Showing papers on "Petrography published in 1993"

Magnetic susceptibility of the Mont‐Louis andorra ilmenite‐type granite (Pyrenees): A new tool for the petrographic characterization and regional mapping of zoned granite plutons

Abstract: We have observed remarkably consistent patterns of concentric zonation in the values of low-field magnetic susceptibility measurements over the Variscan age Mont-Louis Andorra granite pluton of the eastern Pyrenees of Andorra, Spain and France. This zonation is a function of the rock petrology since a close correlation is shown between the petrographic nature (as defined by chemical analysis) and the magnetic susceptibility magnitude of the rocks. It reflects the dominantly paramagnetic nature of the granite, carried by Fe-bearing silicates, and this is demonstrated by the fact that low-field susceptibilities of representative specimens are almost equal to (1) their high-field susceptibilities and (2) their calculated susceptibilities using Fe contents, assuming a null ferromagnetic contribution. We conclude that this technique accurately reflects the modal abundances of ferromagnesian phases in rocks of the ilmenite series and that it represents a powerful and efficient tool for the reconnaissance surveying of petrological variations in granitoid plutons.

Thermobarometry in a subgreenschist to greenschist transition in metabasites of the Abitibi greenstone belt, Superior Province, Canada

Abstract: Mineral equilibria in the system CaO–MgO–Al2O3–SiO2–H2O provide a basis for mapping of four reaction isograds and one bathograd in the low-pressure transition from subgreenschist to greenschist facies. Most of the Matachewan area of the Abitibi greenstone belt is in the lower-pressure bathozone, as indicated by the widespread occurrences of the subassemblage Prh–Chl. The higher-pressure bathozone is indicated by two occurrences of Pmp–Act–Ep–Qtz, but in these samples the bathograd is displaced to anomalously low pressure by the high Fe content of the coexisting minerals. This illustrates the need to analyse coexisting minerals, calculate activities of end-member species, and compute P–T curves for individual samples before interpreting the isograd/bathograd pattern. Petrographic and microprobe analysis indicates that great care must be taken in the selection of ‘equilibrium’ assemblages. Pyroxene phenocrysts in one sample are replaced by the assemblage Pmp–Act–Ep–Chl–Qtz, whereas Prh–Act–Ep–Chl–Qtz occurs in the groundmass. Compositional variation may be more cryptic, as in a sample of metabasaltic hyaloclastite that contains two spatially distinct ‘univariant’ assemblages, Prh–Pmp–Ep–Chl–Qtz and Prh–Act–Ep–Chl–Qtz, within the devitrified matrix. Whereas chlorite compositions are similar in both assemblages, prehnite and epidote in the latter assemblage are significantly richer in Fe and poorer in Al. Accordingly, the rock is interpreted to contain two distinct ‘univariant’ assemblages, rather than one ‘invariant’ assemblage (Prh–Pmp–Act–Ep–Chl–Qtz). The displaced ‘univariant’ curves for this sample intersect at 2.2 kbar and 250°C. Taking account of all thermobarometric implications, the low-grade limit of the greenschist facies is at 250–270°C and 2–2.5 kbar, corresponding to depths of 7–8 km. Comparison of apparent P–T conditions on both sides of the Larder Lake – Cadillac break, a regional CO2-metasomatized fault zone that is spatially associated with many Archaean gold deposits, provides an upper limit of not more than c. 1 km for post-metamorphic south-side-up, dip-slip displacement.

Significance evaluation of factors controlling river water composition

Abstract: In the present paper the factors controlling river water chemistry are described and their significance is evaluated, including: climate, petrography, soils/sediments, geomorphology of the catchment area, hydrography, pollution, biological activity of rivers, and rainwater composition. The results of this study indicate that the dominant factors influencing riverwater chemistry are the climate and the petrography of the catchment area. In the case of heavily polluted rivers, pollution appears to be the dominant factor. The approach followed is based on the watershed information, e.g., climatic, geomorphological, petrographical (percentage distribution of geochemical important petrographic units), soil/sediment composition, rainwater composition, as well as the hydrographic and the hydrochemical data of the rivers. In order to achieve a better understanding of the nature of the factors influencing river water composition as well as to specify them quantitatively, multivariate statistical analyses (factor and cluster analyses) were performed on the rain-corrected hydrochemical and petrographic information of the watersheds.

Silicification of Evaporites in Permian (Guadalupian) Back-Reef Carbonates of the Delaware Basin, West Texas and New Mexico

Abstract: Outcrops of the Seven Rivers, Yates, and Tansill Formations contain widespread evaporites replaced by quartz and calcite. The original evaporites consisted of discrete horizons, scattered nodules, enterolithic layers, and individual crystals or crystal fragments of gypsum and/or anhydrite within a finely crystalline dolomite matrix. Based on field and petrographic observations, silicification of evaporites proceeded from the exterior to the interior of the nodules. The earliest replacement was by euhedral, black megaquartz that contains abundant fluid inclusions (water and hydrocarbons) and solid inclusions (mainly anhydrite and dolomite). The siliceous replacements were followed by precipitation of equant, blocky calcite spar, which filled pores left by latestage dissolution of evaporites. The fluid inclusions in the replacive megaquartz are primary, and many contain both hydrocarbons and water. Daughter minerals of halite, gypsum, or possibly antarcticite (CaCl2·6H2O) are also found within the aqueous inclusions. Homogenization-temperature data for hydrocarbon and aqueous fluid inclusions average 67.7°C and 67.1°C, respectively. Hydrocarbonbearing and aqueous inclusions are thus interpreted to have formed simultaneously from the same fluids. Eutectic melting and final melting temperatures for aqueous inclusions indicate that the fluids were concentrated brines consisting of CaCl2 and NaCl. Oxygen-isotope values for the megaquartz replacements averaged 28.4 (SMOW), indica ing precipitation from evaporative waters with an isotopic composition of +2.9 (SMOW). Evaporite silicification was coeval with or slightly postdated hydrocarbon migration. The fluid-inclusion data provide a record of the fluid temperatures and compositions that prevailed during silica precipitation. These data, coupled with regional stratigraphy and published geothermal gradients, suggest a burial depth of approximately 1.3 km during silicification. The source of the silica for evaporite replacement is problematic. We postulate, however, that silica may have been derived from dissolution of siliciclastics in back-reef units. Organic acids that form from breakdown of hydrocarbons increase the solubility of quartz by bonding with silicic acid. As these products (and associated brines) moved into cooler oxidizing zones, the organic acids broke down, releasing silica into solution. This organically mobilized silica eventually precipitated as megaquartz replacements of preexisting evaporites.

Influence of Provenance and Burial History on Diagenesis of Lower Cretaceous Frontier Formation Sandstones, Green River Basin, Wyoming

Abstract: The Upper Cretaceous Frontier Formation on the Moxa Arch in the western Green River Basin, Wyoming, has had a varied diagenetic history that was controlled in part by differences in composition of detrital framework grains and in burial history. Petrographic examination of 247 thin sections from 13 cores from the south-plunging arch and adjacent deep basin is the basis for diagenetic investigation of sandstones ranging in depth from 2 km to almost 5 km. Major diagenetic events were (1) mechanical compaction by grain rearrangement and deformation of ductile grains, (2) formation of illite and mixed-layer illite-smectite rims, (3) precipitation of quartz overgrowths, (4) precipitation of calcite cement, (5) generation of secondary porosity by dissolution of feldspar, chert, biotite, and mudstone grains and calcite cement, (6) precipitation of kaolinite in primary and secondary pores, and (7) chemical compaction by intergranular pressure solution and stylolitization and additional precipitation of quartz cement. The northern and southern ends of the Moxa Arch differ in the magnitude of each of these diagenetic events. Provenance differences caused more abundant ductile rock fragments and feldspar to be deposited at the northern end of the Moxa Arch. As a result, Frontier sandstones from the northern Moxa Arch underwent more extensive mechanical compaction. In addition, feldspar dissolution and albitization buffered acid-rich basinal fluids at the northern end, resulting in greater development of secondary porosity and precipitation of calcite cement than at the southern end. Deeply buried sandstones at the southern end of the arch and in the basin contain the most abundant quartz cement because intergranular pressure solution and stylolitization liberated silica for overgrowths.

Effects of Submarine Cementation on Microfabrics and Physical Properties of Carbonate Slope Deposits, Northern Bahamas

Abstract: Petrographic evidence from slope rocks in the northern Bahamas reveals the widespread alteration of sedimentary facies by interstitial precipitation of fine-grained magnesian calcite. Most cementation occurs at slope depths within the permanent thermocline (200 to 800 m) and in areas of well-developed, long-slope currents.

Neolithic rock resources across the western alps: Circulation data and models

Abstract: Western Alpine rocks feature prominently among the polished stone implements of the Southern French Neolithic. Detailed petrographic examination of about 2600 artifacts from that area and neighboring regions in France, Italy, and Switzerland, coupled with field investigation of both primary and detrital outcrops, now provides conclusive evidence that (i) only selected rocks were used, (ii) specific rock resources were traded east to west across the Western Alps, and (iii) Ligurian rocks from the Voltri Massif were an important part of such circulation. Exploited Alpine rocks were glaucophane schists, eclogites sensu lato, and jadeitites. Serpentinites were only used in Switzerland and the Jura. A regional network for glaucophane schists in French Provence can be contrasted with long-distance circulation of eclogites, based on primary and/or secondary “quarry” areas in Liguria and Piedmont in northwestern Italy. A preliminary discussion is made of exchange and circulation models for raw materials, blanks, or finished tools, among Neolithic societies around and across the Western Alps. © 1993 John Wiley & Sons, Inc.

The stratigraphy and setting of the Precambrian rocks of the Llangynog Inlier, Dyfed, South Wales

Abstract: Detailed mapping of the Precambrian rocks of the Llangynog Inlier, made possible by a series of new exposures, has allowed the establishment for the first time of the 1100 m thick succession within the Precambrian rocks of the area. All boundaries of the Precambrian rocks with younger strata in the area are either faults or unconformities. Some 350–400 m of rhyolitic lavas form the oldest rocks seen in the area; they are extensively silicified and devitrified and show little of their original texture, although flow-banding is visible in some outcrops and the presence of perlitic and snowflake textures imply that originally they were glassy in part. These lavas are succeeded by a varied succession dominated by volcaniclastic siltstones showing evidence of shallow water deposition and containing medusoids and trace fossils of Ediacaran age. Interbedded in the sediments are basaltic lavas, autobreccias and rare hyaloclastites displaying textural features characteristic of submarine eruption, and rhyolitic pumiceous and shardic ash-flow tuffs. The name Coomb Volcanic Formation is proposed for this succession, within which are recognized two members, a lower Castell Cogan Rhyolite Member succeeded by the Coed Cochion Volcaniclastic Member. In association are varied intrusions, comprising both dolerites and dacites. A palaeoenvironmental model, built on evidence of petrography, palaeontology and sedimentology is that of shallow water to emergent volcanic islands.

Fluid Inclusions as Carbonate Microfabrics: A Petrographic Method to Determine Diagenetic History

Abstract: Fluid inclusions in diagenetic phases are microfabrics for which petrographic study provides useful clues about diagenetic environment and subsequent thermal history. With properly prepared samples, the first step should be to determine the origin of the fluid inclusions in the context of other geologic and petrographic work. Petroleum-filled fluid inclusions can be identified using ultraviolet (UV) epifluorescence microscopy. For aqueous fluid inclusions, the next step is to describe the ratios of vapor to liquid among genetically related fluid inclusions. Then, petrographic study should identify those members of populations whose vapor-to-liquid ratios have not been altered by necking down or metastability. For those, all-liquid inclusions record entrapment below about 50°C. Inclusions with highly variable ratios of vapor to liquid, together with all-liquid inclusions, indicate entrapment in the vadose zone. A population of all-liquid fluid inclusions indicates entrapment in the low-temperature phreatic zone; however, subsequent thermal reequilibration may generate all-liquid inclusions that occur together with two-phase inclusions with small vapor bubbles. A population of two-phase inclusions with consistent ratios of vapor to liquid indicates entrapment at a single high temperature. Reequilibration of high-temperature fluid inclusions generates two-phase inclusions with only somewhat consistent ratios of vapor to liquid. Thus, determination of vapor-to-liquid ratio in genetically related fluid inclusions is diagnostic of diagenesis in the vadose zone, the low-temperature phreatic zone, and the high temperature environment; thermal reequilibration after initial entrapment can also be identified. The final step involves cracking the crystal to determine fluid inclusion internal pressure. A 1-atm pressure is diagnostic of the vadose zone, whereas high pressures or pressures near a vacuum represent entrapment or reequilibration at high temperature.

Kalsilite in Brazilian Kamafugitic Rocks

Abstract: The general area of this study (nearly 450 km 2) is situated in the western part of the state of Minas Gerais (southeastern Brazil), about 340 km from Belo Horizonte city, in the counties of Carmo do Paranaiba, Arapufi and Tiros (Fig. 1). In this area, the Mata da Corda formation consists of ultrabasic alkaline potassic to ultrapotassic lavas and volcaniclastic (mainly epiclastic) rocks. The total thickness of the rock sequence does not exceed 50 m. The lavas were dated by Hasui and Cordani (1968) at 80 Ma (late Cretaceous) by the ~)K/4~ whole-rock dating method. This formation overlies a non-volcanic sedimentary sequence--the Areado Formation (early Cretaceous), composed of conglomerates, mudstones and sandstones. The two formations constitute the San Franciscan basin, which rests on slightly metamorphosed Proterozoic basement rocks of the Bambuf Group. The lava crops out in the studied area as discontinuous small flows which are commonly highly weathered. Field observations show that this formation is composed largely of massive thin flows (individually not exceeding 0.5 m in thickness), sometimes overlain by, or interbedded with volcaniclastic rocks. The samples of lavas and associated cumualte rock which were analysed in the present work (Table 1 and Fig. 2a-d) come from loose fragments and pebbles collected on the slopes and at the feet of hills where the corresponding altered rocks crop out (Fig. 1). Petrography and Mineralogy

Uranium-Hydrocarbon Association in Francevillian Uranium Ore Deposits, Lower Proterozoic of Gabon

Abstract: The uranium ore deposits of the Francevillian Series (Gabon) represent the oldest (2.0 Ba) high grade uranium ore deposit located in a sedimentary environment. In these deposits, uranium is associated with migrated organic matter which occurs as infillings of secondary porosity in deltaic sandstones. Sedimentologic, tectonic, petrographic, and geochemical studies have permitted a reconstruction of the geologic conditions in which uranium mineralization took place. The combined results indicate that these uranium ore deposits are located in sandstone reservoirs associated with hydrocarbon traps and are capped by impermeable black shales which are also good oil source rocks.

Petrography of alkaline volcanic-subvolcanic rocks from the Brazilian Fernando de Noronha Archipelago, Southern Atlantic Ocean

Abstract: A classification scheme for the volcanic-subvolcanic rocks of the Fernando de Noronha Archipelago is presented, following the IUGS recommendations, based on the petrographic study of a large number of samples. In Fernando de Noronha two main volcanic events were defined by Almeida (1955): an older Remedios Formation (12 to 8 Ma old) composed of domes, plugs and dikes of tephritic-basanitic to trachytic and phonolitic compositions, with essexite porphyries, limburgites and alkaline lamprophyres intruding basal pyroclastic rocks, and a younger Quixaba Formation (3 to 2 Ma), made up mostly by flows of nephelinitic composition (ankaratrites). The lamprophyres were separated in two groups: tephritic or melanocratic lamprophyres and typical lamprophyres, the first ones belonging to a suite characterized by continuous increase in amphibole content. Most of the rocks of the Remedios Formation appear to belong to two distinct petrographic series, one represented by an undersaturated sodic basanite-tephrite (essexite)-phonolite trend and the other by a potassic alkali basalt-trachyandesite-trachyte series, while the limburgites and lamprophyres are of uncertain ancestry. Some ankaratrite and olivine nephelinite dikes that only cut the Remedios rocks are also attributed to the Quixaba Formation. The basanitic flow with mantle xenoliths of the Sao Jose Island, considered by Almeida (1955) to form the youngest Sao Jose Formation, is here tentatively interpreted as representing the waning stages of the Quixaba volcanism.

Petroleum prospectivity of the Clarence-Moreton Basin, eastern Australia: A geochemical perspective

Abstract: The Clarence‐Moreton Basin contains abundant oil‐prone organic matter of terrestrial origin particularly in the Walloon Coal Measures and, to a lesser extent, in the Koukandowie Formation. This oil‐prone character is reflected in the petrographic composition, pyrolysis yields, kerogen elemental composition and extractable hydrocarbon yields of shale and coal. Maturation levels vary from immature to marginally mature in the west to overmature in the eastern part of the basin in New South Wales. Calculations based on Rock Eval data show that significant oil generation occurred in a narrow maturation range (0.8–1.0% vitrinite reflectance) and that migration has been highly efficient. Maturation modelling and fission track analyses indicate that hydrocarbon generation occurred 80–100 Ma during a period of high heat flow when the Tasman Sea spreading ridge was adjacent to the southeastern side of the Logan Sub‐basin. Despite the abundance of oil‐prone source rocks, the basin is considered to be largely gas‐pro...

Evaluation of lunar rocks and soils for resource utilization: Detailed image analysis of raw materials and beneficiated products

Abstract: The rocks and soils of the Moon will be the raw materials for fuels and construction needs at a lunar base. This includes sources of materials for the generation of hydrogen, oxygen, metals, and other potential construction materials. For most of the bulk material needs, the regolith, and its less than 1 cm fraction, the soil, will suffice. But for specific mineral resources, it may be necessary to concentrate minerals from rocks or soils, and it is not always obvious which is the more appropriate feedstock. Besides an appreciation of site geology, the mineralogy and petrography of local rocks and soils is important for consideration of the resources which can provide feedstocks of ilmenite, glass, agglutinates, anorthite, etc. In such studies, it is very time-consuming and practically impossible to correlate particle counts (the traditional method of characterizing lunar soil petrography) with accurate modal analyses and with mineral associations in multi-mineralic grains. But x ray digital imaging, using x rays characteristic of each element, makes all this possible and much more (e.g., size and shape analysis). An application of beneficiation image analysis, in use in our lab (Oxford Instr. EDS and Cameca SX-50 EMP), was demonstrated to study mineral liberation from lunar rocks and soils. Results of x ray image analysis are presented.

Petrography and geochemistry of early Proterozoic granitoid rocks in Wisconsin magmatic terranes of Penokean Orogen, northern Wisconsin

Abstract: Jl Introduction J2 Geologic setting J2 Wisconsin magmatic terranes J2 Pembine-Wausau terrane J6 Marshfield terrane J6 Granitoid rocks J6 Pembine-Wausau terrane J6 Syntectonic granodiorite suite J7 Post-tectonic suite J7 Athelstane Quartz Monzonite J7 1,835 Ma alkali-feldspar granite J9 1,760 Ma granodiorite-granite group J13 Marshfield terrane JtS Syntectonic tonalite-granodiorite suite J16 Gneissic granite near Neillsville J23 1,835 Ma alkali-feldspar granite J24 Summary and conclusions J24 References cited J27 Appendix A. Petrographic descriptions of rock samples listed in table 1 J29 Appendix B. Petrographic descriptions of rock samples listed in table 2 J30

Hydrocarbon potential of sandstone reservoirs in the neogene east slovakian basin part i: a petrographic examination of lithology, porosity, and diagenesis

Abstract: Core description and petrographic analyses were utilized to study East Slovakian Basin sandstone reservoirs. Reservoir development is largely dependent on the original sandstone composition, which is influenced by deposition in a deltaic setting, local sourcing and volcanic activity. Sandstones are texturally and mineralogically immature lithic and feldspathic arenites. The presence of unstable lithic grains and feldspars contributes to low, irregular reservoir porosity, due to deformation by compaction, and susceptibility to chemical alteration. Reservoir quality is also influenced by the subsequent diagenesis of the sandstones, which is driven by high heat flow. Lithic fragments and feldspars alter readily to form authigenic/diagenetic mineral suites, which tend to occlude porosity, however, dissolution of some of these grains also enhances secondary porosity development. Most observed porosity in the basin sandstones is secondary, developing from dissolution of both carbonate cement and unstable framework grains. Porosities suggest a weak decreasing trend with depth of burial. However, detailed examination of several localities reveals that porosity development is strongly influenced by local factors (e.g. structural evolution, sandstone lithology, and the distribution of volcanics). Sandstones of the East Slovakian Basin are generally not good hydrocarbon reservoirs (particularly for liquid hydrocarbons), due to the presence of unstable framework grains, early carbonate cementation and authigenic/diagenetic mineral suites. Exceptions are found when dissolution of cement and framework grains results in significant secondary porosity.