Showing papers on "Petrography published in 2004"

Organic Facies in Devonian and Mississippian Strata of Western Canada Sedimentary Basin: Relation to Kerogen Type, Paleoenvironment, and Paleogeography

Abstract: Petrographic analyses of dispersed organic matter (including macerals and palynomorphs), siliceous and calcareous microfossil assemblages and microtextures (e.g. stromatolitic) have been used to define and interpret five organic facies and regionally map their distribution for the following informal groupings of potential hydrocarbon source rocks in the Western Canada Sedimentary Basin: Upper Devonian Woodbend group, Upper Devonian Winterburn group and Upper Devonian to Lower Mississippian black shales of the Exshaw and Bakken formations. Five petrographic organic facies (A-E) are defined for the potential source rocks based on assemblages of alginites, acritarchs, sporinites, siliceous microfossils and algal mat microtextures. Organic facies A, B (prasinophyte alginites and acritarchs) and C (coccoidal alginite), represent accumulation in relatively deep (basin), intermediate (shelf-platform), and shallow water depths (bank-reef margin to lagoonal). Organic facies D is defined by siliceous microfossils (e.g. Radiolaria) and accumulated in deep basinal to outer shelf settings immediately east of an ancient Pacific Ocean, or south of an ancient Arctic Ocean. This facies may reflect regions of upwelling which extended into intracratonic and epicontinental settings. Organic facies E, characterized by stromatolitic microtextures with or without coccoidal alginite, only occur within Upper Devonian Winterburn Group shallow water, restricted shelf to lagoonal dolostones associated with evaporites. As a whole, the regional distribution of organic facies is related to paleogeography, paleobathymetry or paleostructure in the source rocks. Surprisingly, petrographic organic facies do not show strong positive correlation with kerogen type as defined by Hydrogen-Oxygen indices or TOC-S2 plots.

Intergranular diamonds derived from partial melting of crustal rocks at ultrahigh-pressure metamorphic conditions

Abstract: IntroductionMantle-derivedrocksandtheirassoci-atedplacerdepositsprovidethetradi-tional sources of economic diamondThe occurrence of metamorphic dia-mondasinclusionsinrefractorymin-erals considerably has enlarged thepossiblerock composition forits for-mation(frommetapelitestometacar-bonates) Microdiamonds, occurringasinclusionsinlow-pressuremineralsfrom crustal rocks, were first inter-preted to have a metastable origin(Letnikov, 1983; Nadejdina and Pos-ukhova,1990;Lavrova,1991;Dobrzh-inetskayaet al,1994)Theabundanceofultrahigh-pressure(UHP)metamor-phic relicts in diamond-bearing rocksoftheKokchetavmassifinKazakhstan(Sobolev and Shatsky, 1990; Shatskyet al, 1995) and of the SaxonianErzgebirge in Germany (Massonne,1999; Nasdala and Massonne, 2000)supportsametamorphicoriginofthediamondwithinitsUHPstabilityfieldRecently, the coexistence of diamondand low-pressure minerals as inclu-sionsingarnethasbeeninterpretedtoreflecttheircrystallizationeitherfromsupercritical fluid (Sto¨ckhert et al,2001) or from melt (Hwang et al,2001) The presence of melt at UHPconditions is an important factor be-causeitmayinfluencethescaleorrateof element redistribution and thedynamicsofuplift(BrownandSolar,1998)Partial melting of the Kokchetavdiamond-bearingrocks,firstproposedbygeochemicalstudies(Shatskyet al,1995, 1999), was suggested again byrecent zircon growth analysis (Her-mann et al, 2001) However, clearevidenceformeltingintheKokchetavmassif is still a matter of debate andrecent work (Parkinson et al, 2002)hasposedthequestion:Whydidtheserocksnotundergomelting?Representative sampling of thediamond-bearing UHP sequences ofthe Kokchetav massif has providednew information One of these sam-ples is a layered garnet–clinopyrox-ene–quartz rock, including thinlayers and lenses, composed of therock-forming disequilibrium assem-blage: diamond⁄graphite, quartz andK-feldsparThisprovidesanunparal-leled opportunity to describe theoccurrence of intergranular diamondand to identify the petrographic evi-dence of silicate melt existence inUHProcksLocation of the investigated rocksin the Kumdy-Kol UHP unitThe Kokchetav massif is a tectonicmegamelange(Dobretsovet al,1995b,1998)inwhichdiamond-bearingrocksexclusively occur within the Kumdy-Kol UHP Unit 1, investigated at theBarchi-Kol (Korsakov, 2000) andKumdy-Kol mines This unit mainlyconsists of (1) garnet–biotite gneissesand interlayered carbonates as themain diamond-bearing rocks and of(2)migmatitesandeclogitebodiesthatare diamond-free (Dobretsov et al,1995b; Shatsky et al, 1995; Theunis-senetal,2000)Unit1wasrepresen-tatively sampled along a continuousandwell-exposedsectionintheformerKumdy-Kol mine gallery, where itappears with a constant and steeplytoSE-dippingcompositionallayeringInterlayered tourmaline-rich metaso-matic rocks and well-layered garnet–clinopyroxene–biotite–quartz rocksoccur side by side Diamond in themetasomatic rocks is only preservedas inclusion in zircon, whereas intra-granular diamond is widespread inother rocks However, in addition tointragranular diamond, intergranulardiamond is here shown to occur inquartz–feldspathicaggregatelayersorlensesinthelatterrocksPetrographyEstimated peak metamorphic condi-tions for the UHP Unit 1are in therange T ¼ 950–1000 C and P >40kbar (Sobolev and Shatsky, 1990;Shatsky et al, 1995; De Corte et al,2000; Hermann and Green, 2001a,b)Indolomitemarblesandzoisitegneis-ses (grouped together here under theABSTRACT

An example of alternative correlation techniques in a low-accommodation setting, nonmarine hydrocarbon system: The (Lower Cretaceous) Mannville Basal Quartz succession of southern Alberta

Abstract: Chemostratigraphy and heavy-mineral techniques have been applied to the Lower Cretaceous Basal Quartz in the Western Canada sedimentary basin. The aim of the study is to demonstrate that these two techniques can be used to help understand the complex stratigraphy of reservoirs deposited in low-accommodation fluvial settings. The Basal Quartz is an ideal unit to demonstrate their applicability in stratigraphic studies of hydrocarbon reservoirs because extensive mapping and petrographic studies have enabled the establishment of a rigorous stratigraphic framework despite its complexity resulting from deposition in a low-accommodation fluvial setting. The three component units analyzed in the Basal Quartz (Horsefly unit, Bantry–Alderson–Taber [BAT] unit, and Ellerslie unit) each have unique geochemical and heavy-mineral characteristics. Chemostratigraphic analysis shows that silty claystones from the Horsefly, BAT, and Ellerslie units have distinctly different geochemistry from one another, with the variations being caused by changes in clay mineralogy and other components, such as feldspar, apatite, and zircon. The geochemistry also suggests periodic volcanogenic input influenced the silty claystones of the Basal Quartz. Heavy-mineral analysis shows that sandstones from the three units can be distinguished on the basis of ratio parameters, such as apatite/tourmaline, rutile/zircon, and zircon/tourmaline, which are controlled by differences in provenance and intensity of weathering during transport.

Accessory minerals as tracers in the provenancing of archaeological marbles, used in combination with isotopic and petrographic data*

Abstract: Thirty-eight archaeological marbles (Roman and medieval) from Modena and Reggio Emilia (northern Italy) were provenanced using the accessory minerals as tracers and the results were compared with those inferred for the same artefacts using isotope composition and MGS (maximum grain size of calcite grains). The number of inferred possible sources is generally lower when using the mineralogical method, which therefore seems to be suited to marble provenancing.

U-Pb dating of zircons from quartz diorite and its enclaves at Tongguanshan in Anhui and its petrogenetic implication

Abstract: The genesis of intermediate-silicic intrusive rocks and their enclaves in Tongling are closely related to Fe, Cu mineralization, which may provide petrologic information on lithospheric dynamics in this region. Tongguanshan quartz diorite and its enclaves, representatives of the Mesozoic intermediate-silicic intrusive rocks in the Tongling area, were studied using approaches of petrography, electron microprobe mineral chemistry and the LA-ICPMS zircon U-Pb dating. The chemical compositions of pyroxene and amphibole in pyroxene-amphibole cumulate, quartz-diorite host rock from Tongguanshan show correlative variations. The Al contents of the minerals in pyroxene-amphibole cumulate are higher than those in the host rocks, indicating that the cumulate crystals were crystallized before the emplacement of host magma. The consistent mineral chemical compositions between the Tongguanshan quartz diorite and the microgranular diorite enclave indicate that the enclave and host rock were crystallized under similar conditions and therefore are closely genetically related. The LA-ICPMS zircon U-Pb dating suggests that the crystallization age of Tongguanshan quartz diorite is 137.5±1.1 Ma, consistent with the previous dates by different methods. However, the present U-Pb dating also revealed the presence of late-Archean relict cores, demonstrating that older lower crustal materials were involved in the generation of Mesozoic magmatic rocks in this area. Furthermore, the dating results show that the crystallization age of the microgranular diorite enclave is 137.5 ± 2.4 Ma, the same as the crystallization age of the host magma. By integrating the previous Sr-Nd-Pb isotope data, it is also discussed the petrogenesis of quartz diorite and its enclaves in Tongguanshan.

UHP-metamorphic rocks from Dora Maira/Western Alps and Kokchetav/Kazakhstan New insights using cathodoluminescence petrography

Abstract: Thin sections of ultrahigh pressure (UHP) metamorphic rocks from the Dora Maira Massif (Italy) and the Kokchetav Massif (Kazakhstan) were investigated using the hot cathode cathodoluminescence (CL) technique. Coloured images of important, but otherwise invisible growth features could be easily identified with this tool within seconds. These features are in excellent correlation with chemical variations of minerals revealed by electron microprobe (EMP). Generally, CL is induced by activator-elements (e.g. Mn and REE) and lattice defects whereas so-called quencher-elements like Fe may reduce or even extinct luminescence. Since X-ray-intensity mapping images (MAPS) of minerals can take up to 50 hours, the CL-method represents an ideal and rapid approach prior to chemical characterization. In addition to typical carbonates such as calcite, Mg-bearing calcite and dolomite, a number of rock forming and accessory minerals including Mg- and Mg-Ca-garnets, diopsidic and jadeitic pyroxenes, kyanite, K-feldspar, quartz, coesite, diamond, zircon, apatite, and bearthite were examined. Features observed in garnets include small-scale oscillatory zoning patterns, changes in morphology during growth as well as different crack generations which were partly annealed. SiO2 phases (coesite, quartz, chalcedony) as well as exsolution textures of dolomite and Mg-bearing calcite are easy to distinguish due to their different CL-colours. Pyroxene displays complex zonation patterns and -to some extent- exsolution-textures of K-feldspar. Kyanite reveals distinct growth zones; in combination with mineral inclusion studies it is possible to discriminate between different kyanite-forming reactions. The different crystallographical orientation of twinned kyanite crystals leads to various luminescence colours, thus, the suture of the twin plane is well defined. Prior to SHRIMP analyses, knowledge of the internal structures of zircon is indispensable. Even very tiny coesite crystals are easy to distinguish from quartz or chalcedony by their disparate luminescence colours. Accessory luminescent minerals like diamond, apatite, bearthite are easy to identify in thin section even if they occur in very small abundance within the matrix or as inclusions. The CL method presented here for UHP-metamorphic rocks is recommended as a pathfinder for the discovery of internal structures of minerals prior to their chemical characterization using EMP.

Provenance of Palaeozoic sandstones from the Carnic Alps (Austria): petrographic and geochemical indicators

Abstract: An integrated approach of petrographic analysis, whole rock geochemistry and microprobe analysis has been applied to obtain information on the geodynamic development and the provenance for Ordovician to Permian siliciclastic successions exposed within the Carnic Alps (Austria). Sandstone detrital mode and geochemical results refine previous geodynamic interpretations. Late Ordovician samples indicate a stable craton and recycled orogenic and, possible, extensional setting. The Early Carboniferous is interpreted to represent a compressional environment, followed by a Late Carboniferous molasse-type foreland basin, and a Permian extensional geodynamic setting. Contrasting geochemical patterns of post-Variscan and Permian sequences suggest a rift setting. Electron microprobe data of detrital white mica also indicate changes in the provenance. Compositional data reflect a shift from low- to medium-grade metamorphic (Ordovician) to high-grade metamorphic (Carboniferous) to low- to medium-grade metamorphic and plutonic source rocks (Permian). Additionally, our data show that various chemical discrimination diagrams do not include all possible ranges of sandstones, and that high contents of detrital mica and ultra-stable heavy minerals may cause misclassification. Consequently, we propose the use of multi-method approach for provenance studies, including the control of geochemical data by modal analysis and heavy mineral investigations.

Impact‐related dike breccia lithologies in the ICDP drill core Yaxcopoil‐1, Chicxulub impact structure, Mexico

Abstract: Petrographic descriptions of three dike breccia lithologies from drill core Yaxcopoil-1 (Yax-1) are presented. They occur within allochthonous units of displaced sedimentary megablocks of the Chicxulub impact structure. The suevitic dike breccias are the uppermost dike lithology. They contain melt rock particles and melt injections into the dike groundmass. Shock features occur ubiquitously and indicate a strong thermal annealing. Flow textures suggest a highly energetic emplacement process, possibly during the excavation stage as a ground-surge related deposit. The impact melt rock dikes are present in a strongly brecciated megablock interval as flow textured, anastomozing veinlets of impact melt rock that were altered to clay minerals. The melt impregnated a dolomitic host rock, indicating a low viscosity and, thus, high initial temperatures. Brecciation of the impact melt rock dikes occurred while they were still below the glass transition temperature, suggesting that dynamic conditions prevailed shortly after the emplacement process. Major element data indicates that the impact melt rock dikes differ in composition from the homogenized impact melt rock of Chicxulub. This could point to an emplacement during the late compression or early excavation stages of cratering. The clastic polymict dike breccias are coeval with pervasive brittle fracturing of the host rocks. They bear clasts including some crystalline basement and possible melt rock particles in a fine-grained dolomite matrix with turbulent flow textures. Fabric and texture indicate a granular flow at ambient pressures. Such conditions could be envisaged for the excavation phase while the transient cavity grew and fractures opened.

Influence of the mineralogical composition and textural properties on the quality of coarse aggregates

Abstract: To evaluate the influence of the petrographic variables on the quality of coarse aggregates consisting of granitoid (granite to tonalite) rocks, 17 samples selected from the Swedish part of the Baltic shield have been studied concerning their petrographic properties, for example, mineral composition, grain size, grain boundaries, and the frequency of micro-cracks. All of the samples selected also have been studied in mechanical tests used to evaluate the quality of aggregates in Sweden. The quality has been determined by means of flakiness, impact value, abrasion value I, and abrasion value II. An analysis of the influence of the mineral composition and textural properties on the aggregate quality has been performed using statistical correlation and linear models. The results indicate that an increasing content of feldspar negatively influences the strength against impact, while an increasing content of mica (tested to 35 vol.%) combined with a diminishing grain size and more irregular grain boundries has a positive influence on the resistance of granitoids to mechanical impact. Abrasion value II seems to be mainly influenced negatively by an increasing frequency of micro-cracks. The practical implementation of the results is suggested.

Petrographic and trace element analysis of uranium-rich tufa calcite, middle Miocene Barstow Formation, California, USA

Abstract: An integrated petrographic and spectroscopic (X-ray diffraction, phosphor imaging and synchrotron X-ray absorption spectroscopy) study of tufas from the Miocene Barstow Formation, California, relates sample morphology, mineralogy and geochemical composition. The tufas, composed mainly of calcite that formed at the interface between an ancient alkaline lake and ground or spring waters, have textures similar to those of microbially mediated terrestrial stromatolites and travertines. The tufas have elevated concentrations of a number of trace elements including Mn, Fe, Sr and U. Synchrotron X-ray fluorescence analyses show that U concentrations can exceed 500 p.p.m. X-ray absorption spectroscopy indicates that the U in these samples is incorporated as U(IV). It is suggested here that alkaline lake waters had a high U/Ca ratio and tufa calcite formed where groundwater or (possibly epithermal) springs brought in Ca and trace elements such as Sr. The rapid, and possibly microbially mediated, precipitation of calcite allowed for incorporation of high concentrations of trace metals as either structural substitutes or extremely fine-scale inclusions.

First petrographic results on impactites from the Yaxcopoil-1 borehole, Chicxulub structure, Mexico

Abstract: The ICDP Yaxcopoil-1 (Yax-1) borehole located 60 km south-southwest of the center of the Chicxulub impact structure intercepted an interval of allogenic impactites (depth of 795-895 m). Petrographic analysis of these impactites allows them to be differentiated into five units based on their textural and modal variations. Unit 1 (795-922 m) comprises an apparently reworked, poorly sorted and graded, fine-grained, clast-supported, melt fragment-bearing suevitic breccia. The interstitial material, similar to units 2 and 3, is permeated by numerous carbonate veinlets. Units 2 (823-846 m) and 3 (846-861 m) are groundmass-supported breccias that comprise green to variegated angular and fluidal melt particles. The groundmass of units 2 and 3 comprises predominantly fine-grained calcite, altered alkali element-, Ca-, and Si-rich cement, as well as occasional lithic fragments. Unit 4 (861-885 m) represents a massive, variably devitrified, and brecciated impact melt rock. The lowermost unit, unit 5 (885-895 m), comprises highly variable proportions of melt rock particles (MRP) and lithic fragments in a fine-grained, carbonate-dominated groundmass. This groundmass could represent either a secondary ydrothermal phase or a carbonate melt phase, or both. Units 1 and 5 contain well-preserved foraminifera fossils and a significantly higher proportion of carbonate clasts than the other units. All units show diagnostic shock deformation features in quartz and feldspar clasts. Our observations reveal that most felsic and all mafic MRP are altered. They register extensive K-metasomatism. In terms of emplacement, we suggest that units 1 to 3 represent fallout suevite from a collapsing impact plume, whereby unit 1 was subsequently reworked by resurging water. Unit 4 represents a coherent impact melt body, the formation of which involved a significant proportion of crystalline basement. Unit 5 is believed to represent an initial ejecta/ground-surge deposit.

Lunar highland meteorite Dhofar 026 and Apollo sample 15418: Two strongly shocked, partially melted, granulitic breccias

Abstract: Studies of lunar meteorite Dhofar 026, and comparison to Apollo sample 15418, indicate that Dhofar 026 is a strongly shocked granulitic breccia (or a fragmental breccia consisting almost entirely of granulitic breccia clasts) that experienced considerable post-shock heating, probably as a result of diffusion of heat into the rock from an external, hotter source. The shock converted plagioclase to maskelynite, indicating that the shock pressure was between 30 and 45 GPa. The post-shock heating raised the rock's temperature to about 1200 °C; as a result, the maskelynite devitrified, and extensive partial melting took place. The melting was concentrated in pyroxene-rich areas; all pyroxene melted. As the rock cooled, the partial melts crystallized with fine-grained, subophitic-poikilitic textures. Sample 15418 is a strongly shocked granulitic breccia that had a similar history, but evidence for this history is better preserved than in Dhofar 026. The fact that Dhofar 026 was previously interpreted as an impact melt breccia underscores the importance of detailed petrographic study in interpretation of lunar rocks that have complex textures. The name "impact melt" has, in past studies, been applied only to rocks in which the melt fraction formed by shock-induced total fusion. Recently, however, this name has also been applied to rocks containing melt formed by heating of the rocks by conductive heat transfer, assuming that impact is the ultimate source of the heat. We urge that the name "impact melt" be restricted to rocks in which the bulk of the melt formed by shock-induced fusion to avoid confusion engendered by applying the same name to rocks melted by different processes.

SHRIMP zircon U-Pb ages of garnet pyroxenite and Fushui gabbroic complex in Songshugou region and constraints on tectonic evolution of Qinling Orogenic Belt

Abstract: “Qinling Group (?)” in Songshugou region in the north of the Shangdan Fault consists predominantly of amphibolite with or without garnet and amphibole-plagioclase gneiss and intercalated with lensiod blocks of garnet pyroxenite, where the famous Songshugou dunite body occurs (Fig. 1). Petrographic study reveals that garnet pyroxenite is the representative rock of early

Fluid systems and mineralization in the north German and Polish basin

Abstract: The North European Basin hosts mineral deposits like the Kupferschiefer and the Mississippi Valley Type deposits in the Silesian sub-basin in Poland. The basement to this basin, exposed in the Harz Mts and in the Flechtingen and Calvorde Blocks, contains Mesozoic Pb–Zn vein mineralization and barite–fluorite deposits as well as massive hematite veins in the Rotliegend volcanics. A comparison of the mineralizing models of these deposits with results from a basin-wide petrographic, fluid inclusion and stable isotope study shows that the genesis of the mineral deposits can be explained by fluid systems that were active during different stages of basin evolution. These comprise syn- to post-magmatic fluids derived from or mobilized in the course of the Rotliegend magmatism, fluids convecting in the Rotliegend units during the extensional basin subsidence in the Permo-Triassic and originating from progressive devolatilization of the basin sequence and fluids derived from the overlying Zechstein evaporites. Deep-reaching fault systems developing during the Cretaceous tectonic reactivation enhanced fluid percolation from the surface to the deep sections of the basin sequence. Identification and correlation of these fluids across the basin and in the mineralizations provide the base for a basin-wide metallogenetic model.