中元古代晚期浅海高能沉积环境中的海绿石:以天津蓟县剖面铁岭组为例

海绿石是一种富钾、富铁的含水层状铝硅酸盐矿物,在沉积学领域常被作为一种普遍的指相矿物。多年研究的结果表明,现代海绿石主要形成在慢速、弱还原的较深水环境中,而且还可以作为"凝缩段"的识别标志之一。天津蓟县剖面中元古界铁岭组第二段灰岩中的海绿石,产在高能叠层石岩礁之中,主要以胶体形式富集在叠层石和均一石的边界上,代表较为典型的原地海绿石;较高的氧化钾含量(大于8%)而显示出高成熟海绿石的特点。很明显,铁岭组二段灰岩中的原地高成熟海绿石,不但不能作为"凝缩段"的识别标志,而且也不是长时间地层间断的产物。由于形成在正常高能浅海环境,而且处于中元古代末期,与现代沉积中的海绿石存在较大的差异,可能代表了中元古代末期的正常浅海还处于含氧量不够充分的弱还原状态,最终使铁岭组灰岩中的海绿石成为前寒武纪海绿石产出的一个典型代表,也间接的表明了在漫长的地质历史演变过程中海绿石产出的多样性特点。     

苏南龙潭组的海绿石特征和沉积环境

<正> 苏南龙潭组为一套海陆过渡相碎屑岩含煤沉积,某些层段的中、细粒砂岩、粉砂岩及砂质灰岩中常见到形态各异的海绿石,虽丰度不大,却分布广泛,与特定的沉积条件和沉积环境息息相关。长期以来,海绿石被认为是一种典型的海相自生矿物,作为鉴定海相和海陆过渡相地层的标志之一,它在含煤地层中的出现,是分析含煤岩系古地理环境的重要     

奥连特盆地白垩系海绿石成因类型及沉积地质意义

厄瓜多尔奥连特盆地白垩系Napo组UT段发育一套分布广泛的富含海绿石的硅质碎屑岩,针对海绿石的岩相、矿物学、地球化学及时空属性进行分析,可以揭示海绿石的组分、成熟度、形成及成因类型,结合地质约束有助于理解其形成的沉积地质意义.利用偏光显微镜、X射线衍射、电子探针及Qemscan对海绿石矿物的岩相、矿物组成和主量元素进行系统地分析.暗绿色、呈弯曲玫瑰花状的海绿石具有高的K₂O含量(平均值为8%,质量百分比),是形成于海相低沉积速率环境的高演化成熟型海绿石云母矿物或狭义范畴的海绿石.化学组分和时空属性揭示研究层段的海绿石经历了一定程度风暴流和/或潮汐流作用的搬运改造,属于层内准原地海绿石.UT段海绿石含量向上的增大趋势和成熟度的变化,以及横向上从盆地东部斜坡区埋深2~3 km到西部盆缘露头区相距约120 km的海绿石在形态和化学成分上具有相似性,指示其主要是层内准原地海绿石的特点.UT段垂向上海绿石含量增大的趋势同时反映外陆棚物源区原地海绿石向岸方向的短距离迁移,反映了相对海平面持续上升的海进过程;而且同时期海绿石平面上的广泛分布指示沉积时期的环境属于构造稳定的陆表海.      

内蒙古大青山逆冲推覆体系中生代逆冲构造活动的40Ar-39Ar定年

内蒙古大青山呼和浩特段北侧发育3条走向北东东、指向北西的逆冲断层,并与被其分割的3个逆冲席体及一个原地系构成大青山逆冲推覆体系。逆冲断层上盘底部发育较深层次的糜棱岩,下盘顶部多发育低温的千糜岩。本研究在构造地质调查基础上,结合宏-微观岩石矿物学分析,采用⁴⁰Ar-³⁹Ar定年对该逆冲体系的活动时间进行约束。逆冲断层带内3个千糜岩绢云母⁴⁰Ar-³⁹Ar年龄范围为120~119Ma,另一样品给出了120Ma的概率统计峰值年龄。千糜岩为低温同变形变质产物,细粒绢云母为同变形新生矿物,其⁴⁰Ar-³⁹Ar年龄可代表变形年龄。侵位在断层内弱变形的花岗闪长岩为同构造晚期侵位,角闪石⁴⁰Ar-³⁹Ar年龄限定其冷却时间下限为121Ma,概率统计峰值年龄为119Ma。逆冲断层上盘底部发育较高温的糜棱岩,而低温千糜岩的形成时间应属于变形后期。因此,120Ma至119Ma期间,大青山逆冲推覆体系的逆冲作用应已是处于变形晚期。     

钻孔岩心磁化率及PXRF 测量在智利 月亮山铁铜矿区应用与找矿预测

[摘 要]以智利月亮山铁氧化物铜金型矿床为例,利用磁化率K 对铁磁性矿物及蚀变岩的现场识别能力和X 射线荧光分析仪(PXRF)快速分析元素含量功能,结合矿床地质以闪长岩(5.5×10^-3SI< K <17.9×10-3 SI)、角砾岩(0.35×10^-3SI< K <0.7×10^-3SI)、磁铁矿(K>753. 4×10^-3SI)、磁赤铁矿(313.3×10^-3SI < K <753.4×10^-3SI)、赤铁矿(0.78×10^-3SI< K <1.62×10^-3SI)、镜铁矿(0.67×10^-3SI< K <0.78×10^-3SI)等划分闪长岩亚相、角砾岩亚相、磁铁矿微相、磁赤铁矿微相、赤铁矿微相、镜铁矿微相;以PXRF现场测量铁含量>30%,铜含量>0.5%为含矿(化)界限,确定磁化率-铁铜含量对应关系:高磁化率-高铁含量-磁铁矿型、低磁化率-高铁含量-赤铁矿型(镜铁矿)、低磁化率-低铁含量-蚀变岩型,及岩相学找矿标志-矿物标志、构造标志、闪长岩标志、蚀变分带标志和矿物蚀变标志等,对月亮山矿区进行深部找矿预测。     

豫西晚元古代洛峪群沉积作用及环境演化

洛峪群本身构成一个完整的海侵-海退沉积序列。早期海侵使得洛峪群底部滨岸沉积逐渐被陆棚沉积所超覆,并造成典型的海绿石砂岩凝缩层。随后的海退导致明显的海岸进积,在垂向上形成外陆棚-内陆棚-滨岸/障壁岛-泻湖沉积序列。洛峪群中的硅质碎屑岩沉积主要受风暴控制,形成各种特征的丘状和洼状交错层砂岩以及浅海浊积岩。洛峪群是华北地块南缘中、晚元古代裂谷期的产物,并可能处于裂谷热沉降阶段的后期。     

对“指相矿物”海绿石的重新认识

长期以来，海绿石一直被认为是在特定海洋条件下形成的自生矿物，被作为海相标志，随着陆相海绿石的不断发现，有必要对这一观点进行重新认识。本文通过对不同地区、不同环境中的海绿石的统计研究认为：海绿石既可以形成于海洋环境，也可以形成于陆相湖泊环境，海绿石不能作为海洋环境的指相矿物；一般来说，形成于陆相湖泊环境的海绿石与形成于海洋环境的海绿石相比，在化学成分上具有Ａｌ２Ｏ３、Ｋ２Ｏ含量高而ＦｅＯ含量低的特点     

天津蓟县中元古界铁岭组叠层石灰岩中原地海绿石的沉积学意义

天津蓟县剖面中元古界铁岭组二段叠层石灰岩中普遍发育海绿石。野外观察表明,海绿石主要分布在叠层石柱体间的泥晶灰岩中,呈薄膜状富集在叠层石鞘外缘;微观特征分析表明,海绿石呈不规则状的胶体形式,显示了原地海绿石的基本特点。电子探针的组分分析表明,铁岭组中的海绿石为中成熟度的海绿石。由于产在潮下高能柱状叠层石灰岩中,铁岭组中的海绿石并不反映低沉积速率或沉积间断的沉积条件,与现代海绿石的形成环境具有明显的差异。因此,中元古界铁岭组叠层石灰岩中的中成熟度原地海绿石是特殊沉积背景下的独特产物,为研究海绿石在地质历史时期产出的多样性提供了一个重要实例。     

松辽盆地青山口组橄榄粗安岩：40Ar/39Ar年龄、地球化学及其成盆、成烃和成藏意义

在松辽盆地上白垩统青山口组二段湖相泥岩中识别出橄榄粗安岩夹层,高精度⁴⁰Ar/³⁹Ar坪年龄t_p=88.0±0.3Ma,属晚白垩世Coniacian阶。岩石具玻基少斑斑状结构,斑晶为橄榄石、辉石和斜长石。岩石富钠、富铝、富轻稀土（∑LREE/∑HREE=8.0~9.7）和U、Th、Pb、Rb、Sr、Ba等不相容的大离子亲石元素,Rb/Sr比高、Sm/Nd比低,δEu =0.95~1.36,高镁（Mg^#=0.61~0.64）,高氧化度（Ox^o=0.62~0.68）,低分异指数（MDI=3.4~5.3）。MgO/Al₂O₃和标准矿物压力计算得到岩浆源区深度大于60km。岩浆源于富集型地幔,属板内火山岩,是成盆过程中裂谷作用发育到最大深切割期的产物。火山活动热效应及其高区域地温场有利于成烃,造成局部类礁相环境形成的大量生物灰岩沉积是重要烃源岩,火山岩之下的牵引构造、之上的差异压实局部高部位以及其周围的浊积体是成藏有利部位。     

云南泸水钨锡矿床微量钾矿物40Ar-39Ar马鞍形年龄谱的含义

采用⁴⁰Ar-³⁹Ar计时方法测定滇西泸水钨锡矿床的白云母、石英、电气石和绿柱石等矿物的年龄，其中微量K矿物（石英、电气石和绿柱石）均形成含过剩⁴⁰Ar的马鞍形年龄谱。研究表明鞍形谱最小年龄的含义比较复杂，有些样品的最小年龄接近于成矿年龄，而有些样品的最小年龄大于甚至远大于成矿年龄。     

徐州大北望寒武系徐庄组海绿石成因探讨

徐州大北望寒武系徐庄组发育不同类型海绿石，为探讨不同类型海绿石的成因，基于海绿石的矿物学、地球化学及沉积环境分析，利用偏光显微镜、X 射线衍射及电子探针等方法对各含海绿石岩层及不同类型海绿石进行系统对比研究查明海绿石的指相意义。结果表明，徐庄组发育颗粒状、碎屑假象状和胶团状三种类型海绿石，其中颗粒状海绿石具有中—高含量钾、高含量铁、砂岩中—高含量铝、石灰岩中低含量铝等特征；碎屑假象状海绿石具有变化大的钾、铁、铝，且保持着其母质形态特征；胶团状海绿石具有高钾、高铁、低铝特点。颗粒状海绿石成因符合颗粒绿化理论，砂岩中海绿石易受环境影响；碎屑假象状海绿石成因符合层状晶格理论及假象替代理论，受到交代和溶蚀作用，因受较强的水动力条件影响成分变化较大；胶团状海绿石是凝胶状海绿石通过胶结作用沉淀，形成于稳定环境中。     

RbSr and KAr ages of paleozoic glauconites from Ohio—Indiana and Missouri,U.S.A.

Glauconite pellets from the Lower Silurian Brassfield Formation on both limbs of the Cincinnati Arch in Ohio and Indiana give a RbSr age of 370 ± 11 Ma, which is substantially younger than the stratigraphic age of the formation. The age is concordantcwith conventional KAr ages of 355 ± 6 and 368 ± 5 Ma for two of the same glauconites. Concordant ages were also obtained from the Viburnum Trend in Missouri, where glauconite pellets from the Davis Formation in an ore-zone collapse structure into the underlying Bonneterre Formation give a RbSr age of 387 ± 21 Ma and conventional KAr ages of 368 ± 5 and 369 ± 5 and 369 ± 5 Ma. A third suite of glauconite from the Bonneterre Formation in the Old Lead Belt ～ 10 km from the nearest ore body has given a RbSr age of 423 ± 7 Ma and slightly older conventional KAr ages of 434 ± 6, 445 ± 6 and 441 ± 11 Ma.Because these glauconite-bearing rocks have been buried to depths of less than 1 km, thermal resetting of the RbSr and KAr systematics appears unlikely. The initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the RbSr isochrons are similar to the ratios for vein- and vug-filling dolomite and calcite. This is consistent with resetting of the RbSr and KAr systems during diagenetic changes which included the isotopic equilibration (perhaps by cation exchange) of the Sr in the glauconite with that in the diagenetic and Mississippi Valley-type ore fluids. This interpretation implies that the age of the Mississippi Valley-type mineralization in the Viburnum Trend is Devonian rather than Carboniferous—Permian as has been inferred from paleomagnetic measurements.Cation-exchange experiments with a dilute Sr-bearing solution and an artificial oilfield brine indicate that glauconite adsorbs large amounts of Sr, some of which is sufficiently strongly attached to the glauconite lattice as to resist leaching with ammonium acetate. The introduction of this strongly attached Sr may be the first step in the resetting of the RbSr systematics of glauconite by cation exchange.     

Origin and geochemical characterization of the glauconites in the Upper Cretaceous Lameta Formation,Narmada Basin,central India

This study presents geochemical characteristics of glauconites in estuarine deposits within the Maastrichtian Lameta Formation in central India. Resting conformably over the Bagh Group, the Lameta Formation consists of ~4-5 m thick arenaceous, argillaceous and calcareous green sandstones underlying the Deccan Traps. The sandstone is friable, medium-to coarse-grained, well-sorted and thoroughly crossstratified, and contains marine fossils. Detailed petrography, spectroscopy and mineral chemistry indicates unique chemical composition of glauconite with high K_2O, MgO, Al_2O_3 and moderate TFe_2O_3. Glauconite is formed by the replacement of K-feldspars, initially as stringers in the cleavages and fractures of feldspars. Incipient glauconite subsequently evolves fully, appearing as pellets. Fully-evolved glauconite pellets often leave tiny relics of K-feldspar. XRD exhibits characteristic peak of 10A from basal(001)reflection of glauconite, indicating the "evolved" character. The K_2O content of glauconites in the Lameta Formation varies from 5.51% to 8.29%, corroborating the "evolved" to "highly-evolved" maturation stage.The TFe_2O_3 content of glauconite varies from 12.56% to 18.90%. The PASS-normalized-REE patterns of glauconite exhibit a "hat-shape" confirming the authigenic origin of glauconites. The slightly-negative to slightly-positive Ce anomaly value and the moderate TFe_2O_3 content of glauconite agree well with a suboxic,estuarine condition. The replacement of K-feldspar by the glauconite contributes towards the high K_2O content. Compositional evolution of glauconites in the Lameta Formation is similar to those observed in many Precambrian sedimentary sequences.     

济阳坳陷新生界海绿石的存在及其地质意义

大量的薄片鉴定资料表明,济阳坳陷沾车凹陷新生界地层中含有海绿石矿物,并且均为原生海绿石。尽管其沉积时代不同,但分布范围相对集中,即主要分布于古近系沙四段上部沙一段的底部。结合古生物化石的分析,认为济阳坳陷沾车凹陷在陆相湖盆沉积的大背景下,曾遭受过小规模的海侵,对当时的沉积环境产生了一定影响,由此在一定程度上产生了生物的变异和海绿石的沉积。对海绿石的成因分析将有助于确定当时的沉积环境。     

An explanation for low radiometric ages from glauconite

K-Ar and Rb-Sr ages from glauconites are about ten to twenty per cent lower than the age of sedimentation. Previous studies have indicated that these low ages are not attributable to normal diffusion loss of Ar from glauconite crystallites.The possibility of argon loss from ‘open’ potassium sites, such as on crystal surfaces and from expanded layers, was investigated by acid dissolution techniques. These studies show that potassium is removed from glauconites with low expandabilities at three different rates. The highest dissolution rate corresponds to cation exchange and comprises five to ten per cent of the total potassium. About five per cent of the total potassium is removed at a much slower rate than that of cation exchange, but at an order of magnitude faster than the bulk of the potassium.Activation energies calculated from rate constants determined at 50° and 80°C, for one sample gave values of 19 kcal/mole for the lowest dissolution rate and 14 kcal/mole for the intermediate rate.It appears that low radiometric ages from glauconites can be largely explained by the presence of potassium in sites where argon is readily lost, although such factors as late epigenetic gain of potassium by glauconite may also contribute to their low radiometric ages. A method is described for making quantitative corrections for such daughter product loss in radiometric age determinations.     

The origin and maturation of lagoonal glauconites: a case study from the Oligocene Maniyara Fort Formation,western Kutch,India

An integrated study of the sedimentology, micropalaeontology, mineralogy and geochemistry of glauconites in the Oligocene Maniyara Fort Formation (western Kutch, India), has been undertaken. Authigenic glauconites, mostly of evolved type, formed within a back‐barrier lagoonal environment. Foraminifera help constrain the biostratigraphy and along with sedimentological evidence, provide information on the depositional conditions. Glauconite in the Maniyara Fort Formation occurs either as infillings within intra‐particle pores of larger foraminifers, or as an altered form of faecal pellets. X‐ray diffraction studies reveal the less mature nature of glauconite infillings compared to the glauconite pellets. Electron microprobe investigation confirms a relative enrichment of K₂O and total Fe₂O₃ in the latter. Both varieties of glauconite formed by initial authigenic precipitation of K‐poor glauconite and subsequently matured by addition of potassium in the interlayer sites and fixation of total iron in the octahedral sites; calcium, magnesium and aluminum were released from the glauconite structure concomitantly. Alkaline conditions during the entire process of glauconite formation did not allow dissolution of foraminiferal tests. Mineralogical and chemical characteristics of the Maniyara Fort Formation glauconites are more similar to deep marine glauconites than those reported from other shallow or marginal marine settings. A low negative cerium anomaly, as well as abundant pyrite, suggests formation of glauconite in sub‐oxic micro‐environments, created by decay of organic matter associated with foraminiferal chambers and faecal pellets. Sub‐oxic condition apparently prevailed relatively longer within the Maniyara Fort Formation lagoons. Copyright © 2011 John Wiley & Sons, Ltd.     

南海北部湾海绿石的研究

样品产于南海北部湾滨岸浅滩胶结较松散的长石石英砂岩中,属于上第三系下洋组。本文从海绿石形态、矿物的物理化学测试结果以及形成介质条件等方面,探讨该区海绿石特征和形成环境。