赣西湘东菊花状天青石的特征及其成因

赣西湘东地区的天青石,主要产于下二叠统由生物屑泥晶灰岩或泥灰岩构成的瘤状灰岩中,少数产于钙镁质页岩中。晶体常沿a轴或b轴延伸呈柱状,集合体在三度空间由内向外呈放射状排列,形似盛开的菊花,是一种比较少见的天青石结晶习性。本文通过沉积学、矿物学、地球化学的研究阐述了它的特征和成因,提出它是在成岩作用过程中碳酸盐灰泥中的SrSO_4围绕雏晶或胶态SiO_2核心生长而成。这种天青石的发现不仅具有矿物学和沉积学的意义,而且可作为我国华南地区沉积型海泡石的找矿标志。     

华南二叠系栖霞组菊花状天青石锶同位素特征及其地质意义

菊花状天青石是华南地区栖霞组内一种特殊的矿物集合体,形成于早期成岩作用阶段。在研究其成因过程中,系统分析了湖北黄石和湖南浏阳栖霞组内菊花状天青石的Sr同位素组成以及湖南浏阳、湖北黄石、江西永丰和广西来宾四处天青石假象、天青石围岩（泥晶灰岩）和围岩中方解石脉的Sr同位素组成。Sr同位素分析结果表明,天青石中的87Sr/ 86Sr最高值为0.707525,最低值为0.706981,平均值为0.707215。与天青石相比,天青石假象、天青石围岩和方解石脉的87Sr/ 86Sr值整体相对富含87Sr。由于天青石假象和方解石脉明显为晚期成岩作用成因,它们相对富含87Sr的锶同位素特征应是晚期成岩作用叠加的结果,与其类似的泥晶灰岩的87Sr/ 86Sr值也可能同样受到了晚期成岩作用的影响。栖霞组菊花状天青石比同组泥晶灰岩更好地记录了栖霞期海水的Sr同位素组成。     

浙西地区奥陶系砚瓦山组瘤状灰岩及其成因探讨*

对浙江常山黄泥塘剖面奥陶系砚瓦山组瘤状灰岩中瘤体和基质的矿物组成、结构、构造、化学成分及碳氧同位素等做了系统研究,并从其基本特征、尤其是碳氧同位素特征出发,结合前人研究成果,对瘤状灰岩的成因进行了初步探讨。研究表明,瘤状灰岩的瘤体和基质存在明显差异：瘤体主要由方解石组成（70％～98％）,基质则以黏土矿物为主（40％～78％）,方解石次之（20％～58％）;瘤体和基质的δ¹³C_VPDB值分别为0.5‰～1.4‰和-0.4‰～1.1‰,平均值分别为0.9‰和0.2‰, δ¹⁸O_VPDB 值分别为-12.8‰～-8.0‰和-11.7‰～-9.0‰,平均值分别为-10.6‰和-10.3‰;每个样品中瘤体的碳同位素值和形成温度均比基质高。黄泥塘剖面砚瓦山组瘤状灰岩是沉积物沉积时期周期性底流溶解和成岩过程中差异性压实和压溶共同作用的产物,瘤体原岩中非碳酸盐组分含量及后期压溶作用强度的差异导致瘤状灰岩不同的存在形式,有条带状、断续状和杂乱状3种。     

川东地区锶矿床地质特征及成因探讨

川东是我国锶矿资源远景最大，质量最佳的成矿区。铝矿产于下三叠统嘉陵江组二、四段的灰岩－白云岩－天青石膏质藻层纹白云岩－石膏岩（或灰岩）韵律层中。天青石膏质藻层纹白云岩是含矿层。准同生阶段已有天青石沉积，局部富集形成矿源层。在后生阶段的热力、压力驱动下．四川盆地的热卤水（２２０～３００℃）沿含矿层交代形成条纹条带状天青石矿层；第二期热卤水（１５０～２２０℃）在含矿层及其附近的碳酸盐岩裂隙中充填交代，形成四块状、脉状天青石；第三期热卤水（＜１５０℃）沿断裂充填形成天青石、重晶石脉。一、二期热卤水成矿作用常相互叠加，形成工业矿床。川东褶皱带西部背料部位具良好的找矿前景。     

滇西上寒武统保山组瘤状灰岩成因

从瘤体与基质的矿物组成、岩石的结构构造出发,深入探讨了滇西施甸地区保山组瘤状灰岩的沉积环境及其形成机理。研究表明,滇西施甸地区上寒武统保山组瘤状灰岩存在3种成因:一种是呈互层状的泥质和灰质沉积物在半固结或弱固结状态下,经差异压实-压溶作用而形成瘤状构造,岩石中各组分含量及后期压溶作用强度的差异导致了瘤状灰岩不同的存在形式;另一种是泥质和灰质沉积物在半固结或弱固结状态下经斜坡滑塌作用形成串珠状、砾屑状灰岩;第三种则是泥灰质沉积层间歇性地暴露干裂,移位磨蚀,并被后期沉积物充填胶结形成瘤状灰岩。      

重庆地区天青石型锶矿床成矿特征

重庆地区的锶矿产于下三叠统嘉陵江组二、四段的灰岩－白云岩－天青石膏质藻层白云岩－石膏岩韵律层中。天青石膏质藻层白云岩是含矿层。准同生阶段已有天青石沉积,形成矿源层。在后生阶段的热力、压力驱动下,第一期热卤水（２２０￣３００℃）沿含矿层交代形成条纹条带状天青石矿层;第二期热卤水（１５０￣２２０℃）在含矿层及其附近的碳酸盐岩裂隙中充填交代,形成团块状、脉状天青石;第三期热卤水（〈１５０℃）沿断裂充填形     

安徽沿长江地区下三叠统瘤状灰岩成因研究

在研究安徽沿长江地区中、下三叠统碳酸岩-蒸发岩时,发现早三叠世地层的中部,普遍不同程度地发育着结核灰岩(或称瘤状灰岩)。各地露头经风化后,常在灰岩层或泥灰岩层面呈现有疙瘩状凸起,作瘤状,在岩层的断面上可清晰地见到瘤体平行于层理断续排列,瘤体大小常为厘米级,并与泥质物交替成层。     

下扬子地区下三叠统蠕虫状灰岩及其成因

华南下三叠统中上部普遍发育蠕虫状灰岩,宏观上可以分为深色“蠕体” 和浅色“基质”。对于 其成因的认识一直存在很大的争议,主要的观点有化学凝聚说、生物扰动说和机械成因说等。笔者通过对下扬 子地区5条典型剖面进行野外详细分层描述、采样、薄片观察以及实验分析,并与中上扬子地区进行对比研究, 对蠕虫状灰岩进行了详细的定义,并将蠕虫状灰岩分为4类：连续成层蠕虫状灰岩、生物爬迹／钻孔蠕虫状灰 岩、呈似层状／杂乱条带状排列的蠕虫状灰岩和混染状蠕虫状灰岩,其在剖面上具有典型的变化规律。蠕虫状灰 岩发育的层段少见宏体化石,遗迹化石、微生物的分布与蠕虫状灰岩之间具有一定的联系。早三叠世缺氧、H_２S和CO_２ 含量增加、海水呈碱性、喜氧生物匮乏等为蠕虫状灰岩的形成创造了前提条件,灰泥在早三叠世特殊的 海洋化学条件下开始沉积分异,后经历了同沉积成岩作用和不同程度的宏体生物、微生物作用,因此形成了不 同类型的蠕虫状灰岩。蠕虫状灰岩发育时为海洋缺氧、有毒气体释放的后期,海水X（Mg）／X（Ca） 值开始降低, 碳同位素值回升,随后生物逐渐活跃起来,环境也逐渐改善。因此蠕虫状灰岩可以被认为是生物复苏、环境改 善的标志。     

广西桂林庙头上泥盆统融县组中的灰岩脉和角砾灰岩体及其成因

广西桂林庙头上泥盆统融县组台地边缘相具鸟眼或溶孔砂屑生屑藻粘结灰岩中发育了大量灰岩脉，灰岩脉呈岩墙状近直立地切过围岩，露头上可见连续垂直延伸长度达十余米，脉内仍是上泥盆统融县组灰岩组分，仅比围岩稍晚或同时代的：角砾状退白云石化藻粘结灰岩、角砾状藻粘结灰岩（快速堆积，角砾多数来自围岩）、细鲕粒颗粒灰岩、含钙球砂屑粒泥灰岩、藻砂屑腹足泥粒灰岩（正常沉积或液化变形软沉积流，均高于围岩层位）及大量栉壳状方解石脉（张性环境）等，灰岩脉内可见软沉积物变形痕迹及液化脉（地震液化），晚期灰岩脉可穿插早期灰岩脉；在其西侧同层位共生有一套楔状（地裂缝）、不规则状角砾灰岩体，角砾棱角状，大小不等，成分是灰色中厚层角砾状藻粘结灰岩、含钙球砂屑粒泥灰岩等。笔者等注意到灰岩脉均出露在北东向正断层的下盘（东侧），而角砾灰岩体则均出露在该断层的上盘（西侧），事实上，灰岩脉及角砾灰岩体分布走向与北东向断层走向一致，角砾灰岩体中的角砾成分显示其来自较灰岩脉围岩稍高层位，与灰岩脉内充填岩性相近，因此，是该断层控制了当时的沉积，此断层是晚泥盆世台地边缘同沉积正断层；灰岩脉是与断层伴生的张性裂隙被围岩角砾或稍晚时段的沉积物充填；角砾灰岩体是同沉积正断层形成的断层崖崩落角砾岩，指示一个消失殆尽的晚泥盆世碳酸盐岩同沉积正断层陡崖，因而，桂林台地，至少西段台地边缘，是与右江各孤立台地边缘一样——在地质图上应标示为同沉积正断层边界。其构造意义是：庙头地区的灰岩脉、角砾灰岩体及同沉积断层是桂林台地对晚泥盆世构造伸展作用的响应，从而说明广西晚古生代的板块拉张，不是从二叠纪才开始的，而是至少从晚泥盆世就开始了；晚古生代至早三叠世持续的孤立台地与深水盆地相间的古地理格局，是在晚泥盆世就奠定了基础。     

下扬子巢湖地区下三叠统瘤状灰岩成因再探讨

巢湖地区下三叠统整体沉积于较深水环境,广泛发育瘤状灰岩。针对目前该瘤状灰岩成因机制认识争议较大的现 状,文中依据野外实地观测及室内宏观、微观的系统观察,结合沉积环境和成岩作用与瘤状灰岩形成的关系分析,对瘤状 灰岩的成因机制进行综合的探讨。研究表明瘤状灰岩的形成是沉积和成岩作用综合作用的产物,沉积时期不平整CCD界面 的周期性浮动是造成瘤状灰岩细颈化的基础,而CCD界面的周期性浮动可能与早三叠世气侯冷暖周期性变化以及海底底流 作用有关。后期基质泥岩中酸性孔隙水的溶蚀是造成瘤状灰岩细颈处折断的重要原因,压溶作用产生的局部应力场变动造 成瘤状灰岩瘤体杂乱状分布。而前人总结的3种瘤状灰岩,即顺层串珠型、断续型和杂乱型瘤状灰岩,是在沉积与成岩改 造作用逐渐加强条件下依次形成。     

华南栖霞组菊花石假象内正延性玉髓的成因及其地质意义

正延性玉髓常被视为干旱蒸发沉积环境的标志。正延性玉髓亦广泛分布于华南地区栖霞组菊花石 (目前绝大多数为天青石假象 )内。本文在天青石后期矿物交代序列识别的基础上,通过对菊花石内各期次碳酸盐交代矿物的电子探针分析,发现菊花石内正延性玉髓形成于富含硫酸根离子和镁离子的成岩介质条件中。结合已有的正延性玉髓研究报道和栖霞组的沉积、成岩作用环境特征分析,认为华南地区栖霞组菊花石内正延性玉髓的形成环境与高蒸发沉积环境或高盐度成岩环境无关。因此,仅根据正延性玉髓的出现不足以确定沉积或成岩环境的盐度条件。     

Geochemistry of Sedimentation and Diagenesis in Qixia Formation (Early Permian) of Badong,Hubei Province, China: Implications for T-R Cycle Interpretation

ＩＮＴＲＯＤＵＣＴＩＯＮＳｅｄｉｍｅｎｔａｔｉｏｎａｎｄｄｉａｇｅｎｅｓｉｓａｒｅｔｗｏｍａｊｏｒｆａｃｔｏｒｓｃｏｎ－ｔｒｉｂｕｔｉｎｇｔｏｇｅｏｃｈｅｍｉｃａｌｃｈａｒａｃｔｅｒｉｓｔｉｃｓｏｆｓｅｄｉｍｅｎｔａｒｙｃａｒｂｏｎ－ａｔｅｓ．Ｐｒｏ...     

湖北省巴东栖霞组沉积成岩作用地球化学特征研究

在详细的沉积作用、成岩作用研究基础上,利用多元统计分析方法重点对湖北省巴东水布垭剖面栖霞组碳酸盐岩地球化学特征进行了分析。研究表明,该组碳酸盐岩中 CaO 、Al₂O₃ 、K₂O 、Fe₂O₃ 、P、Ba 和 Mn的含量主要与沉积环境或原岩岩性有关,地层中SiO₂、MgO 、Sr 和 Na 的含量主要与成岩作用有关。建立了栖霞组沉积成岩作用地球化学模式。该研究合理地解释了本组岩石特殊的地球化学特征,深化了对该组岩石形成环境,特别是成岩环境的认识,区分了沉积作用和成岩作用对该组岩石地球化学特征的影响,对合理地选取有关地球化学参数研究该组岩石及类似地层的沉积作用和成岩作用具参考价值。     

Evinosponges in the Triassic Esino Limestone (Southern Alps): documentation of early lithification and late diagenetic overprint

A carbonate buildup of Middle Triassic age, the Esino Limestone, outcrops in the Southern Calcareous Alps of Lombardy (N Italy). Along its margin and within the open subtidal facies, the Esino Limestone contains calcite cement-filled cavities of cm to m size. These features, known as evinosponges, may form pervasive networks within the host rock. The filling consists of concentric, isopachous layers of fibrous low-Mg calcite crystals characterized by strong undulose extinction and bent cleavages. The cement crusts are non-luminescent under cathodoluminescence, but both cements and host rock are cross-cut by micro-fractures filled with bright-luminescent calcite, related to late void-filling sparite. Mixing of different carbonates is reflected in stable isotope data. On the hand specimen scale, the oxygen and carbon isotope compositions of cements and host rock show little variation. When compared on a regional scale, the values cover a broad range from δ¹⁸O(PDB)=?5‰ to ?12‰ and from δ¹³O =0‰ to +3‰. The linear covariant trends defined by the oxygen and carbon isotope data for different sampling regions reflect the admixture of late, isotopically depleted calcite with an isotopically enriched non-luminescent calcite of early diagenetic origin. The Esino Limestone fibrous cements, which were probably precipitated in the marine or marine-meteoric phreatic environment, were affected by late diagenetic processes that caused mineral deformation and isotopic depletion through recrystallization and the admixture of a later calcite. These later calcites precipitated from penetrative fluids possibly related to Late Triassic volcanic activity and/or to the Late Cretaceous/Early Palaeogene alpine orogeny.     

Mineralogical and isotopic record of biotic and abiotic diagenesis of the Callovian-Oxfordian clayey formation of Bure (France)

The Callovian-Oxfordian (COx) clayey unit is being studied in the Eastern part of the Paris Basin at depths between 400 and 500 m depth to assess of its suitability for nuclear waste disposal. The present study combines new mineralogical and isotopic data to describe the sedimentary history of the COx unit. Petrologic study provided evidence of the following diagenetic mineral sequence: (1) framboidal pyrite and micritic calcite, (2) iron-rich euhedral carbonates (ankerite, sideroplesite) and glauconite (3) limpid calcite and dolomite and celestite infilling residual porosity in bioclasts and cracks, (4) chalcedony, (5) quartz/calcite. Pyrite in bioturbations shows a wide range of δ³⁴S (−38‰ to +34.5‰), providing evidence of bacterial sulphate reduction processes in changing sedimentation conditions. The most negative values (−38‰ to −22‰), measured in the lower part of the COx unit indicate precipitation of pyrite in a marine environment with a continuous sulphate supply. The most positive pyrite δ³⁴S values (−14‰ up to +34.5‰) in the upper part of the COx unit indicate pyrite precipitation in a closed system. Celestite δ³⁴S values reflect the last evolutionary stage of the system when bacterial activity ended; however its deposition cannot be possible without sulphate supply due to carbonate bioclast dissolution. The ⁸⁷Sr/⁸⁶Sr ratio of celestite (0.706872-0.707040) is consistent with deposition from Jurassic marine-derived waters. Carbon and oxygen isotopic compositions of bulk calcite and dolomite are consistent with marine carbonates. Siderite, only present in the maximum clay zone, has chemical composition and δ¹⁸O consistent with a marine environment. Its δ¹³C is however lower than those of marine carbonates, suggesting a contribution of ¹³C-depleted carbon from degradation of organic matter. δ¹⁸O values of diagenetic chalcedony range between +27‰ and +31‰, suggesting precipitation from marine-derived pore waters. Late calcite crosscutting a vein filled with chalcedony and celestite, and late euhedral quartz in a limestone from the top of the formation have lower δ¹⁸O values (∼+19‰), suggesting that they precipitated from meteoric fluids, isotopically close to present-day pore waters of the formation. Finally, the study illustrates the transition from very active, biotic diagenesis to abiotic diagenesis. This transition appears to be driven by compaction of the sediment, which inhibited movement of bacterial cells by reduction of porosity and pore sizes, rather than a lack of inorganic carbon or sulphates.     

青海大风山锶矿床中天青石的成分特征

青海大风山是中国典型的陆相碳酸盐-硫酸盐沉积组合锶矿床,也是中国目前储量最大的锶矿床.文章对大风山矿床中不同产状的天青石进行了系统的电子探针测试,分析归纳了不同类型矿石中天青石的成分变化规律,发现同心圆状天青石中的SrO含量最高,其次是细纤维状、花朵状、钟乳状、假层纹状、粗纤维状天青石,隐晶质天青石含量最低.通过与江西省萍乡包公庙、宜春慈化、湖南浏阳永和海相碳酸盐沉积组合锶矿床、重庆大足、玉峡及干沟等海相碳酸盐-硫酸盐沉积组合锶矿床、云南金顶硫化物-天青石层控伴生锶矿床天青石的对比,总结了不同类型矿床中天青石的特征,指出青海大风山天青石矿床成分简单,除SrSO4较高外,其余均相对较低,有利于碳酸锶的提取和回收,具有很高的工业价值.     

祁连山冻土区天然气水合物伴生碳酸盐C、O同位素特征及其地质意义

伴生碳酸盐矿物在海底与天然气水合物伴生是一种普遍现象,但在陆上冻土区中报道较少。以近两年在祁连山冻土区发现天然气水合物伴生的碳酸盐矿物为研究对象,根据对含碳酸盐样品的显微镜观察及矿物分析,确定了伴生碳酸盐的矿物种属及赋存状态。按碳酸盐矿物组成及地质产状的不同,其赋存状态分成4种类型,即白色薄层状、烟灰色菱形晶簇状、深灰色薄壳状、微细浸染状。根据不同赋存形态碳酸盐的C、O同位素特征,认为烟灰色菱形晶簇状方解石或呈(云烟状)微晶方解石可能与天然气水合物分解有关。碳酸盐C、O同位素随深度变化特征表明在一定深度处可能存在着烃类物质的活动,即天然气水合物分解,导致碳酸盐的矿物生成。     

Calcite cementation during bacterial manganese, iron and sulphate reduction in Jurassic shallow marine carbonates

Faunally restricted argillaceous wackestones from the Middle Jurassic of eastern England contain evidence of early diagenetic skeletal aragonite dissolution and stabilization of the carbonate matrix, closely followed by precipitation of zoned calcite cements, and precipitation of pyrite. Distinctive cathodoluminescence and trace element trends through the authigenic calcites, their negative δ¹³C compositions and the location of pyrite in the paragenetic sequence indicate that calcite precipitation took place during sequential bacterial Mn, Fe and sulphate reduction. Calcite δ¹⁸O values are compatible with cementation from essentially marine pore fluids, although compositions vary owing to minor contamination with ¹⁸O-depleted ‘late’cements. Mg and Sr concentrations in the calcites are lower than those in recent marine calcite cements. This may be a result of kinetic factors associated with the shallow burial cementation microenvironments. Bicarbonate for sustained precipitation of the authigenic calcites was derived largely from aragonite remobilization, augmented by that produced through anaerobic organic matter oxidation in the metal and sulphate reduction environments. Aragonite dissolution is thought to have been induced by acidity generated during aerobic bacterial oxidation of organic matter. Distinction of post-oxic metal reduction and anoxic sulphate reduction diagenetic environments in modern carbonate sediments is uncommon outside pelagic settings, and early bacterially mediated diagenesis in modern platform carbonates is associated with extensive carbonate dissolution. High detrital Fe contents of the Jurassic sediments, and their restricted depositional environment, were probably the critical factors promoting early cementation. These precipitates constitute a unique example of calcite authigenesis in shallow water limestones during bacterial Mn and Fe reduction.     

Sedimentological and palaeohydrological responses to tectonics and climate in a small, closed, lacustrine system: Oligocene As Pontes Basin (Spain)

ABSTRACT A small, closed, lacustrine system developed during the restraining overstep stages of the Oligocene As Pontes strike‐slip basin (Spain). The increase in basin accommodation and the headward spread of the drainage, which increased the water input, triggered a change from shallow, holomictic to deeper, meromictic conditions. The lower, shallow, lacustrine assemblage consists of mudstone–carbonate cycles recording lacustrine–palustrine ramp deposition in a saline lake. High Sr content in some early diagenetic calcites suggests that aragonite and calcite made up the primary carbonate muds. Early dolomitization took place together with widespread pedogenic activity. The upper, deep, freshwater, lacustrine assemblage includes bundles of carbonate–clay rhythmites and fine‐grained turbidite beds. Primary calcite and diagenetic siderite make up the carbonate laminae. The Mg content of the primary carbonates records variations in Mg/Ca ratios in lacustrine waters. δ¹⁸O and δ¹³C covariance trends in calcite reinforce closed drainage conditions. δ¹⁸O data indicate that the lake system changed rapidly from short‐lived isotopically light periods (i.e. from seasonal to pluriannual) to longer steady‐state periods of heavier δ¹⁸O (i.e. from pluriannual to millennial). The small δ¹³C changes in the covariant trends were caused by dilute inflow, changing the contributions of dissolved organic carbon in the system and/or internal variations in lacustrine organic productivity and recycling. In both shallow and deep carbonate facies, sulphate reduction and methanogenesis may account, respectively, for the larger negative and positive δ¹³C shifts recorded in the early diagenetic carbonates (calcite, dolomite and siderite). The lacustrine system was very susceptible to high‐frequency, climatically forced water balance variations. These climatic oscillations interfered with the low‐frequency tectonic and morphological changes in the basin catchment. This resulted in the superposition of high‐order depositional, mineralogical and geochemical cycles and rhythms on the lower order lacustrine infill sequence.