Showing papers on "Petrography published in 1980"

Evaluation of multispectral middle infrared aircraft images for lithologic mapping in the East Tintic Mountains, Utah

Abstract: Six channels of multispectral middle infrared (8 to 14 micron) aircraft scanner data were acquired over the East Tintic mining district, Utah. This area has high relief and moderate vegetation and consists mainly of Tertiary silicic igneous rocks and Paleozoic quartzite and carbonate rocks that have been locally hydrothermally altered. These digital-image data were computer processed to create a color-composite image based on principal component transformations. Color differences in this image are related to the spectral differences in the surface material and allow discrimination of several rock types, depending primarily on their silica content. When combined with a visible and near infrared color-composite image from a previous flight, with limited field checking, it is possible to discriminate quartzite, carbonate rocks, quartz latitic and quartz monzonitic rocks, latitic and monzonitic rocks, silicified altered rocks, argillized altered rocks, and vegetation.

Refined Techniques for Staining Plagioclase and Alkali Feldspars in Thin Section: METHOD PAPER

Abstract: Staining of thin sections for plagioclase and alkali feldspar has not become a universally applied petrographic tool, probably due to difficulties encountered in effectively staining plagioclase. Modifications of standard techniques are described for rapidly producing stained thin sections of high quality. Critical steps in the procedure include pre-polishing with wet 600-grit abrasive paper, using adequate etching times, and applying a K-rhodizonate solution of 0.02 g per 30 ml water, or weaker.

Refined techniques for staining plagioclase and alkali feldspars in thin section

Abstract: Staining of thin sections for plagioclase and alkali feldspar has not become a universally applied petrographic tool, probably due to difficulties encountered in effectively staining plagioclase. Modifications of standard techniques are described for rapidly producing stained thin sections of high quality. Critical steps in the procedure include pre-polishing with wet 600-grit abrasive paper, using adequate etching times, and applying a K-rhodizonate solution of 0.02 g per 30 ml water, or weaker.

Chronology of the structural and petrologic development of the southwest Sierra Nevada foothills, California: Summary

Abstract: Metamorphic country rocks of the southwest Sierra Nevada foothills represent the southernmost exposures of the western foothills metamorphic belt. The rocks of the foothills belt in this region are disrupted and metamorphosed by the Cretaceous batholith to a much greater extent than to the north, probably owing to a deeper level of exposure. The oldest country rocks in the region consist of the Carboniferous to Triassic Kings—Kaweah ophiolite belt which served as basement for Jurassic volcanic arc and continent-derived clastic sequences (Saleeby, 1978, 1979). The history of ophiolite generation, sea-floor transport, continental margin accretion, and volcano-plutonic arc growth is recorded by numerous petrogenetic and deformational episodes. Useful time constraints have been derived for a significant number of these episodes by U/Pb and K/Ar geochronometry. These time constraints have facilitated the construction of an absolute chronology for the structural and petrologic evolution of the region. In this paper, the critical geological and geochronological relations are summarized. The isotopic data and their interpretation, and the detailed field relations of the rocks dated along with supporting petrographic and petrochemical data are presented in the complete article in Part II.

Howardites - Samples of the regolith of the eucrite parent-body: Petrology of Frankfort, Pavlovka, Yurtuk, Malvern, and ALHA 77302

Abstract: Modal petrographic methods have been applied to the meteorites Frankfort, Pavlovka, Yurtuk, Malvern, and ALHA 77302, to determine some of the characteristics of the regolith of the eucrite parent body. Lithic clasts in the meteorites fall into three major groups: pyroxene + plagioclase rocks, orthopyroxenites, and fused-soil clasts. Lithic clasts make up a small proportion of the soil; mineral clasts from orthopyroxenites dominate the coarse-grained fraction; and the fine-grained fraction contains minerals from both orthopyroxenites and plagioclase + pyroxene rocks. The eucrite regolith appears to have the following characteristics: the source rocks are friable, the soils are immature, comminution is the major soil-forming process, and the soil is well mixed.

The Organic Petrology of the Upper Cretaceous Boundary Creek Formation, Beaufort-Mackenzie Basin

Abstract: A detailed petrographic study of the organic material in the Upper Cretaceous Boundary Creek Formation of the Beaufort-Mackenzie Basin has been used to establish a general picture of a) its depositional environment, b) variation in the type of organic material present, and c) its petroleum source potential. The Boundary Creek appears to have accumulated in a depositional setting which had both marine and seasonal terrestrial input and predominantly anaerobic bottom conditions. It contains the types and quantities of organic material that would make it an excellent source rock. However, it has not yet been sampled in an area where it is thermally mature enough to realize its full oil-generating potential.

The distinction of pristine from meteorite-contaminated highlands rocks using metal compositions

Abstract: Pristine highlands rocks, i.e., those which have retained the chemical characteristics they acquired from igneous processes, contain metal grains whose Ni and Co contents are distinct from those in most polymict, meteorite-contaminated rocks. The difference is mainly a result of the bulk Ni/Co ratios of pristine rocks being much lower than those of chondritic meteorites. The compositions of metal grains thus provide a rapid and effective criterion for the recognition of pristine highlands samples.

Oil geochemistry and potential source rocks of the officer basin, south australia

Abstract: Oil shows observed in Cambrian Observatory Hill Beds, intersected during recent stratigraphic drilling of SADME Byilkaoora-1 in the Officer Basin, indicate that oil has been generated within the basin. Shows vary in character from "light" oils exuding from fractures through to heavy viscous bitumen in vugs in carbonate rocks of a playa-lake sequence. The oils are immature and belong to two primary genetic families with some oils severely biodegraded. The less altered oils are rich in the C13 - C25 and C30 acyclic isoprenoid alkanes. Source beds within the evaporitic sequence contain 0.5 - 1.0% total organic carbon and yield up to 1900 ppm solvent-extractable organic matter. Oil-source rock correlations indicate that the oils originated within those facies drilled; this represents the first reported examples of non-marine Cambrian petroleum. The main precursor organisms were benthonic algae and various bacteria. Studies of organic matter in Cambrian strata from five other stratigraphic wells in the basin reveal regional variations in hydrocarbon source potential that relate to differences in precursor microbiota and/or depositional environment and regional maturation. Micritic carbonates of marine sabkha origin, located along the southeast margin of the basin, are rated as marginally mature to mature and good to prolific sources of oil. Further north and adjacent to the Musgrave Block, Cambrian siltstones and shales have low organic carbon values and hydrocarbon yields, and at best are only marginally mature. Varieties of organic matter recognised during petrographic studies of carbonates in the Officer Basin include lamellar alginite (alginite B) and "balls" of bitumen with reflectance in the range 0.2 to 1.4%.

Genesis of the Smartville arc-ophiolite, Sierra Nevada foothills, California

Abstract: Rare earth element analyses of metavolcanic rocks from the Smartville, Calif. ophiolite divide the uniform suite of pillowed and massive lavas into the lower part consisting of massive, brecciated tholeiites, an intrusive dike-sill complex, and part of a plutonic suite. The tholeiites are light REE depleted with a (Ce)N range of 6.5 to 26.0 and (Yb)N of 6.0 to 30.0; it is suggested that the tholeiites were produced by partial melting of a LREE depleted source similar to MORB. The upper part of the volcanic pile is comprised of basaltic-andesitic flows and interbedded coarse to fine volcanic sediments; these 'calc-alkaline' rocks are light REE enriched with the (Ce)N range of 17.0-28.0 and (Yb)N of 8.0 to 12.0. Finally, petrographic investigations of volcanic sediments and sulfide ore deposits in the lava show that the ophiolite formed near a group of active submarine and subareal volcanoes.

Lamprophyres from Indian Gondwanas - K-Ar Ages and Chemistry

Abstract: Lamprophyric dyke rocks occurring in the Indian Gondwanas (Permian-Lower Cretaceous) form a consanguinous suite ranging from ultrabasic mica-peridotite to micro-syenitic assemblages. Chemical characters include low SiO 2 ( ca. 40% or less), high (FeO+MgO) and high MgO/FeO and K 2 O/Na 2 O ratios. The petrographic and chemical characters have been discussed in a broad framework after Rock (1977). K-Ar radiometric ages on four biotite separates and one whole rock from three different localities range from 105 to 121 Ma and are believed to represent true crystallisation ages.

Petrography of possible ophiolitic rocks along the qift-quseir road, eastern desert, egypt

Abstract: Serpentinites in the central Eastern Desert of Egypt along the Qift-Quseir road are possibly members of a Precambrian ophiolite assemblage. The sequence of components from bottom to top includes ultramafic rocks, gabbroic rocks, sheeted dykes, and basic volcanics which are pillowed at the top.

A contribution to the nomenclature and classification of volcanic rocks

Abstract: At present one is not able to devise a single, standard classification of igneous rocks that is acceptable to all petrologists. This is mainly because igneous petrology consists of an uneasy union between the descriptive science of petrography and the explanatory science of petrogenesis, and at present the latter discipline is unable to provide a coherent model, or series of models, that can account for the origin and evolution ofall igneous rocks. Some of the more significant concepts used in the classification of volcanic rocks are examined, and three separate, yet complementary, classifications are presented.

Origin of discontinuity surfaces in limestones; isotopic and petrographic data, Pleistocene of Barbados, West Indies

Abstract: In order to compare the validity of petrographic and isotopic techniques in determining the depositional and diagenetic history of limestones, both techniques were used on samples from a Pleistocene outcrop on Barbados, where depositional and disgenetic models are relatively simple and have well-documented recent analogues. The outcrop consists of a reasonably homogeneous grainstone cut by a discontinuity surface. The carbonate immediately above and below the discontinuity forms well-cemented layers, but the rest of the grainstone is highly permeable. Only the lower well-cemented layer contains isopachous, bladed cement. Carbon of this layer is isotopically light, and oxygen is slightly heavy. In addition, 18O distinctl changes across the discontinuity. Based on the cement morphology, the lower cemented layer formed as a submarine hardground; however, the isotopic analyses indicate evaporative precipitation of calcite at a subaerial exposure at or just above the submarine hardground following a regression. This regression and following transgression formed the discontinuity. The distinct offset of 18O values across the discontinuity indicates that the limestone above and below the discontinuity underwent mineralogical stabilization in solutions of different isotopic compositions. Diagenesis of the unit below the discontinuity preceded deposition of the unit above. The upper cemented layer resulted from cementation in a perched phreatic environment resting on the discon inuity. We consider these results to be generally applicable to other limestones. Stratigraphic variation in 18O is especially useful in identifying discontinuity surfaces and in mapping individual depositional/diagenetic packages within otherwise similar limestones. Integration of petrographic and isotopic data is necessary to determine the origin of discontinuity surfaces.

Spinel cataclasites in 15445 and 72435 - Petrology and criteria for equilibrium

Abstract: The problem of establishing the existence of equilibrium among the coexisting phases in the rock is addressed by presenting petrographic and mineral chemistry data on a new spinel cataclasite from 15445 (clast H) and data more extensive than those previously available on two clasts in 72435 Criteria useful in reconstructing the original petrology of these and other spinel cataclasites are analyzed by considering equilibrium among the different phases, that is, the mono- or polymict nature of these cataclasized samples Finally, the role of impact processes in disturbing the equilibria is discussed

Zeolitization of Miocene volcaniclastic rocks, southern Desatoya Mountains, Nevada

Abstract: Heulandite, clinoptilolite, mordenite, analcime, thomsonite(?), erionite, and chabazite(?) occur with other authigenic minerals in the thick Miocene volcaniclastic sequence of the southern Desatoya Mountains, Nevada. The mineral assemblages formed diagenetically in an open system probably at depths of 2,500 m or less and in a variety of rock types, including rhyolite, quartz latite, dacite, latite, and andesite. The authigenic minerals developed mainly through the breakdown of the inherently unstable glassy components, and the original porosity and permeability of the rocks were major factors controlling the degree of alteration. Determination of the temperatures of diagenesis depends greatly on ascertaining the time of diagenesis and, thus, the geothermal gradient at that time. The paragenetic sequence of events between primary and secondary materials and among authigenic mineral species is (1) partial replacement of glass by clay minerals, (2) solution of remaining glass, (3) precipitation of heulandite and/or clinoptilolite (± silica) and/or other authigenic minerals, and (4) deposition of opal or chalcedony in cavities or veinlets. Twenty-seven electron microprobe analyses are given for single crystals and crystal clusters of heulandite, clinoptilolite, and thomsonite(?). Chemical comparisons between (1) petrographically identified glass and its adjacent alteration products and (2) unaltered and/or partially altered and completely altered whole rocks from the same unit indicate that Si, Ca, Na, K, and H 2 O were mobile components. It is inferred from petrographic observations that Fe and Mg also were mobile. Na, and possibly some K, apparently were lost during diagenesis. Mineral development and chemical changes seem to have followed the general pattern for leaching in an open system.

Impregnation technique using colored epoxy to define porosity in petrographic thin sections

Abstract: This vacuum-assisted technique for colored impregnation of all rock types clearly defines porosity, even in low-permeability, very fine grained, or semifriable material. The technique is also suitable for use with water-soluble and heat-sensitive material. The technique is trouble-free and provides a practical way of visually reproducing rock pore space less than 1 μm in size.

Petrology and radiogeology of the stripa pluton

Abstract: To better define the character of the rock encompassing the thermomechanical and hydrological experiments at the Stripa mine in central Sweden, and to help determine the size of the Stripa pluton, detailed studies were conducted of the petrology and radiogeology of the quartz monzonite and adjacent rocks. Petrologic studies emphasized optical petrography, with supplementary X-ray diffraction, X-ray fluorescence and microprobe analyses. Radiogeologic investigations were based primarily on surface and underground gamma-ray spectrometric measurements of uranium, thorium and potassium, supplemented by laboratory gamma spectrometric analyses and fission-track radiographic determinations of the locations and abundance of uranium in the rock matrix. Both the quartz monzonite and the metavolcanic leptite which it intruded are strongly fractured. Two stages of fracture filling are evident; an earlier stage encompassing quartz, sericite, feldspar, epidote, and chlorite, and a later stage dominated by carbonate minerals. The Stripa quartz monzonite is chemically and mineralogically distinct from other plutons in the region. Muscovite is the predominant mica in the quartz monzonite; biotite has been altered to chlorite, hornblende is absent, and accessory minerals are scarce. In contrast, in other plutons in the Stripa region biotite and hornblende are prominent mafic minerals and accessory minerals are abundant. The Stripa quartzmore » monzonite is also considerably more radioactive than the leptite and other plutons in the region. Uranium and thorium abundances are both- 30 ppm, considerably higher than in "normal" granitic rocks where the thorium-to-uranium ratio generally exceeds 2. Potassium-argon dating of muscovite from the Stripa quartz monzonite indicates that this rock may be older, at 1691 million years than granitic rock of the neighboring Gusselby and Kloten massifs, whose ages, based on K-Ar dating of biotite, are respectively 1604 and 1640 m.y. Heat flow and heat productivity considerations show that although Stripa quartz monzonite contains high abundances of radioelements, the pluton has little effect on the regional heat flow. If it occurs in a layered plutonic setting, it is not more than 1.5 km thick; otherwise it may comprise a stock, dike, or border phase that is relatively small compared with the large granitic plutons exposed in the region.« less

Conglomerate and Sandstone Petrography, Deep Sea Drilling Project Site 445, Philippine Sea

Abstract: Conglomerates and sandstones in lithologic unit V at DSDP Site 445 comprise lithic clasts, detrital minerals, bioclasts, and authigenic minerals. The lithic clasts are dominantly plagioclase-phyric basalt and microdolerite, followed by plagioclase-clinopyroxene-phyric basalt, aphyric basalt, chert, and limestone. A small amount of hornblende schist occurs. Detrital minerals are dominantly plagioclase, augite, titaniferous augite, olivine, green to pale-brown hornblende, and dark-brown hornblende, with subordinate chromian spinel, epidote, ilmenite, and magnetite, and minor amounts of diopside, enstatite, actinolite, and aegirine-augite. Bioclasts are Nummulites boninensis, Asterocyclina sp. cf. A. penuria, and some other larger foraminifers. Correlation of cored and dredged samples indicates that the Daito Ridge is mainly composed of igneous, metamorphic, ultramafic, and sedimentary rocks. The igneous rocks are mafic (probably tholeiitic) and alkalic. The metamorphic rocks are hornblende schist, tremolite schist, and diopside-chlorite schist. The ultramafic rocks are alpinetype peridotites. Mineralogical data suggest that there were two metamorphic events in the Daito Ridge. The older one was intermediateto high-pressure metamorphism. The younger one was contact metamorphism caused by a Paleocene volcanic event, possibly related to the beginning of spreading of the west Philippine Basin. The ultramafic rocks suffered from the same contact metamorphism. During the Eocene, exposed volcanic and metamorphic rocks on the uplifted Daito Ridge may have supplied pebble clasts to the surrounding coast and shallow sea bottom. The steep slope offshore may have caused frequent slumping and transportation of the pebble clasts and shallow-water benthic organisms into deeper water, forming the conglomerates and sandstones treated here.

Petrology and mineral chemistry of 67667, a unique feldspathic lherzolite

Abstract: The petrography and mineral chemistry of 67667 lherzolite suggests cataclasis of a fine-grained high-temperature rock, perhaps formed as a cumulate in a high-level pluton. With the exception of the Sr content of plagioclase, the mineral chemistry fits with that of major rock types ascribed to the lunar crust. No evidence is found to favor a relationship between 67667 and present-day meteorites falling on the earth.

Petrographic and Geochemical Properties of Tephras in Deep Sea Drilling Project Cores from the North Philippine Sea

Abstract: Refractive index and chemical composition were determined for glass shards contained in more than 100 tephra layers in DSDP Leg 58 sediment cores collected in the Shikoku Basin, North Philippine Sea. The refractive index is consistent with chemical composition. Refractive index and total iron show a linear relationship. Tephra in Pleistocene and Pliocene sediments is mostly rhyolitic and dacitic (non-alkali), whereas tephra in the Miocene shows wide composition variations in the eastern part of the basin. Basaltic tephra is recognized in Miocene sediments at Sites 443 and 444, but not at Site 442, west of the other two sites. This indicates that the basaltic tephra came from eruption relatively close to those drill sites (perhaps the Kinan Seamounts and the Shichito-Iwo Jima volcanic arc), although the exact source has not been identified.