1,721,014 research outputs found
Petrogenesis of oligocene plutonic rocks in western Anatolia (NW Turkey): Insights from mineral and rock chemistry, Sr-Nd isotopes, and U-Pb, Ar-Ar and (U-Th)/He geochronology
No full textAN INVESTIGATION OF NATIVE COPPER IN PLAGIOCLASE, LAKE AND HARNEY COUNTIES, OREGON
Full text linkMafic igneous rocks are commonly associated with various metals and there are many examples of basalts that host small amounts of native copper. Such copper is more common among the matrix phases of basalts and could represent a final stage of a melt or a secondary alteration. Worldwide, there are only a few occurrences of copper within early-crystallizing phenocrysts of plagioclase, and this fact makes the ‘sunstones’ of Oregon as interesting as they are spectacularly beautiful. The native copper in these ‘sunstones’ occurs as thin platelets (copper schiller) with crystallographically-controlled orientations. These copper platelets appear to have formed via the exsolution of metallic copper and are typically found in the cores of the highest-grade gemstones. The age of the sunstone host basalts has previously been uncertain. Also, there has been little petrographic or geochemical characterization of the basalts hosting the sunstones. The objectives of this thesis were to determine the age and provide an improved petrographic, petrologic and geochemical characterization of the host basalt. These labradorite megacrysts (~An67) have strikingly homogeneous major and trace element distributions and internally homogeneous 87Sr/86Sr ratios, similar to those observed in plagioclase phenocrysts of the Columbia River Basalt Group's Steens Basalt (~16.7 Ma). The homogeneous nature of all these data suggests that following copper exsolution, the crystals have not experienced significant chemical change (diffusive mass transport, alteration, weathering). This research determined the age of the basalt hosting sunstones. The means of four matrix plateaus is 9.16±0.12 Ma (95% c.l., MSWD=1.13). These late Miocene plateau ages are comparable to lavas of the High Lava Plains Trend but are distinctly younger than the Steens Basalt. In addition, this research illustrated a simple sunstone development hypothesis
Metamorphism, magmatism, and exhumation history of the Tavşanlı Zone, NW Turkey: new petrological constraints
No full text40Ar/39Ar Ages of Feldspar and Muscovite from the Source and Detritus of the French Broad River, North Carolina
Full text linkAs the westernmost metamorphic belt of the Appalachians, the Blue Ridge has been the subject of many geochronology studies. The Blue Ridge experienced high-grade deformation and peak metamorphism during Taconic orogeny, followed by a low-grade metamorphic overprint during the Acadian orogeny. The Alleghanian orogeny is the last collisional stage of the Appalachians and associated regional metamorphism and ductile deformation is documented along most of the Piedmont and the Carolina Slate belt. There is still debate, however, as to the extent of Alleghanian metamorphism in the western Blue Ridge. This concern is made more difficult to evaluate because previous work generally did not characterize the history of low-temperature metamorphism of the Blue Ridge in the region between western North Carolina and Tennessee.
To address the cooling history of the Blue Ridge, samples were collected in the area of the French Broad River catchment in North Carolina. Single crystals of muscovite from basement and stream sediment samples and K-feldspar from the basement, were dated in this project to avoid the ‘inherited’ ages often associated with high-temperature geochronometers. Muscovite from basement rock samples in the catchment area yield single crystal 40Ar/39Ar ages that typically range from 315 Ma to 400 Ma. K-feldspar crystals from basement rocks yield single crystal ages as young as ca. 270 Ma and up to ca. 1100 Ma. Results for the two mineral phases show similar age distribution patterns: easterly basement samples, within and near the Brevard fault zone, yield Carboniferousage distributions characterized by simple, single modes, but basement samples collected near and west of Asheville are more complex. The incremental heating experiments for K-feldspar show variable discordance. The variation in ages of microcline and orthoclase crystals from a single basement sample collected west of Asheville covers a wide range of 800 million years from late Mesoproterozoic to Permian. Comparing the results of the present study to published data for higher temperature thermochronometers (e.g., U-Pb ages for zircon, 40Ar/39Ar ages for hornblende and micas), it is remarkable that the low temperature record of K-feldspars can be used to characterize a greater range of cooling history rather than the higher temperature thermochronometers. The 40Ar/39Ar age signature of detrital muscovite mineral samples collected along the French Broad River catchment becomes dramatically more complex within lower grade rocks downstream (northwest). Our work on basement samples in the catchment shows the complexity is not only due to the increasing sediment input of various local tributaries into the trunk stream, but also to the intra-sample complexity of polymetamorphic history recorded in the metamorphic rocks west of the Brevard fault zone
40Ar/39Ar age variations among cogenetic feldspars from the Benson Mines, New York
Full text linkThe alkali feldspar group constitutes one the most common mineral groups of Earth’s crust. The 40Ar/39Ar method for dating potassium feldspars has proven to be a powerful analytical tool for evaluating low-temperature thermochronologic histories. Conventionally, for a vast number of studies, 40Ar/39Ar ages are determined for bulk concentrates of alkali feldspar of a particular grain size, separated and prepared following crushing of the host rock. In early studies of orthoclase from the Benson Mines in the northwestern Adirondack Highlands, Foland (1974) concluded that argon diffusion occurred over the physical scale of orthoclase crystals, a result reaffirmed by more recent studies of Foland & Xu (1990) and Cassata & Renne (2013). If the diffusion length scale is defined by physical grain size, the crushing and grain size reduction requisite for the analysis of bulk feldspar samples can sensibly be expected to alter and misrepresent the natural distribution of 40Ar ⃰.
The primary objective of the present study is to obtain 40Ar/39Ar ages of cogenetic orthoclase crystals for different lithologies of the Benson Mines. Argon analyses were accomplished via the incremental heating of mineral grains with either a CO2 laser, or a diode laser in conjunction with a thermocouple to control temperature. To further assess the impact of grain size and other physical characteristics on resultant 40Ar/39Ar ages, this study utilizes the analysis of single crystals of potassium-bearing mineral phases from lithologies recollected at the Benson Mines. Lithologies sampled from the Benson Mines include magnetite-rich orthoclase-sillimanite gneiss, magnetite-rich orthoclase-garnet gneiss, biotite-hornblende syenite, and pegmatite. In addition to orthoclase from various samples, single crystals of hornblende and biotite were also analyzed, and yield plateau ages of ca. 970 Ma and ca. 900 Ma, respectively. These results are consistent with a cooling history (≤ 1oC/m.y.) through the retention temperatures of various phases following late stages of the Mesoproterozoic Grenville orogeny.
Argon release spectra produced from step-heating single crystals of orthoclase in the present study reveal significant discordance between cogenetic crystals of individual samples and also systematic differences in age between feldspar populations of different samples. Notable variation is also observed at the scale of individual grains between initial and final heating steps. Release spectra for single crystals of orthoclase from the sillimanite gneisses yield the oldest 40Ar/39Ar ages observed and generally are less discordant between crystals, considering all of the lithologies analyzed, with total gas ages ranging from 875 to 850 Ma. A highly perthitic feldspar from a hornblende syenite yields much younger ages ranging from ca. 750 to 670 Ma. In contrast, K-feldspars from post-metamorphic pegmatites yield the most discordant spectra for individual crystals, with ages ranging from ca. 800 to 500 Ma and total gas ages ranging from ca. 765 to 677 Ma. Differences in the age distributions observed for feldspars of a given rock (more or less discordance, or differences in absolute age) are consistent with analysis of fragments differing in size and shape that present variable representations of their natural grain-scale diffusion geometries. The younger and more discordant ages for the feldspars from the syenite and pegmatite are interpreted to result from a higher local water activity during cooling than was the case in the gneisses. Results of the present study expand the earlier work of Foland and others for the Benson Mines, and emphasize the importance of analytical strategies to recover natural diffusion gradients in radiogenic 40Ar* concentrations at the scale of individual feldspar grains
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Geology of the 1:24,000 Roanoke East Quadrangle and investigations of the Long Island Creek Gneiss within the southernmost Appalachians, Alabama and Georgia
No full textThe geology of the 1:24,000 Roanoke East, Alabama, Quadrangle has been determined to be of high mapping priority by the State Geologic Mapping Advisory Committee due to the rapid development along the I-85 corridor west of Atlanta, Georgia. Objectives for this study are: (1) to map and describe lithologies and their distributions; (2) to analyze structures and fabrics; (3) to produce a vector ArcGIS geologic map of the Roanoke East, Alabama, Quadrangle; (4) to characterize the Long Island Creek Gneiss through both geochronological and geochemical analyses, and; (5) to synthesize the geologic history. The key findings are seven-fold. (1) The lithologies of the Jacksons Gap Group within the Roanoke East Quadrangle are not easily divided into separate, mappable units as a result of their lithological similarities and their inter-gradational nature. The current author divides the lithologies of the Jacksons Gap Group within the Roanoke East Quadrangle into two main lithofacies types: a structurally lower section defined mostly by garnetiferous-graphitic-quartz-biotite schists and phyllites interlayered with micaceous quartzites; and an upper section of variably graphitic, garnetiferous-sericite-chlorite schists and phyllites, with no interlayered quartzites. The Long Island Creek Gneiss intrudes these units. (2) The lithologies of the Jacksons Gap Group within the Roanoke East Quadrangle are also not easily distinguishable from lithologies of the immediately adjacent units (the Emuckfaw Group and schists within the Waresville Schist). Outcrops of cataclasites defining the Abanda and Katy Creek faults bounding the Jacksons Gap Group were not observed and, hence, placing the upper and lower boundaries of the package proved to be a difficult task. (3) The current author defines the upper limit of the Emuckfaw Group within the Roanoke East Quadrangle based on the presence/lack of metagraywackes, which occur together with rare, thin amphibolites. I also have defined the lower limit of the Waresville Schist by the occurrence of amphibolites, interpreting them to be part of the metavolcanics of the Dadeville Complex. This contrasts with the placement of the same contacts by workers in Georgia, who instead place these amphibolites within lithologies of the Brevard shear zone. (4) The Long Island Creek Gneiss that intrudes the Jacksons Gap Group does not appear to cross over the boundaries of the Brevard shear zone within the area of the Roanoke East Quadrangle. (5) Early D1 fabrics and lithologic contacts are locally truncated at the Katy Creek fault where syn- to late-D1 fabrics parallel it, implying the juxtaposition of the Dadeville Complex and Jacksons Gap Group during a syn- to late-D1/M1 event. An inverted metamorphic gradient along the Katy Creek fault may have been formed as a result of down-heating that occurred during the emplacement of the hot Dadeville Complex on top of the cooler Jacksons Gap Group. (6) The age of Rock Mills Granite Gneiss remains to be constrained geochronologically, but it is younger than the 466 Ma Waresville Schist it intrudes (VanDervoort et al., 2017). (7) Based on available evidence, the Long Island Creek Gneiss is a highly fractionated melt emplaced at 293.1 +/- 5.3 Ma during the end of the period of Alleghanian plutonism spanning between ~330 and 295 Ma (Lin, 2015), placing a maximum on the timing of right-slip movement along the Brevard shear zone
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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