Showing papers on "Petrography published in 1970"

Polytypism of Chlorite in Sedimentary Rocks

Abstract: Four chlorite polytypes of Bailey and Brown (962) have been identified by X-ray diffraction in clay-size chlorites of soils, sediments, and sedimentary rocks: (1) IIb, the polytype of common metamorphic and igneous chlorites; (2) Ib(s = 90°); (3) Ib(s = 97°); (4) Ia. An additional stacking arrangement. Iba, is defined herein as disordered chlorite which lacks an h0l diffraction band in the 2.4–2.5 A region. Most type-I chlorites are authigenic, as demonstrated by thin-section petrography. Type-I chlorites form during diagenesis, or less commonly during halmyrolysis, at temperatures and pressures less than those of low-grade metamorphism. A type-1 crystallization sequence is proposed, from least to most stable: Iba → Ib(s = 97°) → Ib(s = 90°). Conditions of low-grade metamorphism usually are necessary to cause conversion of Iba(s = 90°) to IIb, the most stable and common polytype. Chemical composition has little or no influence upon polytype relative stabilities; temperature is much more important. Sediment source areas with high relief, abundant rainfall, cold climate, and which contain IIb-chlorite-bearing metamorphic rocks, may yield essentially unweathered IIb chlorite to sites of deposition. Thus, clay-size IIb chlorite in unmetamorphosed sedimentary rocks can be interpreted as detrital. Caution is required, however, because IIb may be able to form authigenically at submetamor-phic temperatures, because it is the most stable polytype. Petrographic evidence is useful in such cases. Chlorite polytypism as a geothermometer can be applied to several geologic problems: (1) the authigenic versus detrital origin of clay minerals in sedimentary rocks, particularly in graywacke matrix; (2) the recognition of diagenetic facies or gradients, areally and stratigraphically, within given geologic provinces; (3) the detection of hydrothermal and incipient metamorphic effects. Chlorite polytypism merits general application as an interpretive tool.

Compositional relations in minerals from kimberlite and related rocks in the Moses Rock dike, San Juan County, Utah

Abstract: The Moses Rock dike, a well-exposed, kimberlite-bearing breccia intrusion, crops out in gently dipping beds of the Permian Cutler Formation, in eastern Monument Valley, Utah. Petrographic, bulk chemical, and electron microprobe analyses of kimberlite and its constituent minerals reveal this highly serpentinized microbreccia contains a primary mineral assemblage consisting of olivine (Mg/Mg+ Fe), 87 to 93), orthopyroxene and clinopyroxene (falling into two compositional ranges after correction for Na-pyroxene molecules-one with Al_2O_3 between 0.5 and 1 percent another, 2 to 5 percent), spinel, chrome-rich pyrope garnet, ilmenite-geikielite, titanoclinohumite and one or more micas. Diamonds are not known. We conclude (1) mineral grains in kimberlite are unlike associated dense rock fragments, except rare lherzolite: (2) kimberlite was emplaced as discrete angular mineral clasts, not a silicate melt; (3) P-T assignments based on clinopyroxenes compositions suggest derivation over a depth range in the upper mantle extending to 150 km or more, at temperatures near or below the experimentally determined garnet-lherzolite solidus: (4) the kimberlite was derived by physical disaggregation of both Al-poor and Al-rich pyroxene bearing peridotite in the mantle (garnet- and spinel-lherzolite, respectively); (5) titanoclinohumite is present in both assemblages and may be an important mineralogical site for volatiles in the upper mantle: (6) dense rock fragments (except lherzolite) are unrelated to the kimberlite and are chunks of the vent wall from the crust and possibly the upper mantle sampled during the eruption.

Tenoumer crater, Mauritania - Age and petrologic evidence for origin by meteorite impact

Abstract: Inclusions of granitic basement rocks, collected from the sparse ‘lava’ exposed at the 1.8-km-diameter Tenoumer crater, Mauritania, exhibit distinctive petrographic features considered indicative of shock-wave action and diagnostic for meteorite impact. The observed effects include (1) distinctive planar features in quartz, oriented parallel to ω{ 101¯3 } and to other specific planes; (2) intense selective vesiculation of quartz and felspar grains; (3) presence of lechatelierite apparently formed by fusion of quartz grains; and (4) partial fusion and decomposition of biotite grains. These effects are quite distinct from those produced from conventional fusion of granitic xenoliths in lavas. The evidence from these inclusions supports the theory of a meteorite impact origin for the Tenoumer crater. The lava, which is chemically similar to the basement rocks and which has apparently been rapidly quenched, is interpreted as an impact melt formed by fusion of the basement rocks at the time of impact and injected into fractures in the crater wall, carrying with it highly shocked inclusions of basement rock. Values of Sr87/Sr86 determined for the lava are approximately 0.720; they are almost identical with values determined for basement rock samples and are too high to represent primary melt derived from the mantle. Potassium-argon age determinations on the melt rock establish the age of the Tenoumer crater as 2.5±0.5 m.y.

Petrology, provenance and silica diagenesis of the Penrith sandstone (lower Permian) of northwest England

Abstract: The Penrith Sandstone (Lower Permian) of north west England is a continental red bed sandstone, deposited as barchan sand dunes in a hot, arid desert environment. The rock displays a high degree of mineralogical and textural maturity, being a coarse, well sorted orthoquartzite, composed of rounded unstrained, strained and polycrystalline quartz grains, with subordinate feldspar and rock fragments. Distinct petrographic features of the detrital grains suggest derivation from the Millstone Grit (Upper Carboniferous) sandstones of Yorkshire, and perhaps County Durham. In the southern part of the area non-silicified sandstones are interbedded with calcite cemented alluvial fan breccias (brockrams). This contrasts markedly with northern areas where brockrams are absent, in that the sandstones are tightly cemented with secondary quartz, occurring as overgrowths of optically continuous, bipyramidal quartz crystals around detrital grains. Many varieties of overgrowth are present, their structure being pre-determined by the internal structure of the detrital grains. Thus, unicrystaline unstrained and strained, and polycrystalline overgrowths can be recognised. Overgrowths of secondary feldspar occur occasionally on detrital orthoclase and microcline. It is suggested that the source of the secondary quartz is due to the solution of the siliceous dusts produced during aeolian abrasion of the quartz grains. Alkaline desert groundwaters are considered to be responsible for taking the siliceous dust into solution, which, upon evaporation, precipitated the silica as optically continuous quartz overgrowths around the detrital grains. The inverse relationship between silicification of the Penrith Sandstones and brockram distribution is attributed to differences in compositon of the groundwaters in the two facies.

Lithologic Controls on the Development of Solution Porosity in Carbonate Aquifers

Abstract: Movement of ground water in maturely karsted limestone aquifers is commonly through solution conduits of considerable size. The object of this study was to determine the lithologic controls on the distribution of conduits within a heterogeneous sequence of carbonate rocks. The volume of all accessible caves more than 100 feet in length in the Nittany Valley area of central Pennsylvania was measured. Most caves are entirely within limestones, cave development in dolomite is extremely rare. Within the limestone sequence the bulk of the cave volume is concentrated in a few members. Chemical and petrographic analyses of the carbonate rocks were associated with cave volume by bar graphs and components analyses. The more cavernous limestones seem to be those with low amounts of dolomite, clay, and other impurities, and a high micrite grain fraction. Coarse limestones and dolomites are less cavernous.

Kimberlite and Related Rocks: A Critical Reappraisal

Abstract: A discussion of the most important petrographic features of kimberlite is presented and a new definition of kimberlite is proposed. The discussion emphasizes those features of kimberlite which are of petrogenetic significance and points out that alnoitic and lamprophyric rocks associated with alkaline rock-carbonatite complexes are not true kimberlites. Consideration of the proposed relation of kimberlites to mica peridotites, carbonatites, basalts, potassic volcanic rocks, and melilitites reveals that kimberlites are not simply or closely related genetically to any of these rocks.

The Green Beds of the Scottish Dalradian Series: Geochemistry, Origin, and Metamorphism of Mafic Sediments

Abstract: The Green Beds of the Scottish Dalradian Series are greenschists and amphibolites of metasedimentary origin. Field relations, petrography, and chemical data are used to demonstrate that these rocks, approximately of basaltic composition, were derived by metamorphism of basic tuffs deposited in an area of pelite sedimentation. The Dalradian Epidiprites, a series of orthoamphibolites associated with the Green Beds are less siliceous and less magnesian, in general, than the metasediments. Pelitic material is more abundant in the Green Beds of the Southwest Highlands than to the northeast. Amphibole occurrence and abundance are related to calcium content of the rocks. Fifty-eight metasediments and three Epidiorites were analyzed for thirty elements each, by X-ray fluorescence methods.

Anatomy of a Meteorite Impact Crater: West Hawk Lake, Manitoba, Canada

Abstract: West Hawk Lake is a near-circular, 11,700-ft -wide, deep lake located 110 mi east of Winnipeg, Canada. The lake now completely submerges an ancient meteorite impact crater. A central drillhole passes through nearly 1080ft of slump and fallback breccia into 655 ft of rupture zone bedrock below a well-defined crater base. Petrographic analysis of core from this hole, a hole near the original crater rim, and an off-center hole reveals numerous shock features which, supplementing earlier geophysical studies, demonstrate an impact origin for this structure. These results are significant: (1) The original crater was approximately 8000 ft wide; the rim has recessed by erosion; the target rock consists of steeply dipping units of metasediments and metavolcanics. (2) Microscope criteria for shock include: abundant planar features in quartz; diagnostic parallel extension fractures; extensive isotropization of tectosilicates in quartz-mica schists (analcite is a common product of recrystallization of the isotropized phases); variable decomposition of biotite and actinolite; abundant glassy fragments and matrix-binding “melt,” now generally devitrified; shock-induced twinning and granulation in pre-impact carbonates. (3) Distribution of shock features is strongly dependent on mineral composition and particle size. Unless intensely shocked, amphibole-rich rocks are usually devoid of shock evidence, even where fragments are intermixed with shocked schistose metasediments. (4) The vertical distribution within the central hole core of intensity of shock in the breccias, as plotted on a shock log, is notably erratic. (5) Autobrecciation, cataclastic fragmentation, and injection of shocked breccia into open fractures account for observed effects in the rupture zone. (6) Zoisite and secondary calcite develop extensively within the breccia lens by solution and neo-mineralization; serpentine var. antigorite forms with the breccia matrix, and in a network of veinlets in fragments and in the rupture zone; sodium-calcium zeolites form in possibly replaced melt material at the crater base. (7) Analysis of strongly shocked rock fragments from the breccias by infrared absorption, X-ray fluorescence, electron microprobe, and argon loss methods demonstrates significant changes in the crystal structure and chemistry of constituent minerals in these materials as compared with unshocked equivalents collected off-structure. West Hawk Lake is compared with the Brent structure, the best-studied model of a simple impact crater in crystalline rocks.

Luminescence petrography and properties of lunar crystalline rocks and breccias

Abstract: Apollo 11 lunar crystalline rock and breccias petrography and luminescent properties, spectral analysis, color, emission bands and shock effects

Petrology of some eclogites and metagabbros of the Oetztal Alps, Tirol, Austria

Abstract: Petrographic, chemical and mineralogical data are presented on the Oetztal eclogites and their co-existing minerals. The available evidence indicates that they constitute the metamorphic derivates of an original gabbroic rock, the plagioclase and clinopyroxene of which reacted to form the garnet, omphacite and kyanite components of the eclogites. According to the available subsolidus experimental data these reactions are believed to have taken place in a 6–10 kb pressure range at about 550°–750° C.

Petrogenesis of Mylonites of High Metamorphic Grade in the Peninsular Ranges of Southern California

Abstract: A fairly continuous, narrow belt of mylonite gneisses extends approximately 60 miles across southern California and crops out prominently at Coyote Mountain, near Borrego Springs, San Diego County. At Coyote Mountain, both prebatholithic rocks and igneous rocks lithologically similar to rocks from the nearby southern California batholith have been deformed in the mylonite zone—a deformation that is the last plutonic event recorded in the rocks. Petrographic evidence within these mylonites at Coyote Mountain indicates that sillimamte-K feldspar-muscovite-quartz assemblages remained stable or recrystallized (or both) during mylonitization. In addition, a maximum “set” temperature of 580° C, inferred from the MgCO 3 content of calcite in deformed dolomite marbles, was determined. The physical conditions probable at the metamorphic peak accompanying mylonitization are: P-T conditions inferred from experimentally studied systems, T = 580° to 660° C and Pxotai = PH, O( ? ) = 3.4 to 7.0 kb; high H 2 O activity, as indicated by the continued stability of muscovite at such high temperatures. Mylonitization within this belt may be related to a rise of magma to higher crustal levels from within the southern California batholith.

Mineralogy and petrology of coarse particulate material from lunar surface at tranquillity base.

Abstract: Five grams of coarse fines (10085,11) contains 1227 grains, mostly mafic holocrystalline rock fragments, microbreccia, and glass spatter and agglomerates with less abundant anorthosite fragments and regularly shaped glass. The crystalline lithic fragments in the coarse fines and microbreccias represent a closely related suite of gabbroid igneous rocks that have a wider range of modal analyses and textures than seen in the larger crystalline rock samples returned by Apollo 11. Petrographic evidence of shock metamorphism is common, and the abundant glass is almost all shock-produced. None of the glass observed is similar to tektite glass.

Moessbauer spectroscopy of Apollo 11 samples

Abstract: Apollo 11 lunar dust, breccia and igneous rocks, using Mossbauer spectroscopy and petrographic techniques

High voltage /800 kV/ electron petrography of Type B rock from Apollo 11

Abstract: Apollo 11 lunar rock clinopyroxene, plagioclase and ilmenite internal substructure, using high voltage transmission electron microscopy

Petrography and origin of lunar breccias and glasses

Abstract: Petrography and shock vaporization origin of Apollo 11 lunar breccias and glasses compared to terrestrial impactites and chondrites

Electron microprobe analysis of Apollo 11 lunar samples

Abstract: Electron microprobe and petrographic analyses of crystalline rock and separates from apollo 11 lunar soil samples

Main aspects of the epigenesis problem

Abstract: SUMMARY This paper presents the main trends in epigenetic research in the U.S.S.R. in the last years: 1 Study of epigenetic zonality in thick series of sedimentary rocks of various ages and tectonic regions, (zones of unaltered clay matrix, of altered clay matrix, of quartzitic structures and hydromica-chloritic cement, and of prick-like structures and phyllite-like schists). 2 Principles of stadial analysis and mineral indicators of different epigenetic stages (clay minerals, organic matter, zeolites, feldspars, sulfides). 3 Facies of regional epigenesis connecting the main petrographic families of unaltered terrigenic rocks (quartz sandstones with kaolinitic matrix and the same composition of associated clays, acidic arkoses with hydromicaceous clays, lithoclastic sands with several matrix components and associated clays, and volcanogenic greywackes) with “related” facies of regional metamorphism. 4 Some practical aspects of epigenesis are: for petroleum geology: maximum depth of burial according to porosity, interruption of quartz, feldspar, sphene, epidote, carbonate, biotite, and clay mineral epigenesis by oil migration; and for ore geology: (a) initial epigenesis: ore impregnation (e.g., volkonskoite in wood remains) due to intrastratal solution of heavy minerals and leaching of rock fragments; (b) deep epigenesis: e.g., galena impregnations due to release of trace elements during dissolution of rock forming minerals e.g., feldspars; and (c) metagenesis: “Alpine” veins across and along bedding, filled, e.g., with copper ores.

Mössbauer Spectrometry as an Instrumental Technique for Determinative Mineralogy

Abstract: As a result of current availability of Mossbauer spectra for a large number of minerals, including examples from many of the important groups of rock-forming minerals, it is at present feasible in many cases to examine and interpret the Mossbauer spectra of rocks, sediments, soils, and other multiphase natural mineral aggregates. We have measured Mossbauer spectra for a considerable number of specimens of igneous rocks and in addition for samples of marine sediments, and a selection of such spectra are presented. Comparisons are made between the preliminary results of Mossbauer spectrometry on standard silicate rocks and data from chemical analyses and modal information from petrographic examinations. Further topics include total iron determinations, ferric-to-ferrous ratio measurements, improvement of petrographic modal information using Mossbauer spectrometry, identification of igneous rock types on the basis of Mossbauer spectral patterns, detection and identification of coarse-grained magnetite and hematite, with applications in the area of paleomagnetism, and the characteristics of Mossbauer spectra of marine sediments.

Late Tertiary Paleoclimate of Eastern United States

Abstract: Paleoclimatic information for the Miocene and Pliocene is scarce or lacking in eastern United States because of a general absence of fossils in the nonmarine upper Tertiary sedimentary rocks. Information previously available was based chiefly on floral evidence from a few widely scattered locations in the Atlantic and Gulf coastal plains, or represented extension of climatic zones which were determined from fossiliferous units in the Mid-Continent or Western Interior. X-ray and petrographic information from land-derived Miocene and Pliocene deposits in northeastern and southeastern United States clearly reflects climatic conditions during the late Tertiary and verifies the conclusion that subtropical conditions did prevail as far north as northern New Jersey during the Miocene and Pliocene. In addition, mineralogic evidence from Miocene sedimentary rocks in the Gulf Coast strongly indicates that the sediments deposited on the Florida platform were not derived from material eroded in the Rocky Mountain drainage basin, as previously has been proposed, but instead represent detritus carried southward by rivers from the southern Appalachians and Piedmont and distributed by longshore currents.

Micropaleontological studies of lunar samples.

Abstract: Optical and electron microscopic studies of rock chips and dust from the bulk sample box returned by Apollo 11 and of petrographic thin sections and acid-resistant residues of lunar material have yielded no evidence of indigenous biological activity. vestigations.

Selected Porphyroidal and granitic rocks at Tennant Creek, Northern Territory

Abstract: The Lower Proterozoic sedimentary succession of the Farramunga Group, Northern Territory, Australia is composed essentially of shales, gre~rackes and conglomerates with minor haematite shales and cherts having an estimate total thickness of 25,000 feet. At Tennant Creek, the lower part of the succession contains conformable horizons, discontinuous lenses, dykes and other isolated bodies of porphyroidal rocks. The rocks show varying degrees of recrystallization and deformation due to an incipient greenschist facies regional metamorphism. Two porphyroidal bodies have been studied in detail and are generally similar with respect to petrography, mineralogy and chemistry. They contain between 28% and 45% of megacrysts of quartz, potassium feldspar, plagioclase and minor biotite set in recrystallized, dominantly quartzofeldspathic groundmasses in which relict textural patterns indicate the former presence of close-packed shards, perlitic cracks and possible amygdaloidal patches and identify the porphyroids as volcanic pyroclastics. The Great Western Porphyroid forms a grossly conformable, perhaps composite, horizon at least twelve miles in length and up to 1,500 feet thick and is of ash-flow origin. The Creek Bed Porphyroid is half a mile long, several tens of feet thick and appears to be a dyke. The shape of the embayments and cavities, extensively developed in the quartz and potassium feldspar megacrysts, appear different from the more regular skeletal or dendritic patterns which are known to result from irregular growth. The close association between curved grain perimeters and embayments suggests that the two are genetically connected and is consistent with weak magmatic corrosion of euhedral grain habits resulting the embayments and cavities. The presence of Carlsbad and Baveno twinning in the potassium feldspar further supports an igneous derivation for these mineral grains. Optical and X-ray data indicate that both the potassium feldspar and plagioclase are in a highly-ordered structural condition. In conformity with their structural states, the compositions of the potassium feldspar and plagioclase are consistent with recrystallization at temperatures typical of greenschist facies metamorphic, rather than magmatic, condition. The major element compositions of the porphyroids are similar to those of calc-alkaline rhyolites apart from a variable alkali ratio and a low CaO content due to the redistribution of K, Na and Ca, probably as a result of hydrothermal alteration. The petrographic, mineralogical and chemical similarity of the porphyroids to high-level granites (and selected enclaves) of the neighbouring Tennant Creek Complex suggests a genetic association. Compositional and structural data on the feldspars are compared throughout the various environments of porphyroid, granite and enclave. The major element composition of the potassium feldspar and the structural states of both the potassium feldspar and the plagioclase are generally similar in the different rock types. This is a result of secondary process of hydrothermal alteration and/or metamorphic recrystallization,which make it impossible to correlate primary order-disorder and compositional feature with environment of occurrence. Delicate, oscillatory zoning preserved in the andesine from the different rock types reveals that the similar plagioclase composition is an original magmatic characteristic. It is concluded that the close similarity of the Rb, Sr and Ba abundances in the potassium feldspar from the various environments is a consequence mainly of the generally similar abundances of these elements in the porphyroid, granite and enclave bulk compositions.

Quantitative mineralogy of an Apollo 11 lunar sample

Abstract: Apollo 11 lunar rock mineralogy, examining petrographic and chemical features by light microscopy, electron microprobe microanalysis and detector system

Petrography and Geochemistry of some Granitoids associated with Gold Mineralisation at Mpohor area in southeastern Ashanti Belt of the Birimian, Ghana

Abstract: Mpohor area in Western Region of Ghana is located within the Birimian at the southeastern corridor of Ashantibelt. Diamond drilling in the area which encloses Father Brown concession intersected steeply dipping igneous rocks on which petrographic and whole rock XRF analyses were conducted to describe the rock types according to mineralogy, texture, alteration and mode of mineralisation. The study showed that gabbro and diorite are weakly deformed, partially altered and of greenschist facies. These rocks intruded into belt granitoids which accompanied metavolcanic rocks (well exposed at a coastal region at the southeast). Subsequent tonalite and granodiorite intrusives, into sheared areas of gabbro are deformed with foliation being parallel to the major foliation in the host rock. This ductile deformation was accompanied by brittle deformation and infiltrated by hydrothermal fluids leading to silicification, carbonatisation, sericitisation, pyritisation of second and third generations and gold mineralisation. On the contrary, chloritisation with fine chalcopyrite together with pyrite of first generation characterise host gabbro and diorite. Hence deformed pyrite porphyroblast of second generation is found in tonalite and partly in contact with magnetite and also associated with gold. The ore zone (Au grade between 1.78 to 48.80 g/t) in tonalite is characterised by lower TiO 2 content (<0.73 wt %) in addition to Fe/Mg ratio above 2.4 and Zr values less than 100 ppm which may suggest that mineralised fluid was depleted of Ti and Zr and accompanied the more mafic host rocks.

Petrology of Coal, Burnt Coal and Para Lava from Singrauli Coalfield, M. P. & U. P

Abstract: Extensive drilling operations carried out by the Geological Survey of India and the Indian Bureau of Mines, in the north-eastern portion of Singrauli coalfield, MP and VP, have revealed the existence of several thick and persistent coal seams, named as Kota, in the Karharbaris, Turra and Purewa from the bottom, in the Barakars, and Jhingurdah Top and Jhingurdah Bottom in the Raniganj formations Petrological studies of the coals representing these seams in cores and from the quarry faces, have been carried out The Karharbari and Barakar coals are composed of mainly durite, 'intermediates', and fusite, while the Raniganj coals are essentially vitrinite-rich, being composed of a greater proportion of vitrite, clarite and the' intermediates' than durite and fusite The mineral matter is thoroughly disseminated in the organic layers, and it is doubtful if mechanical washing methods can improve the quality of coal The petrographic composition is also unfavourable for making metallurgical coke The sporinite content is quite high and may be useful for hydrogenation The Singrauli coals are similar in petrographic composition to the coals of the adjacent coalfields of Son valley basin It is also inferred that the Singrauji basin formed a part of the large spread of the Gondwana sediments from Talcher in the east to Singrauli in the west, forming the Son-Mahanadi basin The burnt coal occurs at the expected outcrop position of the Purewa seam in asso ciation with para lava -fused Barakar sandstone The presence of cristobalite indicates that temperatures up to 1300°C were reached during spontaneous combustion of the coal seam