蓬莱滩剖面28件硅质岩(分别为中二叠统茅口组3件与上二叠统合山组25件)的主量和稀土元素进行了分析研究,并结合岩石学特征,认为:蓬莱滩剖面合山组硅质岩沉积于受到陆源物质影响的边缘海盆环境,它们的硅质主要来源于硅质生物,为生物成因;而其茅口组硅质岩则沉积于远离陆源物质影响的边缘海盆环境,它们的硅质主要来源于热液,为热液成因.通过与铁桥剖面的热液成因硅质岩的沉积背景对比研究,结果表明:中晚二叠世时期,来宾地区为富硅的边缘海盆环境,而陆源物质输入程度的差异是造成本地区形成不同成因硅质岩的主要因素.     

Hydrothermal and resedimented origins of the precursor sediments to banded iron formation: sedimentological evidence from the Early Palaeoproterozoic Brockman Supersequence of Western Australia

The Early Palaeoproterozoic Brockman Supersequence comprises banded iron formation (BIF), bedded chert, limestone, mudrock, sandstone, breccia, tuffaceous mudstone, ashfall tuff and, in sections not reported here, basalt and rhyolite. Density current rhythms are preserved in sandstones, mudrocks, tuffaceous mudstones and limestones. Relics of similar rhythms in BIF imply that its precursor sediments were also deposited by density currents. Hemipelagic deposits are siliciclastic or mixed siliciclastic–volcaniclastic mudstones. Bedded chert, chert nodules and the chert matrix of BIF preserve evidence for formation by diagenetic replacement. For bedded chert (and chert nodules), silica replacement occurred before compaction close to or at the sediment–water interface, indicating that it is siliceous hardground. The chert matrix of BIF formed during compaction but before burial metamorphism. Original sediments were resedimented from two sources: (1) limestone, mudrock, sandstone, breccia and tuffaceous mudstone from a shelf; and (2) BIF from within the basin realm. Shelf sediments were resedimented to basin-floor fans during third-order lowstands. The precursor sediments to BIF are interpreted to have been granular hydrothermal muds, composed of iron-rich smectite and particles of iron oxyhydroxide and siderite that were deposited on the flanks of submarine volcanoes and resedimented by density currents. Resedimentation occurred by either bottom currents or gravity-driven turbidity currents, and the resulting sediment bodies may have been contourite drifts. The concept that BIF records high-frequency alternating precipitation from ambient sea water of iron minerals and silica is negated by this study. Instead, it is postulated that the precursor sediments to BIF originated in much the same way as modern Red Sea hydrothermal iron oxide deposits, implying that at least the particles of iron oxyhydroxide originated from the oxidation of vent fluids by sea water. Several orders of cyclicity in basin filling establish a relationship between rising to high sea levels, episodic sea-floor hydrothermal activity and BIF that is reminiscent of the link between eustacy and spreading-ridge pulses.     

中扬子地区早寒武世构造-沉积样式与古地理格局

前寒武纪-寒武纪转折期是地球历史演化的重要阶段之一,不仅纪录了后生动物的产生、灭绝以及加速分异的过程,同时也伴随着全球构造重组、长期全球性海洋缺氧、海洋地球化学明显的变化等。为了更好地认识这一重要时期地球表层环境演变及动力机制,我们对鄂西-湘西北地区下寒武统地层(相当于第1、2阶)进行了详细的沉积相和层序分析,在3大岩相组合(碳酸盐岩、硅岩、黑色泥岩/页岩)中识别出了16种主要岩相类型,并根据岩相时空变化识别出4个沉积层序(S1～S4)。在寒武纪开始时,湘西碳酸盐台地发生暴露,而台缘南侧发生裂陷,台地北侧的湘西北地区(如石门杨家坪)也发生大幅沉陷,但往北至三峡地区变浅,过渡为碳酸盐台地,形成两隆两凹的古地貌格局,这种巨大侧向地形差异(起伏)指示伸展构造背景下的断块掀斜作用,这种构造作用在寒武纪早期呈幕式活动,从而控制了该时期的沉积作用、沉积样式及其空间分布,并以沉积层序体现,因此每一层序代表了一幕从构造(拉张)强化到衰减过程的沉积响应。在构造幕的开始阶段,由于强的拉张和断裂掀斜作用,湘西碳酸盐台地发生暴露或变浅,而在该台缘南侧发生断陷,形成断阶,深部富硅热液流体沿断裂向上运移到断阶海底形成浅海热液喷流系统(硅烟囱),所以此处层序底部往往由一套特殊的硅质沉积(如有多管-孔丘状硅岩、脉状硅岩或漏斗状/楔状硅岩体)组成。此带向东南盆地方向,由于热液活动衰减和海水影响的增强,主要发育层状硅岩。从湘西北至三峡地区,层序底部一般由薄层状硅岩(常含泥质)组成,且向台地方向量减少(但碳酸盐岩增多)。在构造幕晚期,由于构造活动衰减,热液硅通量大幅减少,除深水盆地外,碳酸盐工厂恢复生产,因此层序上部主要由碳酸盐岩组成(深水盆地除外)。从第四构造幕热液活动后,盆地基底热衰减迅速,深部硅输入通量大幅减少,并在扬子地块发生大范围均匀挠曲沉降和快速海平面上升,造成碳酸盐台地淹没,形成了牛蹄塘组(或同期)富有机质黑色岩系。从此后,中扬子地区进入了一个新的构造-沉积演化时期。     

Hydrothermal Origin of Early Permian Chert Nodules in theCentral North China Craton Linked to Northern MarginCratonic Activation

Sedimentary chert phases from the Archean to the present are widely used to trace sedimentary environments and tectonic settings. Recently,chert nodules occurring within carbonates have been the subject of possible hydrothermal or biogenic origin,in lieu of a diagenetic origin.However,chert nodules from a vast cratonic basin represent extremely rich silica accumulations but less noted is how they respond to submarine hydrothermal activity (and/or surface siliceous organism productivity). The links between the cratonic-type chert depositions and environmental changes regarding cratonic evolution need to be revisited at a large temporal-spatial scale. The chert nodules are widespread throughout the Lower Permian Taiyuan Formation in the North China Craton (NCC).Several Taiyuan chert-rich successions across the NCC have been selected to study possible links between chert deposition and cratonic evolution in scenario of partial cratonic activation of the northern NCC margin during the Late Paleozoic.Based on stratigraphic correlation,the chert nodules are ubiquitously,evenly distributed throughout the Taiyuan Formation at a large craton-basin scale from the northern to southern interior NCC.Petrological results,elemental abundances,together with silicon and oxygen isotopic compositions of chert samples infer significant hydrothermal contributions for the silica accumulations. Therefore, the cratonic-scale chert depositions of hydrothermal origin infer a giant and remote silica reservoir,linking to the large igneous province and magmatism in the NCC northern margins.The Taiyuan chert nodules could be unique marine sedimentary archives recording the Late Paleozoic NCC partial activation,which also generated continental records of igneous rocks,bauxites and tuffs.The strong tectonics of the northern margin,south-dipping topography and northward transgression of the Early Permian NCC facilitated the chert deposition on the shallow marine platform in the cratonic interior.     

塔里木盆地寒武系白云岩的主要成因类型及其储层评价

塔里木盆地寒武系发育规模巨大的白云岩地层。对覆盖区钻孔岩心的岩石学分析表明,塔里木盆地寒武系白云岩成因主要有六种:潮坪相沉积成因、潟湖相沉积成因、渗透回流交代成因、压实排挤流交代成因、岩溶低温结晶成因和岩溶高温结晶成因。岩心物性分析与铸体薄片分析结果表明,岩溶低温结晶成因白云岩本质上可成为最优质的白云岩储层,储层特征为高孔高渗;潮坪沉积成因与岩溶高温结晶成因白云岩可形成高孔低渗的白云岩储层;潟湖相沉积成因白云岩、渗透回流交代成因白云岩与压实排挤流交代白云岩一般不作为优质的白云岩储层。     

塔里木盆地寒武系层状硅质岩与硅化岩的元素、δ30Si、δ18O地球化学研究

塔里木盆地寒武系发育层状硅质岩和硅化岩,层状硅质岩主要发育于塔东下寒武统深水沉积相区;硅化岩发育于塔东上寒武统斜坡沉积区与西部台地区寒武系白云岩中。根据显微结构特征,可将硅化岩分为放射状硅化岩与交代残余结构硅化岩两种。分析结果表明,层状硅质岩在Al—Fe-Mn三元图中位于正常海水沉积硅质岩区内,在微量元素平均地壳标准化图解上显示平缓右倾的特征;层状硅质岩Si同位素组成护。δ30Si为0．99‰～1．4‰,O同位素组成δ18O为21．4‰～24．4‰,与沉积型硅质岩相吻合,指示了正常海水沉积成因。硅化岩区别于层状硅质岩的典型特征是具有高U异常的特征;罗西斜坡放射状硅化岩具有较高的微量元素和稀土元素含量（∑REE为18．8～96．9μg／g）与较低的Si、O同位素值（δ18O和δ30Si值分别为15．8‰和1．7‰）;西部台地区交代残余结构硅化岩具有较低微量元素和稀土元素含量（∑REE为0．58～2．61μg／g）与较高的δ30Si（1．0‰～3．8‰）、δ18O（21．6‰～27．0‰）值特征。盆地东西部硅化岩的地球化学差异可能与硅化流体的温度差异有关,罗西斜坡放射状硅化岩硅化温度相对更高;另一方面,硅化过程对h430SiO4选择性较高,因而形成的交代石英具有较高的铲勘值。根据古城4井硅化岩包裹体均一温度与交代石英的O同位素值计算得到交代流体的δ18O值为9．1‰,该值与酸性岩浆水的δ18O值相似,指示了硅化流体可能来自于岩浆或变质水;以δ18O值（9．1‰）作为西部台地区硅化流体的O同位素值,计算得到西部台地区硅化岩硅化流体温度为101．3～158．5℃。根据石英O同位素温度计计算的硅化流体温度呈东高西低的趋势,指示了硅化流体可能来自台地东部。     

塔东北库鲁克塔格地区中上寒武统白云岩类型与孔隙演化*

对塔东北库鲁克塔格隆起的乌孜里塔格剖面中上寒武统白云岩的岩石学、成岩作用、裂隙—孔隙发育与充填等特征研究认为：可划分为有序度逐渐增加的粉晶白云岩、粉细晶白云岩、中细晶白云岩和中粗晶白云岩及晶洞中的中粗晶、巨晶和鞍形白云石等4种类型;分别对应于早期准同生—浅埋、中浅埋藏、中等埋藏和构造断裂—热液（热卤水）作用4期的成岩阶段;其中,沿走滑断裂—裂隙带、呈不规则的“侵入体” 具有斑点状、条纹—条带、雁行状—斑马、角砾状和不规则状结构的中粗晶、粗晶、巨晶（部分为鞍形）白云岩（石）,一般不发光或呈昏暗的暗红色,具有与加拿大西部盆地典型的热液白云岩相似的特征;中上寒武统白云岩存在4～5期构造裂隙和3期以上的充填作用;热液白云岩中以晶间孔、晶间溶孔（洞）、裂隙及沿裂隙的扩溶孔洞为主,主要有二世代的白云石和方解石和少量沥青、石英、微量硬石膏和重晶石等多期充填作用。与西加盆地寒武系部分或全部交代充填缝洞及角砾构造、受来自落基山造山带的晚白垩世至古近系拉腊米（Laramide）构造的被排驱的热液流体交代形成的典型热液白云岩稍有不同,海西晚期（晚二叠世）及燕山—喜马拉雅期（晚白垩世—第四纪）沿挤压—走滑断裂排驱的地层热卤水循环—扩散与交代作用可能是本区热液白云岩的主要成因,储集空间主要由中、晚期构造断裂—裂隙（溶蚀）所致,但不排除成岩早期埋藏或晚期大气水作用对其影响。     

The unconformity-related palaeokarst in the uppermost Ediacaran carbonate rocks in the northwestern Tarim Block,NW China: implication for sedimentary evolution during the Ediacaran–Cambrian transition

ABSTRACT

The unconformity between the Ediacaran and Cambrian and its associated palaeokarst was well preserved within the Aksu area of the northwestern margin of the Tarim Block and provides new insights into the sedimentary evolution of the Tarim Basin during the Ediacaran–Cambrian transition. In this contribution, the typical palaeokarst features in the uppermost Ediacaran dolostones at nine outcrops in the Aksu area were documented, including palaeocaves, cave-sediment infills, solution vugs, and pores. Large-scale collapsed breccia assemblages in palaeocaves indicate that strong karstification had occurred in the south belt from the Xiaoerburak to Kule sections. Karst characterized by small-scale caves and solution vugs at the Chigebrak and Kakebashi sections suggests that the karstification in the north belt was not stronger than that in the south belt. Whereas sparse solution pores and small vugs can only be observed at the Sugetbrak and Yuermeinak sections, demonstrating that the karstification was weak in the middle belt. Combined with the thickness of sediments in the base of the Cambrian which subsequently overlay on the Ediacaran–Cambrian unconformity, it is speculated that the palaeo-upland was probably located in the middle belt from the Sugetbrak to Yuermeinak sections and the slope was developed in the south and north belts during the Ediacaran–Cambrian transition. The palaeokarst documented in the uppermost Ediacaran dolostones in the Aksu area illustrates an important sedimentary hiatus of the Tarim Block during the Ediacaran–Cambrian transition, which might be due to the sea-level fall at the end of the Ediacaran.     

塔里木盆地肖尔布拉克剖面下丘里塔格组层序地层及其油气勘探意义

塔里木盆地晚寒武世台地沉积区发育大量的白云岩,其中蕴涵着大量白云岩储层。以柯坪地区肖尔布拉克剖面为例,首次提出了以地层单层厚度为划分依据划分层序地层的方法,结合岩石学分析结果,确定了肖尔布拉克剖面寒武系下丘里塔格组白云岩的层序地层四分方案,塔中地震资料表明该四分方案可能适用于塔里木盆地整个西部台地区。根据肖尔布拉克剖面下丘里塔格组白云岩的岩石学分析,埋藏岩溶作用产生的溶蚀空间为该白云岩段的主要储集空间,因此可将下丘里塔格组白云岩储层定义为"埋藏岩溶型白云岩储层",该储层发育位置与层序界面有密切关系,一般沿层序界面之下发育。根据上寒武统白云岩的层序地层四分方案,与层序地层对储层的控制作用,提出了塔里木盆地西部台地区上寒武统"埋藏岩溶型复式白云岩储层"概念,并给予了阐释。     

Chert occurrences in carbonate turbidites: examples from the Upper Jurassic of the Betic Mountains (southern Spain)

In Upper Jurassic carbonate turbidites of the Betic mountains (southern Spain), chert occurs in three morphologies: bedded chert, nodular chert and mottled chert. The last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock. The three types of chert are formed by replacement of limestones and are associated with different calcareous facies. Turbidite packstones of Saccocoma and peloids, and turbidite lime mudstones of pelagic material contain bedded and nodular cherts. The silicification textures are mainly micro- and cryptocrystalline quartz, with local chalcedonic quartz (both length-fast and length-slow) which is more common in the packstones. Only mottled chert is produced where calcareous breccia beds are silicified. Mottled chert consists of micro- and cryptocrystalline quartz, length-slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz. Beds and nodules are the result of early diagenetic silicification, with silica derived from the calcitization and dissolution of radiolarians and, subordinately, sponge spicules, whereas mottled chert is the consequence of later silicification in a probably Mg-rich environment. Early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones. Turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water. A silica-rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and nodular chert.     

Germanium/silica ratios in diagenetic chert nodules from the Ediacaran Doushantuo Formation,South China

Germanium/silica (Ge/Si) ratios of dolostone- and mudstone-hosted chert nodules from the Ediacaran (635–542 Ma) Doushantuo Formation in the Yangtze Gorges area, South China, are reported. These chert nodules typically have a calcite rim, a pyrite rim, and a silica core, the latter sometimes containing disseminated pyrite. The silica core was precipitated by early diagenetic replacement of carbonate and silty/muddy sediments. Two types of chert nodules are identified based on their mineralogy and geochemistry. Type-1 chert nodules are poor in disseminated pyrite in the silica core. They also have low Al, and show a strong positive correlation between Al contents and Ge/Si with a near-zero or negative intercept. In contrast, Type-2 chert nodules contain abundant disseminated pyrite in the silica core and show a weakly positive correlation between Ge/Si ratios and Al contents (with a large positive intercept on the Ge/Si axis). The Ge/Si of Doushantuo nodules are greater than those of Cretaceous deep-sea cherts, suggesting that the Ge/Si ratio of Ediacaran seawater/porewater was greater than the Cretaceous due to the more effective discrimination against Ge by inorganic opal precipitation relative to biogenic opal precipitation. The positive correlation between Ge/Si and Al can be interpreted using a mixing model with a pure chert (characterized by a low Ge/Si ratio) and an Al-rich endmember (characterized by a high Ge/Si ratio). The latter is most likely represented by a clay component, but the model-based estimate of the Ge/Si ratio inferred for the Al-rich (clay) endmember is much higher than that of Phanerozoic clay minerals. These high Ge/Si ratios for the clay endmember may be related to the generally high Ge/Si ratio of Ediacaran seawater, but could also be related to clay–organic matter interactions. Organic matter absorbed to clays could provide an additional source of Ge because certain organic molecules are known to have a high affinity for Ge due to their strong metal ion-chelating properties. The high Ge/Si ratio of the Al-rich endmember in Type-1 chert nodules suggests that Ge in porewaters from which these cherts precipitated may have been dominated by Ge–organic complexes. The low Ge/Si ratio inferred for the Al-rich endmember in Type-2 chert nodules is therefore taken to indicate that Ge was released from organic matter, perhaps due to anaerobic degradation of organic matter (accompanied by the formation of pyrite), and was redistributed between clay–organic endmembers and pure chert, resulting in a non-zero intercept in the Ge/Si vs. Al₂O₃ plots. These observations suggest that a strong terrestrial influence in a restricted sedimentary basin or a high content of dissolved organic carbon in Ediacaran seawater and porewater may have contributed to the dominance of Ge–organic complexes in the Doushantuo basin in the Yangtze Gorges area.