西秦岭中志留统含铀岩系中硅质岩的岩石学、地球化学及其成因

根据产状和成因,本文将西秦岭中志留统含铀岩系中的硅质岩分为,(1) 成岩过程中硅化交代碳酸盐岩所形成的硅质岩,产于碳酸盐岩和泥质岩类所组成的透镜层的顶部或碳酸盐岩层内;(2) 早期成岩阶段形成的纹层状硅质岩,主要产于碳酸盐岩和泥质岩透镜层底部或呈薄层夹于粉砂岩、泥质岩层中。根据沉积环境分析及硅质岩的化学成分、REE分布模式和氧同位素组成,提出本区硅质岩形成于受大气淡水影响的局限浅海环境。     

The origin and diagenesis of cherts from Cyprus

The Troodos Massif of Cyprus is overlain by a variety of cherts in pelagic chalks, volcanogenic sediments, radiolarites and radiolarian mudstones, all of Campanian to Upper Eocene age. There are two chert types, granular chert and vitreous chert. X-ray diffraction (XRD) reveals the silica polymorphs, disordered cristobalite and quartz. Silicification of the chalks varies from incipient, to bedded, granular cherts, all with disordered cristobalite as the main silica phase. Quartzitic cherts are restricted to the base of Upper Palaeocene and Lower Eocene calciturbidite beds. Disordered cristobalite predominates in the radiolarian mudstones at the foot of the sequence. The form of disordered cristobalite in cavities ranges from microspherules of radiating bladed crystals, the ‘lepispheres’ of the Deep Sea Drilling Project (DSDP) to bladed overgrowths, and fibrous silica. In contrast, within the fine grained matrix, the disordered cristobalite takes the form of partly coalescent crude microgranules and microspherules. Most of the chalcedonic quartz in Cyprus is derived by recrystallization of previously inorganically precipitated disordered cristobalite rather than by direct precipitation. According to the concept of impurity-controlled maturation the composition of host sediment controls the incorporation of exchangeable cations and other impurities into inorganically precipitated disordered cristobalite. With time (up to 100 million years) internal solid state reorganization of the disordered cristobalite is accompanied by gradual expulsion of impurities, until the cristobalite dissolves followed by quartz precipitation. Complete conversion to quartz takes place first in porous calcareous sediments free of impurities, as in the Cyprus calciturbidites; in fine grained clay-rich sediments, like Cyprus radiolarian mudstones, disordered cristobalite persists much longer. Impurity-controlled maturation also helps explain the diagenesis of Cyprus chert nodules.     

Sedimentology of cherts in the Early Proterozoic Wishart Formation, Quebec-Newfoundland, Canada

The siliciclastic Wishart Formation of the Early Proterozoic Labrador trough is a high-energy shelf deposit. Wishart sandstones contain both interstitial chert with textures of void-filling cement and thin chert intercalations contaminated with siliciclastic mud. Although volumetrically minor, these cherts occur in several thin, areally extensive stratigraphic intervals. The Wishart contains intraclasts of both the chert-cemented sandstone and the impure chert layers (as well as several other types of chert sand and gravel). This suggests the cherts formed penecontemporaneously, which is consistent with the absence of any signs of replacement in all but one of the chert types and the clear-cut distinctions between chert types, even where they are side by side in a single thin section. The origin which appears to be most compatible with available evidence is that the cherts represent silica precipitated from thermal waters that rose through the sediments of the Wishart shelf and discharged into suprajacent seawater. A biogenic origin is unlikely in view of the lack of appropriate organisms during the Early Proterozoic and the rapidity with which the cements formed. A volcanogenic origin is unlikely because volcaniclastic textures are plentiful in associated formations but absent from the Wishart. Precipitation induced by evaporative concentration is unlikely in view of the widespread evidence of tidal currents and the lack of evidence of desiccation in the Wishart. Finally, the cherts are not restricted to the lowest-energy facies, and therefore they presumably did not accumulate as a background sediment. Deposition of silica above the sediment/water interface was probably made possible by ambient concentrations of silica that were significantly higher than those of Phanerozoic seawater. Cherts with similar textures occur in other Early Proterozoic sediments, most notably arenitic or granular iron-formations.     

生物化石钙质结构的分类与演化(连载)

绝大部份无脊椎动物和藻类化石由碳酸钙组成。研究生物化石钙质结构是为了对化石进行正确鉴定和合理分类,探索古生物间相互的演化关系。沉积岩,特别是碳酸岩中含有大量的生物组份,研究其结构是确定岩石薄片内生物化石及其碎片类型的必要手段,在研究岩石、岩相和古生态时必需广泛采用。     

Abiogenic silica layers within a fluvio-lacustrine succession,Bolzano Volcanic Complex,northern Italy: a Permian analogue for Magadi-type cherts?

Thin layers of chert occur within a fluvio-lacustrine succession sandwiched between acidic volcanic rocks of the Lower Permian Bolzano Volcanic Complex in northern Italy. These cherts are interpreted as analogues to Magadi-type abiogenic chert known from modern volcanic lake settings. Soft-sediment deformation features, shrinkage cracks, and well-preserved palynomorphs document very early diagenetic silica precipitation, volume loss by dehydration, and consolidation prior to mechanical compaction of the sediment. Transformation to quartz and concomitant (re)crystallization resulted in a variety of silica textures, dominated by microflamboyant quartz. The oxygen isotopic composition of chert samples ranges from +25·4 to +29·0‰ and is negatively correlated with crystal size and ordering.     

论东天山土屋-延东(斑岩)铜矿的容矿岩

通过对土屋-延东（斑岩）铜矿容矿岩的时空分布、物质组分及结构构造研究，认为该矿床的容矿岩是一套由中酸性-基性火山岩（包括沉凝灰岩）经富Na、Si热流体交代而成的钠质酸性-基性火山岩，按岩性及结构分以下4类：（1）具斑状或似斑状结构的钠质酸性-中酸性火山岩或称之为钠长石英斑岩与石英斑岩，它常位于矿体的上部，约占容矿岩的20%；（2）以自形、半自形板状、粒状交织结构为主的（含少量斑晶）钠质中酸性-中基性火山岩，或称之为安山玢岩，它常位于矿体的中部及中下部，约占容矿岩的50%；（3）以自形、半自形板状、粒状交织结构为主的富铝基性火山岩，或称之为高铝玄武岩，它常位于矿体的下部及底板围岩，约占容矿岩的20%；（4）以凝灰结构、碎屑结构为主的钠质中酸性-中基性火山碎屑岩，它常位于矿体的下部及底板围岩，约占容矿岩的10%。     

陆相硅质岩成因类型及构造—气候指示意义

硅质岩形成于特定的地球化学条件,具有重要的构造—气候—成岩指示意义.我国硅质岩的研究主要集中于海相地层,陆相硅质岩虽分布广泛但研究却很薄弱,成因解释由于借鉴海相燧石经验,以地球化学分析为首要手段,结论存在片面性和单一性,可能会引起古老大陆重要气候—构造—环境信息的遗漏缺失.国内外陆相硅质岩全面调研表明,陆相燧石主要存在...     

An attempt to quantitatively reconstruct the paleo-primary productivity by counting the radiolarian fossils in cherts from the latest Permian Dalong Formation in southwestern China

A quantitative reconstruction method of paleo-primary productivity is present on the basis of the investigation on the faunal record in cherts. By obtaining the accumulation rate of radiolarian shells in the cherts and considering all aspects of silica loss before burial, such as dissolution in the eutrophic zone or within the sediments, it is possible to reconstruct the production rate of radiolarian in the surface water, which would be used to estimate the paleo-primary productivity. This method is applied to the end Permian chert sequence from Dalong Formation in Dongpan area, southwestern China. The resulting paleo-primary productivity of the ocean in end Permian south China is about 477.9 g C/m²/a during the sedimentation of cherts, which, to some extent, agrees with the primary productivity reported for some modern oceans.     

Atypical textures in quartz veins from the Simplon Fault Zone

Samples of monomineralic quartz veins from the Simplon Fault Zone in southwest Switzerland and north Italy generally have asymmetric, single girdle c-axis patterns similar to textures measured from many other regions. Several samples have characteristically different textures, however, with a strong single c-axis maximum near the intermediate specimen axis Y (the direction within the foliation perpendicular to the lineation X) and a tendency for the other crystal directions to be weakly constrained in their orientation about this dominant c-axis maximum. This results in ‘streaked’ pole figure patterns, with an axis of rotation parallel to the c-axis maximum. These atypical samples also have a distinctive optical microstructure, with advanced recrystallization and grain growth resulting in a strong shape fabric (S_B) oblique to the dominant regional foliation (S_A), whereas typical samples have a strong S_A fabric outlined by very elongate, only partially recrystallized, ribbon grains. The recrystallized grains of the atypical samples are themselves deformed and show strong undulose extinction and a core-mantle recrystallization structure. The streaked texture is likely to be a direct consequence of lattice bending and kinking during heterogeneous slip on the favoured first-order prism (10 0) (a) system, the heterogeneity itself being due to problems in maintaining coherence across grain boundaries when insufficient independent easy-slip systems are available for homogeneous strain by dislocation glide. Such bending would be particularly prevalent in very elongate, thin ribbon grains, resulting in high internal strain energy and promoting recrystallization. Thus both the texture and the microstructure could be significantly modified by later strain increments affecting quartz grains with an already developed, nearly single-crystal texture.     

Origin of nodular chert in the carbonate rocks of the Kaladgi group (younger precambrian), Karnataka,India

Lensoid, irregular nodules and laminations of chert appear along the bedding planes and laminations of the Vidyanagar Dolomite Member of the Kaladgi Group, Karnataka, India. The field relations of the chert nodules with the host dolomite and evidence on polished hand-specimens clearly demonstrate that the cherts are secondary after dolomite. This contention finds extensive support from thin-sections which reveal floating relict dolomite fragments on a microcrystalline quartz mat, dolomite rhombs in various stages of progressive replacement, polycrystalline quartz rhombs apparently after dolomite. Chertification has also resulted in an aggrading neomorphic recrystallization of dolomicrite into dolomicrospar which rims the dislodged floating fragments of dolomite.Scanning electron microscopic studies revealed three distinct types of surfaces of chert; the polyhedral surface characteristic of microcrystalline quartz, the spongy surface characteristic of chalcedonic quartz (documented in thin-sections as semi-radiating to radiating aggregates of elongated crystals growing on the rhombic surfaces of dolomite, and the intermediate type showing both. The microcrystalline quartz seems to have originated through multinucleation at equally spaced centres on the dolomite mat, whereas the chalcedonic quartz originated by slow but direct and unhindered precipitation into cavities and residual pore-spaces, following dolomitisation. The silica invasion seems to have been accomplished by alkaline interstitial waters charged with silica.The occurrence and relation of chert-nodules with the host dolomite not only helps in unravelling their own origin, but also aids in drawing the dolomitisation curtain and in turn the post-dolomitisation diagenetic modifications of the host carbonates. It may be added, however, that the Kaladgi Carbonates have undergone mineralogical/textural modifications during diagenesis with superimposition of changes affected later, i.e. during tectonism which folded and cross-folded these sediments.     

Garnet coronas in scapolite-wollastonite calc-silicates from East Antarctica: the application and limitations of activity-corrected grids

Calc-silicate boudins within Proterozoic granulite facies gneisses of the northern Prince Charles Mountains, East Antarctica, preserve a number of reaction textures including garnet coronas between calcite and scapolite; garnet-quartz coronas between scapolite and wollastonite and between plagioclase and wollastonite; calcite-quartz intergrowths in wollastonite; and calcite-plagioclase symplectites in scapolite. These textures have been modelled using petrogenetic grids for reactions in the CaO-Al₂,O₃-SiO₂-CO₂ system, but with reduced mineral activities to account for additional components in real mineral compositions. Such fixed-composition reduced-activity grids are strictly valid only at the point in P-T-a_CO2 space where an assemblage last equilibrated, and do not show the true positions of reactions away from this point because mineral compositions change with reaction progress. In this case, however, mineral compositions close to end-member values and low extents of reaction progress mean that compositional change was limited and the grids are good approximations to true pseudosections over the entire P-T-a_co2 range of interest. The grids show that the textures are consistent with near-isobaric cooling from about 850 to 700d? C at 7 kbar, a P-T path compatible with thermobarometric studies of other lithologies from the area. Phase relationships indicate that CO₂ activities were buffered by the local mineral assemblage during peak and retrograde metamorphism, either under fluid-absent conditions or within a non-pervasive fluid phase. Previous studies of garnet coronas in scapolite-wollastonite calc-silicates have used qualitative grids based on limited experimental data to invoke garnet growth during water infiltration at high temperature, but the grids used here show that garnet coronas can form on cooling, without any need for water influx.     

A new sulphur isotopic study of some Iberian Pyrite Belt deposits: evidence of a textural control on sulphur isotope composition

The sulphide deposits of the Iberian Pyrite Belt (IPB) represent an ore province of global importance. Our study presents 113 new sulphur isotope analyses from deposits selected to represent the textural spectrum of ores. Measured ³⁴S values range from −26 to +10‰ mostly for massive and stockwork ores, in agreement with data previously published. In situ laser ³⁴S analyses reveals a close correlation of ³⁴S with texture. Primary diagenetic textures are dominated by relatively low ³⁴S (−8‰ to −2‰), whereas stockwork feeder textures are dominated by higher ³⁴S (∼+3‰ to +5‰). Intermediate textures (mainly coarse textures in stratiform zones) have intermediate ³⁴S, although they are mostly dominated by the high ³⁴S component. Rare barite has a homogeneous ³⁴S around +18‰, which is consistent with direct derivation from Lower Carboniferous seawater sulphate. A dual source of sulphide sulphur in the IPB deposits has been considered. A hydrothermal source, derived from reduction of coeval seawater sulphate in the convective systems, is represented by sulphide in the feeder zones. Here variations in ³⁴S are caused by variations in the extent of the sulphate reduction, which governs the SO₄:H₂S ratio. The second end-member was derived from the bacterial reduction of coeval seawater sulphate at or near the surface, as reflected in the primary textures. A distinct geographical variation in ³⁴S and texture from SW (more bacteriogenic and primary textures) to NE (more hydrothermal textures and ³⁴S) which reflects a variation in the relative input of each source was likely controlled by local geological environments. Given that the sulphur isotope characteristics of the IPB deposits are unlike most VMS and Kuroko deposits, and noting the dominance of a mixed reduced sedimentary and volcanic environment, we suggest that the IPB could represent an ore style which is intermediate between volcanic and sedimentary hosted massive sulphide types. Received: 8 October 1997 / Accepted: 14 May 1998     

Microfabrics of calcite ultramylonites as records of coaxial and non-coaxial deformation kinematics: Examples from the Rocher de l'Yret shear zone (Western Alps)

Microfabrics were analysed in calcite mylonites from the rim of the Pelvoux massif (Western Alps, France). WNW-directed emplacement of the internal Penninic units onto the Dauphinois domain produced intense deformation of an Eocene-age nummulitic limestone under lower anchizone metamorphic conditions (slightly below 300 °C). Two types of microfabrics developed primarily by dislocation creep accompanied by dynamic recrystallisation in the absence of twinning. Coaxial kinematics are inferred for samples exhibiting grain shape fabrics and textures with orthorhombic symmetry. Their texture (crystallographic preferred orientation, CPO) is characterised by two c-axis maxima, symmetrically oriented at 15° from the normal to the macroscopic foliation. Non-coaxial deformation is evident in samples with monoclinic shape fabrics and textures characterised by a single oblique c-axis maximum tilted with the sense of shear by about 15°. From the analysis of suitably oriented slip systems for the main texture components under given kinematics it is inferred that the orthorhombic textures, which developed in coaxial kinematics, favour activity of <10–11> and <02–21> slip along the f and r planes, respectively, with minor contributions of basal-<a> slip. In contrast, the monoclinic textures, which developed during simple shear, are most suited for duplex <a> slip along the basal plane. The transition between the dominating slip systems for the orthorhombic and monoclinic microfabrics is interpreted to be due to the effects of dynamic recrystallisation upon texture development. Since oblique c-axis maxima documented in the literature are most often rotated not with but against the shear sense, calcite textures alone should not be used as unequivocal shear sense indicators, but need to be complemented by microstructural criteria such as shape preferred orientations, grain size estimates and amount of twinning.     

The blueschists along the Indus Suture Zone in Ladakh, NW Himalaya

ABSTRACT Blueschists occur along the Indus Suture Zone in Ladakh as tectonic thrust slices, as isolated blocks within mélange units and as pebbles within continental detrital series. In the Shergol-Baltikar section high-pressure rocks within the Mélange unit lie between the Dras-Naktul-Nindam nappes in the north and the Lamayuru units in the south. The blueschists are imbricated with mélange formation of probably upper Cretaceous age. They are overlain discordantly by the Shergol conglomerate of post Eocene (Oligo-Miocene ?) age. Blueschist lithologies are dominated by volcanoclastic rock sequences of basic material with subordinate interbedding of cherts and minor carbonates. Mineral assemblages in metabasic rocks are characterized by lawsonite-glaucophane/crossite-Na-pyroxene-chlorite-phengite-titanite ± albite ± stilpnomelane. In the quartz bearing assemblages garnet is present but omphacite absent. P-T estimates indicate temperatures of 350 to 420°c and pressures around 9–11 kbar. Geochemical investigations show the primary alkaline character of the blueschist, which suggests an oceanic island or a transitional MORB type primary geotectonic setting. K/Ar isotopic investigations yield middle Cretaceous ages for both whole rocks and minerals. Subduction related HP-metamorphism affecting the Mesozoic Tethyan oceanic crust developed contemporaneously with magmatism in the Dras volcanic are and the Ladakh batholith. Subsequent collision of India with Asia obducted relics of subduction zone material which later became involved in nappe emplacement during the Himalayan mountain building.     

Stepwise fluorination—a useful approach for the isotopic analysis of hydrous minerals

Analytical uncertainties in oxygen isotopic studies of hydrous silica have been investigated using a partial fluorination procedure in which fractional oxygen yields are achieved by reducing the amount of fluorine. Stepwise reaction of opaline silica results in a set of sequential oxygen fractions which show a wide range of δ¹⁸O values due to variable amounts of water, organic matter, and other impurities, δ-values for successive fractions in non-biogenic opal systematically increase as water is reacted away and then remain constant to within ±0.2%. as the remaining silica reacts, δ-values in biogenic silica increase similarly but then decrease when low ¹⁸O oxide(?) impurities begin to react.The troublesome water component in opal is readily removed by Stepwise fluorination. This technique allows more precise oxygen isotope analysis of non-biogenic opal-A, and may improve the analytical precision for biogenic silica and any silicate mineral containing a significant water component.     

Very high 18o enrichment in Archean cherts from South India: implications for Archean ocean temperature

Stable carbon and oxygen isotopic compositions of essentially unmetamorphosed Archean (> 2.6 Gyr old) cherts and carbonates of the Dharwar Sequence of southern India, from the northernmost part of the Dharwar-Shimoga supracrustal belt (Kalche and Nagargali), have been determined. The cherts from the Nagargali area, which preserve oolitic texture and cryptocrystalline silica, show highly enriched δ¹⁸O values ranging from 28 to 31.4%o relative to SMOW. Such values are the highest yet reported from Archean nondetrital sediments, but are similar to those of modern marine cherts. On the assumption of a seawater δ¹⁸O of 0%₀, calculation of temperature based on the maximum δ¹⁸O value of 31.4%₀ yields a value of 40°C. This is significantly less than 70–80°C reported for the Archean oceans based on cherts and chert-phosphate pairs. Diagenetically recrystallized microcrystalline chert-dolomite pairs of Kalche area exhibit a range of oxygen isotopic ratios similar to those reported for Archean cherts and carbonates from other parts of the world. The temperature of diagenesis is estimated to be about 68°C.     

Genesis of the channel iron deposits (CID) of the Pilbara region,Western Australia

Miocene fluvial goethite/hematite channel iron deposits (CID) are part of the Cenozoic Detritals 2 (CzD2), of the Western Australian Pilbara region. They range from gravelly mudstones through granular rocks to intraformational pebble, cobble and rare boulder conglomerates, as infill in numerous meandering palaeochannels in a mature surface that includes Precambrian granitoids, volcanics, metasediments, BIF and ferruginous Palaeogene valley fill. In the Hamersley Province of the Pilbara, the consolidated fine gravels and subordinate interbedded conglomerates, with their leached equivalents, are a major source of export iron ore. This granular ore typically comprises pedogenically derived pelletoids comprising hematite nuclei and goethite cortices (ooids and lesser pisoids), with abundant coarser goethitised wood/charcoal fragments and goethitic peloids, minor clay, and generally minimal porous goethitic matrix, with late-stage episodic solution and partial infill by secondary goethite, silica and siderite (now oxidised) in places. Clay horizons and non-ore polymictic basal and marginal conglomerates are also present. The accretionary pedogenic pelletoids were mostly derived from stripping of a mature ferruginous but apparently well-vegetated surface, developed in the Early to Middle Miocene on a wide variety of susceptible rock types including BIF, basic intrusives and sediments. This deep ferruginisation effectively destroyed most remnants of the original rock textures producing a unique surface, very different to those that produced the underlying CzD1 (Palaeogene) and the overlying CzD3 (Pliocene – Quaternary). The peloids were derived both intraformationally from fragmentation and reworking of desiccated goethite-rich muds, and from the regolith. Tiny wood/charcoal fragments replaced in soil by goethite, and dehydrated to hematite, formed nuclei for many pelletoids. Additionally, abundant small (≤10 mm) fragments of wood/charcoal, now goethite, were probably replaced in situ within the consolidating CID. This profusion of fossil wood, both as pelletoid nuclei and as discrete fragments, suggests major episodic wild fires in heavily vegetated catchments, a point supported by the abundance of kenomagnetite – maghemite developed from goethite in the pelletoids, but less commonly in the peloids. The matrix to the heterogeneous colluvial and intraformational components is essentially goethite, primarily derived from modified chemically precipitated iron hydroxyoxides, resulting from leaching of iron-rich soils in an organic environment, together with goethitic soil-derived alluvial material. Major variations in the granular ore CID after deposition have resulted from intermittent groundwater flow in the channels causing dissolution and reprecipitation of goethite and silica, particularly in the basal CID zones, with surface weathering of eroded exposures playing a role in masking some of these effects. However, significant variations in rock types in both the general CID and the granular ore CID have also resulted from the effects of varied provenance.     

Quaternary palaeoceanographic changes in the northern South China Sea (ODP Site 1146): radiolarian evidence

The abundance and accumulation rates of siliceous microfossils in the northern South China Sea, including radiolarians, diatoms and sponge spicules, increased during most glacial intervals within the past 1100 kyr. Similar trends are observed in the index of thermocline surface radiolarians (TSR), diatom accumulation rates (DAR), charcoal accumulation rates (CAR) and the abundance of radiolarian species Cycladophora davisiana davisiana. Decreasing sea‐surface temperature accompanied by increased seasonality since 900 ka is indicated by a decline in the tropical radiolarian assemblage, including Tetrapyle octacantha and Octopyle stenozona, and by an increase in the subtropical assemblage, including Pterocorys zancleus, Peromelissa phalacra and Ommatartuts tetrathalamus tetrathalamus. Rapid increases at about 800 to 700 ka of siliceous microfossils, charcoal, subsurface and intermediate radiolarians, as well as the TSR index and the DAR, imply a fundamental shift in climate and a shoaling thermocline. Although these fundamental changes in the silicious fauna and flora of the South China Sea take place within the context of a developing 100‐kyr cycle, they do not change in step with changing sea‐level as indicated by marine δ¹⁸O. This is most clearly illustrated by the step‐like increase in silica accumulation (radiolaria, diatoms and sponge spicules) at 680 ka. Rather, these fundamental changes probably reflect intensified surface productivity associated with enhanced East Asian winter‐monsoon circulation. Copyright © 2003 John Wiley & Sons, Ltd.