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Showing papers on "Petrography published in 1975"


Journal ArticleDOI
TL;DR: The only two Nakhlite meteorites, Nakhla and Lafayette, are identical in mineral composition, consisting of augite (Wo39En38Fs23), olivine (Fo32-35), plagioclase (An27), K-feldspar (Or75Ab22An3), titaniferous magnetite with exsolved ilmenite, iddingsite (?), and minor amounts of fluor-chlorapatite, FeS, pyrite, chalcopyrite and K-rich glass.
Abstract: The only two Nakhlite meteorites, Nakhla and Lafayette, are identical in mineral composition, consisting of augite (Wo39En38Fs23), olivine (Fo32–35), plagioclase (An27), K-feldspar (Or75Ab22An3), titaniferous magnetite with exsolved ilmenite, iddingsite (?), and minor amounts of fluor-chlorapatite, FeS, pyrite, chalcopyrite, and K-rich glass. The texture is suggestive of a cumulative origin.

151 citations


Journal ArticleDOI
TL;DR: In this paper, the authors argued that the geochemical and petrographic characteristics of the Troodos massif are compatible with its formation at the axis of a slow spreading ridge within a small, marginal ocean basin.
Abstract: A wide variety of data support the contention that the Troodos massif, Cyprus is a fragment of oceanic lithosphere formed at a constructive margin some 85 m.y. ago. However, Troodos rocks differ significantly in major and trace element content and mineral parageneses from those formed at present day active constructive margins. It is here argued that the geochemical and petrographic characteristics of the massif are compatible with its formation at the axis of a slow spreading ridge within a small, marginal ocean basin.

108 citations


Journal ArticleDOI
TL;DR: Upper mantle (garnet-peridotite facies) xenoliths from the Matsoku kimberlite pipe consist predominantly of olivine, orthopyroxene, clinopyroxenes and garnet, with a wide range of modal values.
Abstract: Upper mantle (garnet-peridotite facies) xenoliths from the Matsoku kimberlite pipe consist predominantly of olivine, orthopyroxene, clinopyroxene and garnet, with a wide range of modal values. In some xenoliths metasomatism, under the ambient mantle P.T. conditions indicated by all xenoliths, has added the primary-metasomatic minerals: phlogopite, ilmenite, rutile, pyrrhotite, pentlandite and chalcopyrite. This metasomatism has probably occurred within the kimberlite magma envelope at depth, and been caused by fluid derived from the kimberlite magma. Omitting metasomatic aspects, the xenoliths show a wide and broadly continuous range of textural varieties, which may be referred to three major groups: coarse-grained (showing restricted deformation and recrystallization), flaser (relatively strongly deformed and showing extensive recrystallization) and even-textured (extensively annealed following deformation). These form a progressive sequence of deformation and annealing, the latter involving recovery, recrystallization and grain growth. It is evident from the textural features, in conjunction with aspects of mineral chemistry, that the xenoliths cannot be in any simple sense cognate with the kimberlite magma, and that a wide variety of textural types occurs within a restricted location in the upper mantle. The deformation and annealing responsible for the textural variety are probably caused both by widespread mantle creep and more localized phenomena occurring within the kimberlite magma envelope at depth. Petrographic and chemical characteristics suggest that the banding seen in some xenoliths is dominantly of cumulate igneous origin.

91 citations


Journal ArticleDOI
TL;DR: Petrographic, rock magnetic, and paleomagnetic studies of fine-grained red sediments of late Cenozoic age in Baja California show that the sediments have variably acquired chemical remanent magnetization (CRM) overprints that have obscured the original magnetization of the deposits.
Abstract: Petrographic, rock magnetic, and paleomagnetic studies of fine-grained red sediments of late Cenozoic age in Baja California show that the sediments have variably acquired chemical remanent magnetization (CRM) overprints that have obscured the original magnetization of the deposits. The chemical remanence in the sediments is carried predominantly by three authigenic minerals — hematite, goethite, and a Mn · Ba compound, herein called “hydropsilomelane” — that occur as pigments and as concretions. Remanence directions generally associated with each mineral are goethite, reverse; hydropsilomelane, normal; and hematite, normal or reverse. Some samples possess a remanence that is strong and normal or strong and reverse; these generally contain only one of the authigenic minerals in abundance. Many samples, however, are weak in intensity and random in direction. When such samples are split into parts and measured, it is generally found that each part is strongly magnetized but that some parts are normal and others are reverse in direction. In such cases, the magnetization of the whole sample is resultant of the multiple components that are generally carried by two or more of the authigenic minerals. The following conclusions can be drawn concerning the acquisition of CRM in the Baja California deposits: (1) The sediments contain chemically unstable iron- and manganese-bearing minerals, such as hornblende and biotite, that are susceptible to postdepositional alteration, and they have provided the parent material for the authigenic magnetic minerals. (2) Authigenic magnetic minerals, growing from crystallites, generally acquired a remanence that was parallel to the Earth9s field when they surpassed the critical grain size. (3) The rate of CRM acquisition has not been uniform in these sediments, probably because the processes of alteration and formation of authigenic magnetic minerals depend on an interplay of highly variable factors such as the chemistry of the interstitial water, hydraulic gradients, and mineralogy of the sediments. (4) Complex variability in acquisition of CRM has led to a remanence stratigraphy that bears little discernible correlation with the geomagnetic field at the time of deposition.

82 citations


Journal ArticleDOI
TL;DR: The South Mountain batholith as mentioned in this paper is an epizonal complex of granodiorite, adamellite and alaskite which was emplaced after the folding and metamorphic events of the Acadian orogeny.
Abstract: Approximately one-third of western Nova Scotia is underlain by granitic rocks of the South Mountain batholith. The pi u ton is an epizonal complex of granodiorite, adamellite and alaskite which was emplaced after the folding and metamorphic events of the Acadian orogeny. Chemical and petrographic data suggest that the rocks comprise a single comagmatic suite related by the fractional crystallization of biotite and plagioclase. The magma for the batholith may have been produced by partial fusion of the lower crust, or through a hybridization of mantle and crustal melts.

68 citations


T. E. Bunch1
01 Jan 1975
TL;DR: In this paper, the results from petrographic observations, electron microprobe analyses of constituent phases, and defocused beam analyses of clasts and glasses were used to investigate the parent body implications of Howardites.
Abstract: The reported investigation is based on the results from petrographic observations, electron microprobe analyses of constituent phases, and defocused beam analyses of clasts and glasses. Howardites consist of mostly angular-to-rounded mineral, rock, and breccia clasts compacted in variable grain-sized matrices of finely comminuted materials. Attention is given to ophitic and subophitic rocks, granular basalts and microgabbros, mafic rocks, Al-rich rocks, breccia clasts, impact melt rocks, glasses, meteorite clasts, bulk composition, particle size distribution, howardite component composition, questions of breccia formation, and parent body implications. It is found that the four main rock types found in howardites do have certain analogies to lunar rocks. It is tempting to speculate that both the moon and the basaltic achondrite parent body formed similar rock suites in their evolutionary processes.

63 citations


Journal ArticleDOI
TL;DR: In this paper, a comparison of data from these three Holocene sequences demonstrates that the most effective environmental segregation based on composition and texture alone is in depositional systems characterized by intermediate or high energy--in systems with a significant proportion of sand-size detritus.
Abstract: The detrital mineralogy of sandstones is affected significantly by depositional environment. The imprint of the environment is recorded as the relative abundance and size of individual detrital species. Differences between environments within any one basin are accompanied by concomitant changes in detrital-sediment composition. Petrographic analyses of Holocene and ancient sedimentary sequences reveal that the environmentally produced compositional changes are process dependent, and are interrelated closely with textural variations. Fluctuations in rock-fragment content among environments indicate that the relative survivability of unstable detrital grains can influence the overall composition of sandstones. A total of 300 samples of varied grain sizes was selected from contrasting environments in three Holocene sedimentary sequences: (1) the Galveston barrier complex, Texas; (2) the lower alluvial valley of the Mississippi River; and (3) the Mississippi delta. In each sedimentary sequence, bivariate and multivariate analyses of petrographic data demonstrate the environmental sensitivity of both composition and texture. A comparison of data from these three Holocene sequences demonstrates that the most effective environmental segregation based on composition and texture alone is in depositional systems characterized by intermediate or high energy--in systems with a significant proportion of sand-size detritus. Four ancient sedimentary sequences, of different age and geographic location, w re selected to complement the Holocene sequences: (1) Wilcox (Eocene) fluvial rocks, Texas; (2) Wilcox (Eocene) deltaic rocks, Texas; (3) Lower Jurassic (Toarcian) marine rocks, England; and (4) Lower Cretaceous Muddy Sandstone marine, transitional, and nonmarine rocks, northeast Powder River basin, Montana and Wyoming. Petrographic data from each sequence demonstrate that the composition of recent detrital sediments and ancient sandstones is environmentally sensitive. Composition is also sensitive to textural changes. In sediments with a wide range of grain sizes, the relation between the abundance and size of detrital monocrystalline quartz is curvilinear. Compositional and textural data derived exclusively from thin-section analyses can be used jointly to segregate depositional environments within a single sedimentary basin, providing no source changes occur, and providing that certain basic sampling restrictions are adhered to. Thus, small samples such as sidewall cores and cuttings may be used in the delineation of depositional environments. Such small samples are available in abundance in the p troleum industry and, if analyzed petrographically, they can render invaluable assistance in the search for stratigraphic traps.

47 citations


Journal ArticleDOI
TL;DR: In this paper, the trace-element abundances are discussed in terms of the petrological problems, particularly the origin of calc-alkali magma, and the calcalkali rocks can be classified into three types which correspond to Kuno's three basalt-magma types.

44 citations


Journal ArticleDOI
TL;DR: Kimberlite breccias from several South African pipes or pipe-like enlargements on dykes contain abundant fragments of country rocks including huge, often brecciated, xenolithic masses which are located at positions well below their original stratigraphic levels.
Abstract: Kimberlite breccias from several South African pipes or pipe-like enlargements on dykes contain abundant fragments of country rocks including huge, often brecciated, xenolithic masses which are located at positions well below their original stratigraphic levels The xenoliths show no pyrometamorphic effects indicative of relatively high temperatures during their incorporation in the kimberlite A striking petrographic feature of these rocks is an emulsion-like texture resulting from the presence of numerous phlogopite-rich, globular segregations, which are set in a matrix which sometimes consists almost solely of primary calcite but in other instances is comprised of variable proportions of calcite and serpentine Evidence is presented which indicates that in the latter cases serpentine has replaced calcite The clear separation of the mica-rich segregations and the essentially calcitic matrix is attributed to the development of low temperature, immiscible K-rich silicate and carbonatitic liquids during emplacement of these rocks The carbonatitic liquids are considered to have acted as the primary transporting fluids during liquid-solid fluidized intrusion and emplacement of the rocks is considered to have taken place as follows After the crystallization of early phenocrysts the residual, ascending kimberlite magma differentiates at relatively high crustal levels into immiscible silicate and carbonatitic liquids accompanied by a coexisting gaseous phase Continued intrusion is accompanied by the separation of the gaseous phase and its accumulation at the head of the magma column Upon further uprise a stage is reached where the internal gas pressure exceeds the lithostatic load and the diatreme is formed by explosive breaching of the cap rocks The resulting rapid pressure drop is accompanied by the upsurge of partly degassed magma fractions which incorporate explosively disrupted cap rock fragments and material which slumps from the walls of the diatreme

43 citations


Journal ArticleDOI
04 Jul 1975-Science
TL;DR: Scanning electron microscopy reveals that carbonate grains in sublittoral sediments in the North Sea have surfaces with a microscopic etch relief, caused by dissolution of calcium carbonate, taking place at the shallow sea floor.
Abstract: Scanning electron microscopy reveals that carbonate grains in sublittoral sediments in the North Sea have surfaces with a microscopic etch relief. In structurally complex skeletal grains, the relief grades into marginal corrosion zones. The etch patterns are caused by dissolution of calcium carbonate, taking place at the shallow sea floor. Carbonate phases affected are calcite, aragonite, and a spectrum of magnesian calcites.

43 citations


Journal ArticleDOI
TL;DR: In the Huntly-Portsoy area, north-east Scotland, staurolite occurs in muscovite-free migmatized semipelites as mentioned in this paper.
Abstract: In the Huntly — Portsoy area, north-east Scotland, staurolite occurs in muscovite-free migmatized semipelites. This occurrence in the Sillimanite-potash feldspar Zone is petrographically distinct from lower-grade staurolite found nearby. Petrographic and chemical data indicate that the staurolite formed stably during the migmatization episode. It was stabilized by high zinc-content. Its formation was probably an early retrograde effect.


Journal ArticleDOI
TL;DR: A detailed description of the petrology and mineralogy of polymict breccia 14321 was given in this paper, which was returned from the Fra Mauro formation on the Apollo 14 mission.


Journal ArticleDOI
TL;DR: In this paper, a petrographic and electron microprobe study of an interbedded calc-mica schist from Gassetts, Vermont reveals the complexities of Fe-Mg crystalline solution and gradients in XH2O/XCO2 during regional metamorphism.
Abstract: A petrographic and electron microprobe study of an interbedded calc-mica schist from Gassetts, Vermont reveals the complexities of Fe-Mg crystalline solution and gradients in XH2O/XCO2 during regional metamorphism. The common association of microcline+diopside ± zoisite formed from biotite +calcite+quartz may be produced by continuous (Fe-Mg) exchange reactions, despite the implied metastability in the CaO-K2O-MgO-Al2O3-SiO2-H2O-CO2 system. Different assemblages are produced in a reaction zone between carbonate and pelite beds of varying thickness. This illustrates the importance of exhaustion of fluid buffering assemblages due to mineral reactions proceeding on a local scale. Siliceous dolomites at the same metamorphic grade have produced significantly different assemblages to the interlayered carbonatepelite sequence.

Journal ArticleDOI
TL;DR: Petrographic and chemical data on five types of intrusive rocks of the so-called Pioneer batholith in the northeastern Pioneer Mountains of southwest Montana show that the rocks range from quartz diorite to granite as mentioned in this paper.
Abstract: Petrographic and chemical data on five types of intrusive rocks of the so-called Pioneer batholith in the northeastern Pioneer Mountains of southwest Montana show that the rocks range from quartz diorite to granite. The volumetrically most important body is a coarse-grained biotite-hornblende “granite” that superficially resembles the Butte Quartz Monzonite of the Boulder batholith, some 60 km to the northeast. Their K 2 O content puts the rocks into Tilling9s “sodic series” of the Boulder batholith. K-Ar dating on biotite and hornblende shows that the date of intrusion of all but the quartz diorite is about 70 m.y. The hornblende age of the quartz diorite indicates that it was intruded 76.5 m.y. ago. Its biotite age of 70.5 m.y. agrees with those of the younger intrusions. High-grade contact metamorphic rocks of the Silver Hill Formation (Cambrian) at one locality yielded the same 70-m.y. age on biotite porphyroblasts. These ages compare closely with those of the Boulder and Philipsburg batholiths and show the same tendency for the less mafic rocks to be younger.

Journal ArticleDOI
TL;DR: In this paper, a petrographic study of the oil-bearing carbonate rocks of the Aptian series in Alamein oil field, the overlying sealing rocks, and the underlying carbonate beds and sandstones of Middle Jurassic age indicated that the dolomites of the AlAMEin are of syngenetic origin and were subjected to a diagenetic dedolomitization process by volume replacement rather than by molecular replacement.
Abstract: In petroleum potential the northern part of the Western Desert of Egypt has become one of the most promising localities of Egypt since the discovery of the Alamein oil field by the Western Desert Operating Petroleum Company (WEPCO). The oil production from the Alamein field comes from Aptian rocks. The Aptian series can be subdivided into two lithostratigraphic units. The lower unit is essentially clastic, whereas the upper one is mainly carbonate rock and commonly is known as the Alamein Dolomite. A petrographic study of the oil-bearing carbonate rocks of the Aptian series in Alamein oil field, the overlying sealing rocks, and the underlying carbonate beds and sandstones of Middle Jurassic age indicated that the dolomites of the Alamein are of syngenetic origin and were subjected to a diagenetic dedolomitization process by volume replacement rather than by molecular replacement. This study revealed that the petrographic characteristics of the Alamein Dolomite are important in the migration and accumulation of petroleum, as well as the source-reservoir relation in the Western Desert of Egypt petroleum province.


Journal ArticleDOI
TL;DR: Petrographic and trace element data on volcanic ash beds from ten southern California localities indicates that the ashes are correlative with the Bishop tuff as mentioned in this paper, and most of the ash beds occur in Colorado River delta sediments or in sediments in structurally controlled basins along strands of the San Andreas fault system.
Abstract: Petrographic and trace element data on volcanic ash beds from ten southern California localities indicates that the ashes are correlative with the Bishop tuff. Most of the ash beds occur in Colorado River delta sediments or in sediments in structurally controlled basins along strands of the San Andreas fault system.

01 Jan 1975
TL;DR: In this paper, an unusually high concentration of granitic fragments, with textures ranging from holocrystalline to glassy, occurs throughout Boulder 1, a complex breccia of highland rocks from Apollo 17, Station 2.
Abstract: An unusually high concentration of granitic fragments, with textures ranging from holocrystalline to glassy, occurs throughout Boulder 1, a complex breccia of highland rocks from Apollo 17, Station 2. Among the minerals included in the granites are enigmatic K-Ca-rich feldspars that fall in the forbidden region of the ternary diagram. The great variability in chemistry and texture is probably the result of impact degradation and melting of a granitic source-rock. Studies of the breccia matrix suggest that this original granitic source-rock may have contained more pyroxenes and phosphates than most of the present clasts contain. Petrographic observations on Apollo 15 KREEP basalts indicate that granitic liquids may be produced by differentiation without immiscibility, and the association of the granites with KREEP-rich fragments in the boulder suggests that the granites represent a residual liquid from the plutonic fractional crystallization of a KREEP-rich magma. Boulder 1 is unique among Apollo 17 samples in its silica-KREEP-rich composition. We conclude that the boulder represents a source-rock unlike the bedrock of South Massif.

Book ChapterDOI
01 Jan 1975
TL;DR: The groundmass texture is intersertal with average grain size 75-100μ. Groundmass Plagioclase (An 45) occurs as fresh laths, interstitial pyroxene occurs as fibrous to skeletal, altered grains as discussed by the authors.
Abstract: 278-35-3,25-35 KT Feldspar-phyric basalt. 21% large phenocrysts of Plagioclase zoned from mottled cores of An 64 composition to An 30 rims. Most phenocryst compositions are in range An so55. Phenocrysts up to 3 mm in long dimension, often occurring in glomerophyric patches up to 5 mm in size. Many phenocrysts contain irregularshaped inclusions of devitrified dark brown glass. Vesicles are rare, usually rounded and rimmed by devitrified dark brown glass, fibrous antigorite, and filled with platy calcite. Irregular vesicles are partly to wholly filled with yellowish serpentine. Groundmass texture is intersertal with average grain size 75-100μ. Groundmass Plagioclase (An 45) occurs as fresh laths, interstitial pyroxene occurs as fibrous to skeletal, altered grains.

Journal ArticleDOI
TL;DR: In this paper, an extensive study of a big number of gneiss specimens with various shock features from the suevite allowed unravelling of the shock behavior of almandite garnets.

Journal ArticleDOI
TL;DR: Weathered whole rock samples of five lithic units from Maine, Virginia, North Carolina and South Carolina were analyzed to evaluate the effect of chemical weathering on the Rb-Sr ages of the original rocks.



Journal ArticleDOI
01 Jan 1975-Lithos
TL;DR: The cordierite-gedrite-cummingtonite as discussed by the authors rocks occur in the zone of sulphide mineralization and are associated with the chlorite ± garnet schists.


Journal ArticleDOI
TL;DR: The origin of carbonate breccia nappes has been investigated in this paper, where the authors show that the coarse grain size and granoblastic texture of calcite marble imply an origin by metamorphism of at least moderate grade.
Abstract: Sheets of carbonate breccia and unbrecciated calcitic carbonate rocks, called carbonate breccia nappes, occur as tectonic slices in nappe piles dominated by pelitic rocks of Early Mesozoic age in the Carson Sink region of Nevada. The most extensive carbonate breccia nappe, the Muttlebury nappe, crops out over 100 $$km^{2}$$. Such sheets are composites of a number of different carbonate rock types, including widespread calcite marble. The coarse grain size and granoblastic texture of the marble imply an origin by metamorphism of at least moderate grade. The degree of apparent recrystallization of marble, however, contrasts markedly with that of most of the limestone with which it is intercalated and is independent of degree of metamorphism, locally nil, of pelitic rocks in adjacent nappes. If the marble is a product of thermal metamorphism, its grade is seemingly incompatible with that of associated rocks. On the basis of field and petrographic studies, the origin of the marble is enigmatic. Carbon and oxy...

01 Jan 1975
TL;DR: In this article, samples of fine 1-mm and 155 1-2 mm particles from several Apollo 17 sites were analyzed for Na, Sc, Cr, Mn, Fe, Co, Ni, Hf, Ta, Th and REE.
Abstract: Samples of fines less than 1-mm and 155 1-2 mm particles from several Apollo 17 sites were analyzed for Na, Sc, Cr, Mn, Fe, Co, Ni, Hf, Ta, Th, and REE. Products of comminution and construction are present in the 1-2 mm particles, and the compositions of the rock fragments clearly indicate the general chemical characteristics of their parent rock types. The likely sources of materials for the glassy particles are considered. Glasses are enriched over their parent soils in Fe, Sc, Mn, and Cr, and are relatively enriched in light REE, so that some chemical fractionation accompanies glass-forming processes. Elements were determined by instrumental neutron activation analysis.

01 Jan 1975
TL;DR: A chemical and petrographic study of the phases from the rock 78235 which was returned on the Apollo 17 mission is reported in this paper. But the analysis of the thin sections from the bounder confirm its cumulate origin.
Abstract: A chemical and petrographic study is reported of the phases from the rock 78235 which was returned on the Apollo 17 mission. Petrographic analysis of the thin sections from the bounder confirm its cumulate origin. In order to develop further the crystallization history for 78235, its subsequent shock history, and its relationship to other lunar crustal rocks, orthopyroxene, plagioclase, glass, and whole-rock samples were prepared and analyzed for major, minor, and trace elements. It is speculated that an early fractional crystallization event producing a layer of orthopyroxene-plagioclase cumulate with varying amounts of trapped liquid took place within 20 km of the surface of the moon.