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Nature and age of pre-Variscan eclogite protoliths from the Low- to Medium-Grade Metamorphic Complex of north–central Sardinia (Italy) and comparisons with coeval Sardinian eclogites in the northern Gondwana context
No full textU–Pb zircon data on retrogressed eclogites sampled in the Giuncana locality from the Sardinian Medium-Grade
Metamorphic Complex yielded a weighted age of 454 ± 6 Ma. This is in agreement with U–Pb zircon ages of 453–460 Ma
obtained from eclogites from the High-Grade Metamorphic Complex. The Giuncana eclogites are very similar to the other
well-known Sardinian eclogites. All of the Sardinian eclogites show positive K, Rb, Ba, U and Pb anomalies and negative
Nb, La, Ce and Sr anomalies. Th is depleted in the Giuncana eclogites and enriched in those from Punta de Li Tulchi and
Punta Tittinosu. All these data reveal clear crustal contamination of the Sardinian Ordovician mantle. REE patterns typical of
normal mid-ocean ridge basalt (N-MORB) characterize all of the Sardinian eclogites. The supply of crustal and calc-alkaline
materials to the Sardinian mantle during the Ordovician is further confirmed by the fact that most Sardinian eclogites plot on
the left side and well above the mantle array in a Th/Yb v. Nb/Yb diagram. In the general Variscan framework of northern
Gondwana, the Sardinian eclogites are witness to the most recent back-arc basins generated by the northward opening of the
Rheic Ocean
Geochemistry of Giuncana eclogite, North Sardinia (Italy) and comparison with coeval Sardinian eclogites
No full textMetamorphic evolution and P–T path of the Posada Valley amphibolites: new insights on the Variscan high pressure metamorphism in NE Sardinia, Italy
Get PDFThe Posada Valley amphibolites in the Inner Zone of Variscan Sardinia are characterized by garnet porphyroblasts containing epidote, titanite, K–feldspar, quartz, clinopyroxene, amphibole and plagioclase concentrated in the garnet core. The matrix is made up of amphibole and plagioclase and fine–grained clinopyroxene and plagioclase. The amphibolites recorded an HP stage, a metamorphic re–equilibration, and a third stage under greenschist facies. P–T conditions of stage I (530–650°C/0.9–1.3GPa for garnet core and 570–690°C/1.0–1.4GPa for garnet rim) were obtained by P–T pseudosection modelling. P–T conditions for stage 2 (T=600–700 °C/P=0.5–0.8 GPa) were obtained applying the same approach to the most retrogressed samples. The Posada Valley amphibolites underwent a cooler evolution, with a thermal peak lower than that recorded by the northernwards eclogites from the High–Grade Metamorphic Complex of Sardinia.
The proposed geodynamic scenario starts with subduction of oceanic crust under the peri–Gondwanan terrane in Upper Devonian. The Posada Valley amphibolites were subducted up to maximum depths of 50 km. Subsequently they reached the subduction channel or were detached from the slab before the eclogite–facies conditions were reached
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Evoluzione metamorfica di rocce eclogitiche e granulitiche della Zona Assiale della Catena Varisica sarda, Sardegna NE
Get PDFThe High Grade Metamorphic Complex of the Axial Zone of the variscan Sardinian chain is characterized by the widespread occurrence of metabasite bodies and lenses within the migmatites and gneisses. These metabasite bodies, decametric to hectometric in size, show polyphase metamorphic evolution and preserve evidences of high temperature (granulite facies) or high pressure (eclogite facies) metamorphism.
This study is focused on the metabasites from NE Sardinia (eclogites and granulites) cropping out a few kilometers north of Olbia. A geological survey was carried out and a schematic geological map of the studied area, comprising Iles, Montiggiu Nieddu, Nodu Pianu and Sos Aranzos localities, was drawn. Several samples from the various lithologies were collected and then analyzed; the petrographic, mineral chemistry and geochemical data were used for the determination of P-T conditions by thermodynamic modelling and conventional geothermobarometry.
The eclogites cropping out at Iles locality are characterized by garnet and kyanite porphyroblasts surrounded by symplectitic and coronitic microstructures. Based on garnet compositional zoning and P-T pseudosection calculations, the P-T path of these rocks reveals a first prograde segment until baric peak P-T conditions of T = 620-690 °C and P = 2.0-2.3 GPa were reached in eclogite facies. Subsequently, the rocks underwent a slight temperature increase accompanied by a pressure decrease until they reached their thermal peak at HP conditions in the granulite/eclogite facies (T = 650-700 and P = 1.4-2.1 GPa). The resulting clockwise P-T path is similar to that observed for other Sardinian eclogites described in the literature.
The amphibolite lens at Mt. Nieddu consists of plagioclase-banded amphibolites and ultramafic amphibolites. The plagioclase-banded amphibolites, characterized by an alternation of withish plagioclase- and dark-green amphibole-rich layers, locally show centimetric-thick layers with several garnet porphyroblasts up to 1 cm in size. The microstructural investigations and thermodynamic modelling on the plagioclase-banded amphibolites and on their garnet rich layers allow to define the following P-T evolution; these rocks firstly recorded an isothermal pressure increase up to the metamorphic peak ( T = 690-740 °C, P = 1.3-1.4 GPa), before the retrograde equilibration in amphibolite facies. The resulting P-T path is anticlockwise.
The ultramafic amphibolites, which are featured by coronitic microstructures around igneous olivine and plagioclase, occur as a lens within the plagioclase-banded amphibolites. The coronitic microstructures consist of single or composite thin corona of orthopyroxene and clinopyroxene around olivine as well as by spinel + clinopyroxene symplectite and garnet coronas around plagioclase. All these minerals were partially replaced by amphibole. The geothermobarometric results obtained from the ultramafic amphibolites indicate P-T conditions of P = 0.2-0.5 GPa and T = 780-850 °C for the igneous crystallization and of P = 1.3-1.7 GPa and T = 680-730 °C for garnet growth. These results are in agreement with the anticlockwise P-T path reconstructed for the adjacent plagioclase-banded amphibolites.
The geochemical data an the P-T paths reconstructed for the studied metabasites are discussed in the geodynamic scenario of the variscan Sardinian chain. A new geodynamic model for the evolution of the studied rocks is also proposed
Variations on the Author
Get PDF“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
Get PDFWe 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
Get PDFWe 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
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