1,721,060 research outputs found
Geochronological and geochemical constraints on the genesis of Cu-Au skarn deposits of the Santa María de la Paz district (Sierra del Fraile, Mexico)
Full text linkCu-Au skarn deposits of the mining district of Santa María de La Paz are located in the Sierra del Fraile, in the northern part of San Luis Potosí, Mexico. Samples of four different intrusions and their related skarn deposits were taken underground, in order to obtain whole-rock and mineral geochemical data. U-Pb LA-ICP-MS zircon data indicate crystallization ages of ca. 37.6–35.0 Ma for skarn-related granitic and granodioritic stocks, which show dominant metaluminous to slightly peraluminous, high-K calc-alkaline compositions. These intrusions were emplaced in a post-Laramide continental arc setting under shallow conditions, as suggested by fast cooling inferred from K-Ar biotite ages of ca. 35–33 Ma. Skarn deposits were contemporaneous with emplacement and crystallization of stocks, as revealed by Re-Os molybdenite geochronological data of exoskarn rocks. Skarn rocks show a general depletion of LILE and REE and a slightly enrichment in HFSE in comparison with intrusive rocks. Pyroxene and garnet show dominant hedenbergite and andradite compositions, respectively. The association of hedenbergite, pyrite, chalcopyrite, native bismuth and pyrrhotite during skarn mineralization suggest reducing conditions, typical for Au but not for Cu skarns. Highest metal contents are concentrated in exoskarn rocks, including anomalies of Cu, Fe, Ni, Sn, Sb, Zn, W and Bi. Metal contents, ore minerals and environmental conditions of skarn rocks, together with whole-rock geochemical data of stocks and pyroxene-garnet compositions, reveal some singularities for the Sierra del Fraile deposits, as they exhibit characteristics of both Cu and Au skarn deposits but might be also compatible with Fe, W, and Zn skarns. Hence, skarn and associated vein deposits of the Sierra del Fraile might have a high potential not only for Cu-Au, but also for several base metals, as reported for comparable post-Laramide skarn and hydrothermal deposits of central and northern Mexico.Fil: Siegesmund, Siegfried. Universität Göttingen; AlemaniaFil: López Doncel, Rubén. Universidad Autonoma de San Luis Potosi; MéxicoFil: Sieck, Pascal. Instituto Potosino de Investigación Científica y Tecnológica; México. Universität Göttingen; AlemaniaFil: Wilke, Hendrik. Universität Göttingen; AlemaniaFil: Wemmer, Klaus. Universität Göttingen; AlemaniaFil: Frei, Dirk. University Of The Western Cape; SudáfricaFil: Oriolo, Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentin
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
Late Eocene to Early Miocene Andean uplift inferred from detrital zircon fission track and U-Pb dating of Cenozoic forearc sediments (15-18 degrees S)
No full textTiming, amount, and mechanisms of uplift in the Central Andes have been a matter of debate in the last decade. Our study is based on the Cenozoic Moquegua Group deposited in the forearc basin between the Western Cordillera and the Coastal Cordillera in southern Peru from similar to 50 to similar to 4 Ma. The Moquegua Group consists mainly of mud-flat to fluvial siliciclastic sediments with upsection increasing grain size and volcanic intercalations. Detrital zircon U-Pb dating and fission track thermochronology allow us to refine previous sediment provenance models and to constrain the timing of Late Eocene to Early Miocene Andean uplift. Uplift-related provenance and fades changes started around 35 Ma and thus predate major voluminous ignimbrite eruptions that started at similar to 25 by up to 10 Ma. Therefore magmatic addition to the crust cannot be an important driving factor for crustal thickening and uplift at Late Eocene to Early Oligocene time. Changes in subduction regime and the subducting plate geometry are suggested to control the formation of significant relief in the area of the future Western Cordillera which acts as an efficient large-scale drainage divide between Altiplano and forearc from at least 15.5 to 19 degrees S already at similar to 35 Ma. The model integrates the coincidence of (i) onset of provenance change no later than 35 Ma, (ii) drastic decrease in convergence rates at similar to 40, (iii) a flat-subduction period at around similar to 40 to similar to 30 Ma leading to strong interplate coupling, and (iv) strong decrease in volcanic activity between 45 and 30 Ma. (C) 2013 Elsevier Ltd. All rights reserved.German Science Foundation (DFG) [EY 23/14, Wo 362/39
Peneplain formation in southern Tibet predates the India-Asia collision and plateau uplift Reply
No full textCretaceous to Cenozoic evolution of the northern Lhasa Terrane and the Early Paleogene development of peneplains at Nam Co, Tibetan Plateau
No full textHighly elevated and well-preserved peneplains are characteristic geomorphic features of the Tibetan plateau in the northern Lhasa Terrane, north-northwest of Nam Co. The peneplains were carved in granitoids and in their metasedimentary host formations. We use multi-method geochronology (zircon U-Pb and [U-Th]/He dating and apatite fission track and [U-Th]/He dating) to constrain the post-emplacement thermal history of the granitoids and the timing and rate of final exhumation of the peneplain areas. LA-ICP-MS U-Pb geochronology of zircons yields two narrow age groups for the intrusions at around 118 Ma and 85 Ma, and a third group records Paleocene volcanic activity (63-58 Ma) in the Nam Co area. The low-temperature thermochronometers indicate common age groups for the entire Nam Co area: zircon (U-Th)/He ages cluster around 75 Ma, apatite fission track ages around 60 Ma and apatite (U-Th)/He ages around 50 Ma. Modelling of the thermochronological data indicates that exhumation of the basement blocks took place in latest Cretaceous to earliest Paleogene time. By Middle Eocene time the relief was already flat, documented by a thin alluvial sediment sequence covering a part of the planated area. The present-day horst and graben structure of the peneplains is a Late Cenozoic feature triggered by E-W extension of the Tibetan Plateau. The new thermochronological data precisely bracket the age of the planation to Early Eocene, i.e. between ca. 55 and 45 Ma. The erosional base level can be deduced from the presence of Early Cretaceous zircon grains in Eocene strata of Bengal Basin. The sediment generated during exhumation of the Nam Co area was transported by an Early Cenozoic river system into the ocean, suggesting that planation occurred at low elevation. (C) 2013 Elsevier Ltd. All rights reserved.German Research Foundation (DFG) [DU 373-5
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
