Showing papers on "Petrography published in 1988"

Geochemical classification of terrigenous sands and shales from core or log data

Abstract: A means of relating geochemical concentrations to existing sandstone classification schemes is based on three chemical parameters: the SiO2/Al2O3 ratio, the Fe2O3/K2O ratio, and the Ca content. In terrigenous sands and shales, the SiO2/Al2O3 ratio separates Si-rich quartzarenites from Al-rich shales, with other sand types showing intermediate values. The ratio of total iron (as Fe2O3) to K2O separates lithic sands (litharenites and sublitharenites) from feldspathic sands (arkoses and subarkoses). In addition, very high Fe2O3/K2O ratios indicate Fe-rich shales (e.g., pyritic, sideritic, hematitic) or Fe-rich sands (e.g., gl uconitic) depending on the silica/alumina ratio. The Ca content is used to differentiate noncalcareous from calcareous sandstones and shales and to separate siliciclastic from carbonate rocks. Sandstones are classified the same by this scheme as by petrographic analysis about 84% of the time, and shales are effectively discriminated from sandstones. The requisite input data can be accurately supplied by geochemical well-logging measurements, enabling unbiased sandstone classification to be displayed on a continuous basis with depth.

Comparison of isotopic and petrographic provenance indicators in sediments from Tertiary continental basins of New Mexico

Abstract: Isotopic variations of Nd in sedimentary rocks provide a new tool for provenance determination. We have compared the isotopic technique with classic petrographic indicators of provenance in sediments from two Tertiary basins of the Rio Grande rift, north-central New Mexico. The siliciclastic sediments in these basins were derived from varying proportions of Precambrian basement and Oligocene volcanic rocks, which are isotopically and petrographically distinct. In the analyzed sediments, a good correspondence exists between petrographic and isotopic provenance indicators. The data also suggest that the isotopic tracer is more sensitive to minor detrital input from a second source than is the framework petrology. The similar Sm/Nd ratios for all analyzed sediments and the identical isot pic values of interlayered mudstone and sandstone indicate that no significant fractionation of Sm and Nd occurs during the formation of the sediments. Thus, the Nd model ages of the sediments preserve the crust-formation age of the source. The Sr isotopic and concentration data show trends generally similar to the Nd data, but they do not indicate the sedimentary provenance as clearly. This ambiguity is a consequence of strong Rb-Sr source heterogeneities and Rb-Sr fractionation during the sedimentary cycle.

Eclogites and eclogite-facies rocks

Abstract: 1. A review of the peculiar mineralogy of the ``Norwegian coesite-eclogite province'', with crystal-chemical, petrological, geochemical and geodynamical notes and an extensive bibliography (D.C. Smith). Introduction. An outline of the regional geological, geochemical and geodynamical contexts of the eclogite-bearing Western Gneiss Region. The Norwegian coesite-eclogite province: a mineralogist's paradise and a petrologist's nightmare. Specific petrogenetic interpretations of all the peculiar minerals in ecolgites and retrogressed eclogites. General comments on the crystal-chemical petrology of the peculiar minerals. The regional context of eclogites in the Western Gneiss Region. Epilogue: geodynamical aspects of pressure in metamorphism. Conclusions. 2. A review of chemical zoning in eclogite garnets (E.D. Ghent). Introduction. Common zoning patterns in eclogite garnet. Estimation of apparent chemical potential gradients. Models for garnet zoning. Types of garnet zoning which do not fit simple zoning models. Isothermal fractionation model. Chemical zoning in garnets due to changes in temperature and/or pressure during crystallization. Exsolution in garnet. Effects of diffusion upon homogenization of garnet and upon garnet-clinopyroxene Mg-Fe exchange geothermometry. Conclusions. 3. A review of the crystal-chemistry of clinopyroxenes in eclogites and other high-pressure rocks (G. Rossi). Introduction. The crystal structure of clinopyroxenes. The crystal chemistry of the end members. Clinopyroxenes of intermediate composition. Methods and techniques in the study of order-disorder phenomena. Structural variations in the system Di-Jd. Cation ordering. Subsolidus phenomena in omphacites. The omphacite phase diagram and its petrological implications. Conclusions. 4. Mineral parageneses in eclogite-facies meta-acidites in northern New Caledonia (P.M. Black et al). Introduction. Petrology of eclogite meta-acidites. Mineralogy of New Caledonian eclogite meta-acidites. Discussion. Conclusions. 5. Meta-acidites and silicic meta-sediments related to eclogites and glaucophanites in northern Sifnos, Cycladic Archipelago, Greece (M. Schliestedt, M. Okrusch). Introduction. General geology. The high-pressure sequence on Sifnos. Petrography. Bulk-rock chemistry. Mineral chemistry. White micas. Epidotes. Deerites. P-T conditions of Eocene high-pressure metamorphism. Conclusions. 6. Trace element and isotopic characteristics of eclogites and other xenoliths derived from the lower continental crust of southeastern Australia and southwestern Colorado Plateau, U.S.A. (R.J. Arculus et al). Introduction. Xenolith localities and geological setting. Xenolith types. Phase chemistry. Bulk rock chemistry. Isotopic data. Wave velocity - density measurements. Pressure-temperature estimates. Discussion. Conclusions. 7.

Petroleum Geology and Geochemistry of Middle Proterozoic McArthur Basin, Northern Australia II: Assessment of Source Rock Potential

Abstract: Five potential source rocks have been discovered in the middle Proterozoic of the McArthur basin. The lacustrine Barney Creek Formation (McArthur Group) and the marine Velkerri Formation (Roper Group) compare favorably in thickness and potential with demonstrated petroleum source rocks in the Phanerozoic. The former contains up to 7% total organic carbon (TOC) and kerogen types I and II. TOC values in the latter range up to 6.5% and the kerogen is type II. Petrographic examination shows the organic matter is mostly lamalginite, which on maturation yields "nonfluorescent lamalginite" and bitumen. Extractable hydrocarbon yields from mature samples indicate that good to excellent source rocks are present and there is ample evidence of migration having occurred. Tmax values derived from Rock-Eval analysis may be used to define maturation levels in much the same way as in Phanerozoic sediments. At low maturation levels, the reflectivities of lamalginite and bitumen are lower than vitrinite reflectance values calculated from methylphenanthrene indices, but converge in the mature zone. This convergence reflects both the inherent lower reflectivity of hydrogen-rich macerals and the source control on methylphenanthrene indices at low maturity. Maturation levels in McArthur Group sediments vary from marginally mature to overmature with abrupt changes over short distances associated with faults. Hydrocarbon generation occurred prior to the deposition of the younger Proterozoic sediments of the Roper Group. In contrast, the source rocks in the Roper Group re marginally mature to mature throughout the study area and are overmature only where they are affected by igneous intrusions. Considerations of burial history suggest that hydrocarbon generation may have occurred during deposition of the Roper Group although an early Paleozoic timing cannot be excluded.

Batholiths, plutons, and suites: nomenclature for granitoid rocks of Westland—Nelson, New Zealand

Abstract: Granitoid rocks of Westland-—Nelson are classified into five geographically distinct batholiths and four suites. Each suite consists of rocks of similar petrographic and chemical characteristics (and probably similar age) which transgress the batholith boundaries. It is suggested that the terms Tuhua Formation and Tuhua Intrusive Group be abandoned, although Tuhua Orogeny is retained as a broad term for mid-Paleozoic orogenic events.

Post-granulite facies fluid infiltration in the Adirondack Mountains

Abstract: Granulite facies lithologies from the Adirondack Mountains of New York contain alteration assemblages composed dominantly of calcite +/- chlorite +/- sericite These assemblages document fluid infiltration at middle to upper crustal levels Cathodoluminescence of samples from the Marcy anorthosite massif indicates that the late fluid infiltration is more widespread than initially indicated by transmitted-light petrography alone Samples that appear unaltered in transmitted light show extensive anastomosing veins of calcite (< 005 mm wide) along grain boundaries, in crosscutting fractures, and along mineral cleavages The presence of the retrograde calcite documents paleopermeability in crystalline rocks and is related to the formation of high-density CO/sub 2/-rich fluid inclusions Recognition of this process has important implications for studies of granulite genesis and the geophysical properties of the crust

Disequilibrium Melting of Granite at the Contact with a Basic Plug: A Geochemical and Petrographic Study

Abstract: Mantle-derived intrusions are potentially an important source of heat for melting in the crust, yet there are few studies of anatectic melts occurring at the contacts of such intrusions to provide natural analogs. This is a study of anatectic melts developed in a Cretaceous granite at the contact with a 12 Ma trachyan-desite plug in the Sierra Nevada batholith, California, where a detailed textural and geochemical study was possible because the melt is now represented as a glass that has undergone limited devitrification. A Rb-Sr isochron age of $11.8 \pm 0.6$ Ma for glasses and granite whole rocks is in agreement with the K-Ar age of $11.7 \pm 0.7$ Ma for a similar trachyandesite plug nearby, confirming that the melting was associated with the trachyandesite intrusion. Melting occurred at about 900°C or higher and at less than 1 kb. The restite minerals consist dominantly of zoned plagioclase and sanidine (both have rims of anorthoclase of similar composition), quartz, and magnetite. The whole-rock major...

Approach to chemical equilibrium in diagenetic chlorites

Abstract: Electron microprobe analyses were made on diagenetic chlorites in sandstones and mudstones from two deep wells according to the petrographic character of the chlorite occurrence: as pseudomorphic phases, rims on quartz or glauconite or as distinct phases in the clay matrix. Chlorite compositions do not depend upon crystallization site (reacting phases) making it apparent that new chlorites can form in an approach to chemical equilibrium at or near the surface (40° C, 1 km depth). Comparison of this data with that for late diagenetic and early metamorphic chlorites indicates that the compositional range for different grains in the same thin section is similar for the samples throughout the 40°–270° C temperature span. Compositional range decreases upon further metamorphism. Al content appears to be a more reliable indicator of temperature variations than other substitutions in the chlorite structure. The clay mineral assemblage which indicates sedimentary facies affects the trends in composition (Al increase or decrease) as a function of temperature. The octahedral site occupancy show a general increase in going from diagenesis to metamorphic conditions in pelitic rocks. The range of Fe-Mg ratios seems to depend more on the chemistry of each sample than the temperature of formation of the minerals.

Sandstone petrographic evidence and the Chugach-Prince, William terrane boundary in southern Alaska

Abstract: The contact between the Upper Cretaceous Valdez Group and the Paleocene and Eocene Orca Group has been inferred to be the boundary between the Chugach and the Prince William tectonostratigraphic terranes. Sandstone petrographic data from the Prince William Sound area show no compositional discontinuity across this contact. These data are best explained by considering the Valdez and Orca Groups to be part of a single terrane—a thick flysch sequence derived primarily from a progressively unroofing magmatic arc with increasing input from subduction-complex sources through time.

Geochemistry of Early Carbonate Cements in the Cardium Formation, Central Alberta

Abstract: Authigenic minerals and diagenetic textures in the marine conglomerates and sandstones of the Cretaceous Cardium Formation in the northeast Pembina area preserve a complex sequence of diagenetic events. Textural relationships observed in thin section and under the scanning electron microscope were used to determine the relative timing of diagenetic events in these marine sediments. Paragenetic sequences are similar for the conglomerate, sandstone, and siderite nodules in the enclosing shales. Extensive cementation by siderite and calcite occurred early in the diagenetic history, before any significant compaction. The geometry and distribution of lithofacies in the Cardium may have influenced the diagenesis and internal stratigraphy of the conglomerate. Overlying shale matrix conglomerate may have trapped upward-migrating fluids increased in buoyancy by dissolved CO2 produced by decay of organic matter in the enclosing shales. These fluids could have infiltrated the upper portion of open matrix conglomerate, causing cementation. Petrographic evidence shows alternating precipitation of siderite and pyrite, implying fluctuating activities of dissolved carbonate and hydrogen sulfide, probably caused by bacterially mediated processes during early diagenesis. Early sequences of siderite and pyrite precipitation can be related to zones of iron reduction, sulfate reduction, and, possibly, decarboxylation during diagenesis. Carbon and oxygen isotopic data indicate a systematic change in the isotopic compositions of calcite and siderite from -25 to -3013C and 018O, for cements early in the paragenetic sequence, to 0 to -513C and -15%18O for cements which are interpreted to occur later in the paragenetic sequence. This observation is consiste t with the influence of meteoric water on later stages of cement deposition in the Cardium, a unit considered to be marine and deposited well offshore on a shallow marine shelf. The intrusion of meteoric water far offshore may be related to changes in sea level. Many variables must be accounted for, but it is feasible, using the estimated regional dip of the Cardium at the time of deposition (0.005°), for a 1-m drop in sea level to push the freshwater--seawater interface seaward on the order of 100 km.

White marble from italy and turkey: an archaeometric study based on minor- and trace-element analysis and petrography

Abstract: In 94 marble samples from 4 quarry districts in Italy (Carrara) and Turkey (Proconnesus, Dokimeion, Usak), minor and trace elements were determined by instrumental neutron activation analysis (INAA) and atomic absorption spectrometry (AAS). The maximum size of the calcite grains (MGS) of the rocks was measured in thin section. For 16 elements considered in this work, the concentration ranges show important inter-district overlaps; this also applies to the maximum grain size. However, the application of cluster analysis, using selected attributes, allows one to discriminate every pair of districts; 90 samples are classified correctly in all classification dendrograms.

Metasomatic origin of spessartine-rich garnet in the South Mountain Batholith, Nova Scotia

Abstract: An occurrence of spessartine-rich (15-25 mole Vo) almandine garnet within a late-stage, polyphase granitic intrusion (18-20 km2) of the 37GMa-old South Mountain batholith, Nova Scotia, is described. Carnet occurs as euhedral to subhedral single grains or spherical quartzgarnet clots and is most abundant near contact zones, along fractures, and in late pegmatites, The gamet has an antipathetic relationship with biotite. Mineral chemistry of the major silicate phases of the host granitoids reflect reequilibration witi a fluid phase. For example, plagioclase compositions range from magmatic values (An11_17) to tlose characteristic of a lower ternperature metasomatic origin (Ano-s), biotite is anomalously enriched in Fe with Fe/(Fe+Mg) values of 0.85, and muscovite is characterized by lower wt.|7o Ti, Fe and Mg than is typical of magmatic muscovite. The petrographic and chemical data are interpreted to reflect a widespread interaction ofhydrothermal fluids ia'ith the host granitoid that resulted in the postmagmatic gowth of garnet. The development of the garnet, inferred to have occurred at temperatures in excess of 500'C based on two-feldspar geothermometry and textural relationships with sillimanile, cannot have occurred without contributions of Al, Fe, Mg and Mn from surrounding metasedimentary rocks. This occrurence of metasomatic garnet, of composition similar to that considered typical ofmagmatic garnets, again raises the problem ofinterpreting what is of primary magmatic origin in granitoid rocks.

Sediment Alteration and Clay Mineral Diagenesis in a Regional Ground Water Flow System, Mississippi Gulf Coastal Plain

Abstract: High Na-HCO3 pore fluids present in many areas of the shallow Gulf Coastal Plain are the product of pervasive, meteoric diagenesis of siliciclastic host sediments. This diagenesis is driven in part by the subsurface generation of CO2 and the reaction of carbonic acid with Na-bearing silicates. While a significant body of water analyses exists, there has been comparatively little study of the mineralogy and petrography of the altered sediments themselves. An analysis of sediment samples from shallow boreholes and wells along a 100-km hydrologic flow path in Upper Cretaceous sands and clays in northeastern Mississippi reveals the presence of spatially systematic variations in sediment alteration and clay mineral diagenesis. Kaolinite is the dominant clay mineral phase in the updip, upgradient portion of the flow system and has apparently been produced in part by the in situ breakdown of feldspars. The relative proportion of kaolinite progressively decreases downgradient, and smectite becomes the dominant clay mineral in the distal portions of the flow system. Smectite is most abundant where meteoric waters are mixing with high-NaCl fluids migrating up from depth. Siderite is a common authigenic cement in the updip portions of the system, and pyrite is present in some downgradient samples. The results of the study are thus in general agreement with the earlier work of Lee (1985), who inferred from systematic variations in the chemistry of waters along this same flow path that mineral diagenesis involving aluminosilicates and Fe-bearing phases exerts an important control on the geochemical evolution of these fluids. Of interest in deeper basinal studies is the role that pervasive, shallow diagenetic precipitation of carbonate cements and authigenic clays, such as we have documented, may play in the subsequent porosity and permeability development of Gulf Coast sediments during deep burial.

Estudo petrográfico das rochas ígneas alcalinas da região de lagoa formosa, mg

Abstract: The first results of a petrographic study about the alkaline rocks from Lagoa Formosa region, Minas Gerais, Brazil, are presented. The rocks belong to the Cretaceous Mata da Corda Formation of the Sao Francisco Basin comprising volcanic breccias, lapillites, tuffs, cinerites, lava flows, volcanic pipes, and dykes. The related petrographic terms are mela-leucitites and minor sienites, alkalipyroxenite, and basalt, Generally, the rocks are porphyritic, mafic with olivine, phologopite and pyroxene phenocrysts in a pyroxene, leucite, phlogopite afanitic matrix.