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Compositional variations of Plio-Quaternary magmatism in the circum-Tyrrhenian area: deep versus shallow mantle processes
No full textThe Tyrrhenian Sea area is the site of complex Plio-Quaternary volcanic activity whose products range from mafic to felsic and from subalkaline to ultra-alkaline. Mafic rocks display variable trends of incompatible-element abundances and ratios that depend on the geographic distribution of volcanoes and delineate several distinct magmatic provinces. These are bounded by important tectonic lines and show distinct geophysical characteristics in the mantle-crust system. The mafic rocks from the various igneous provinces display moderate internal isotopic variations. However, when considered collectively, they define continuous trends that connect distinct extreme compositions. These include high (i.e., high U/Pb) mantle (HIMU), depleted MORB mantle (DMM), and enriched mantle type 1 (EM1) end-members and a strongly radiogenic 87Sr/86Sr, unradiogenic 143Nd/144Nd, and mildly radiogenic 206Pb/204Pb composition that resembles upper-crustal estimates. The smooth trends of radiogenic isotopes have been interpreted as mixing involving deep reservoirs emplaced into the upper mantle by uprising plumes. However, the plume hypothesis is unnecessary and insufficient to explain the compositional characteristics of the igneous rocks in the Tyrrhenian area. Extremely heterogeneous and anomalous upper-mantle compositions and the occurrence of abundant rocks with arc geochemical signatures (i.e., high large ion lithophile element / high field strength element ratios) are best explained by multiple episodes of mantle metasomatism, mostly linked to subduction processes. These likely occurred at various stages in the complex geodynamic evolution of this area, which has been affected by various phases of rifting and subduction over the last 300 m.y. DMM compositions likely derive from depleted mantle sources, which have suffered extraction of mafic melts during rifting phases. The EM1 and upper crustal-like compositions may reveal introduction of crustal material into the upper mantle during pre-Hercynian or Hercynian and Alpine orogenies, respectively. The origin of the HIMU-type composition is more controversial. Its occurrence in most of the Cenozoic igneous rocks in western-central European and in the circum-Mediterranean area requires a wide geochemically uniform reservoir, whose nature is still debated but may represent a rather homogeneous asthenosphere
Tertiary To Present Evolution Of Orogenic Magmatism In Italy
No full textAbstract: Tertiary to present magmatism in Italy is related to the convergence between the African and European
plates. The Alpine orogeny and magmatism are the result of eastward and southward subduction of the Tethys ocean
basin beneath the Adriatic continental plate and of the continental collision and post-collisional relaxation along the
Alpine arc. The Apennine chain and associated magmatism developed as a result of west-directed subduction of the
Adriatic plate beneath the southern European margin. The bulk of the magmatism during this complex geodynamic
evolution is related to melting of mantle sources that were modified by subduction processes. These magmas, which
are referred to as “orogenic”, show high enrichments in Large Ion Lithophile Elements and relative depletion in High
Field Strength Elements. A significant amount of magmas, however, are related to melting of mantle sources that were
not contaminated by subduction. These magmas are referred to as “anorogenic” and have low LILE/HFSE ratios.
Eocene magmatism in Italy was concentrated in the Alpine area, with emplacement of both orogenic and anorogenic
magmas. Orogenic igneous activity may have started also in Sardinia.
In the Oligocene, the climax of Alpine orogenic magmatism was reached. Compositions range from calcalkaline to
ultra-potassic (lamproitic), the latter being restricted to the Western Alps, where subduction of upper continental crust
is documented. Calcalkaline to shoshonitic activity in the Eastern Alps continues up to Late Oligocene.
Younger activity in Italy is essentially related to the Apennine subduction zone. In Sardinia, the bulk of calcalkaline
and high-K calcalkaline activity started around ~28 Ma, and reached its climax at about 21-18 Ma, probably
continuing until ~12 Ma. Successively, orogenic magmatism shifted eastward and southeastward forming several
centres in the Tyrrhenian basin and in the Italian peninsula. At the same time anorogenic magmatism (tholeiitic to Naalkaline)
developed in several places behind the Apennine compression front (Sardinia and Tyrrhenian Sea basin), and
along the northern margin of the Africa foreland (Eastern Sicily and Sicily Channel).
Relationships between magmatism and geodynamics in Italy are complex. Orogenic magmatism is sometimes coeval
with subduction, suggesting mantle wedge melting under the effect of water-rich fluids released by the undergoing
slabs. In other cases, magmatism is younger than subduction, resulting from post-collisional decompression melting of
contaminated mantle wedge.
Petrological and geochemical compositions of orogenic magmatism suggests involvement of variable types of upper
crustal material in its genesis. This was added to the mantle sources during subduction, and was particularly
abundandant in the Tuscany and Roman provinces. Mantle components of deep origin were also involved in the
orogenic magmatism especially along transversal lithospheric faults cutting the subduction front, or in backarc
volcanoes. Asthenosphe and deep-mantle-plume have been suggested as possible sources for these components, but
such an issue is poorly understood and still subject of debate
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
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
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