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Brecce serpentinitiche nel massiccio ultrabasico del Monte Avic (Falda Ofiolitica Piemontese): possibili evidenze di erosione sottomarina
No full textThermobarometry of phengite-bearing eclogites in the Glacier-Rafray Austroalpine unit, Italian Western Alps. Looking for the geothermal gradient of a fossil accretionary prism.
No full textGeochemistry of eclogitised Fe-Ti gabbros from different lithological setting (Aosta valley ophiolites, italian western Alps). Protolith composition and eclogitic paragenis
No full textMantle-cover sequence in the Western Alps metaophiolites : a key to recognize remnants of an exhumed Oceanic Core Complex (OCC)
No full textAlpine ophiolites represent the sutured Jurassic Tethys ocean interposed between the European and African plates. They mostly consist, similarly to modern oceanic lithosphere at slow spreading ridges, of mantle peridotites intruded by gabbros sealed by basaltic lavas and sediments (e.g., Lagabrielle & Cannat, 1990). A true sheeted dike complex has never been recognized. In modern slow spreading ridge systems, mantle rocks and gabbros are commonly exposed at the seafloor along detachment shear zones which exhume Oceanic Core Complexes (OCC). In the hangingwall of detachment faults, syn-tectonic sediments bearing mafic/ultramafic breccias unconformably rest atop mantle rocks or ophicarbonate breccias. In the southern Aosta valley, the Mount Avic serpentinite massif is interpreted as a fossil OCC (Fontana et al., 2013), as attested by the occurrence of ophicarbonate breccias (Tartarotti et al., 1998 and refs.). In the nearby Champorcher valley a primary mantle-cover sequence is tentatively reconstructed. Here, massive to mylonic serpentinite pass upward to ophicarbonate breccias covered by carbonate-rich serpentinitic metarenites and by a chaotic rock unit on which calcschists, embedding cm-sized clasts of actinolite/tremolite-schists, rest unconformably. The chaotic rock unit has a block-in-matrix fabric with rounded to irregularly-shaped blocks of serpentinite, cm-to several dm in size, randomly distributed within a matrix of foliated impure marbles and calschists. The internal fabric of this metasedimentary succession is well consistent with mass-transport processes related to an active tectonic setting in which mantle rocks were progressively and continuously exhumed by faulting. This mantle-cover sequence may represent part of the Mount Avic OCC documenting, as for Balestro et al. (2015) for the Monviso meta-ophiolite Complex, one of the first examples of remnants of OCCs preserved in exhumed eclogitic ophiolites. The correct interpretation of this mantle-cover sequence is crucial for better interpreting intra-oceanic processes that controlled the Tethys ocean floor evolution during the Jurassic extensional stage
Geochemistry of eclogitised Fe-Ti-gabbros from different lithological settings (Aosta Valley ophiolites, Italian Western Alps). Protolith composition and eclogitic paragenesis
No full textEclogitised Fe-Ti metagabbros were collected from the Piemonte ophiolites of the Aosta valley (Italian Western Alps). These rocks are either associated with Mg-metagabbros or serpentinites, and show the same homogeneous metamorphic imprint. Petrographic observations on pseudomorphic replacement in undeformed pods and on mineral assemblages in tectonitic portions of the Fe-Ti metagabbros reveal the presence of three different high-pressure mineral parageneses. The first includes garnet, omphacite and rutile; the second consists of garnet, omphacite and glaucophane; and the third contains garnet, omphacite, tremolite and clinozoisite. Bulk rock chemical analyses show that all the studied samples derive from similar magmatic protoliths with tholeiitic affinity. However, differences in the amounts of major elements, namely TiO2, Na 2O, CaO and REE (particularly LREE) were found between the Fe-Ti metagabbros associated with Mgmetagabbros and those occurring with serpentinites. Different chemical compositions account for the three distinct eclogitic parageneses which developed in the two groups of metagabbros. We infer that both magmatic processes and rock alteration in an oceanic environment may have contributed towards producing these chemical differences. We suggest that Fe- Ti gabbros crystal lizing from similar magmatic protoliths, as a consequence of the different extent of alteration in the oceanic environment, developed distinct mineral assemblages during subduction-related Alpine highpressure metamorphism. This interpretation indicates that the early pre-Alpine history of the oceanic lithosphere in the Piemonte basin played an important role in determining the petrographic and geochemical features of its metamorphic products
Fossil mantle-sediments interface recognized in the Western Alps metaophiolites : a key to unravel the accretion mechanism of the Jurassic Tethys ocean
Full text linkIn the southern Aosta Valley (Italian Northwestern Alps), meta-ophiolites are mainly composed of serpentinized mantle-derived peridotites intruded by gabbros and rodingitic dykes, well exposed in the Mount Avic area, and of smaller amounts of mafic rocks and metatrondhjemite. This rock assemblage recalls the "slow-spreading" lithosphere created at modern mid-ocean ridges. Meta-ophiolites show a dominant early Alpine subduction-related metamorphic imprint under eclogite/blueschist facies conditions, variously retogressed under greenschists facies conditions. In the high Champorcher Valley (SW of Mount Avic) serpentinites are directly covered by a serpentinite mélange followed by flysch-like calcschists with detrital ophiolitic interbeds. Despite the pervasive Alpine tectonic deformation and metamorphic recrystallization through subduction-related stretching and boudinage and collision-related folding, the mélange internal fabric still retains records of a block-in-matrix structure, well consistent with mass-transport processes related to an active oceanic tectonic setting in which mantle rocks were progressively and continuously exhumed by faulting. The products of mass-transport processes and faulting are unconformably sealed by flysch-type calcschists embedding cm-sized clasts of actinolite/tremolite-schists interpreted as detrital ophiolitic material. The serpentinite mélange is interpreted as syn-extensional sedimentary rocks produced at the mantle-sediments interface on the Jurassic Tethys ocean floor and subsequently overprinted by subduction zone tectonics
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
Record of Jurassic mass transport processes through the orogenic cycle: Understanding chaotic rock units in the high-pressure Zermatt-Saas ophiolite (Western Alps)
Full text linkThe eclogite facies Zermatt-Saas ophiolite in the Western Alps includes a composite chaotic unit exposed in the Lake Miserin area, in the southern Aosta Valley region. The chaotic unit is characterized by a block-in-matrix texture consisting of ultramafic clasts and blocks embedded within a carbonate matrix. This unit overlies massive serpentinite and ophicarbonate rocks and is unconformably overlain by layered calcschist. Despite the effects of subduction and collision-related deformation and metamorphism, the internal stratigraphy and architecture of the chaotic unit are recognizable and are attributed to different types of mass transport processes in the Jurassic Ligurian-Piedmont Ocean. This finding represents an exceptional record of the preorogenic history of the Alpine ophiolites, marked by different pulses of extensional tectonics responsible for the rough seafloor topography characterized by structural highs exposed to submarine erosion. The Jurassic tectonostratigraphic setting envisioned is comparable to that observed in present-day magma-poor slow- and ultraslow-spreading ridges, characterized by mantle exposure along fault scarps that trigger mass transport deposits and turbiditic sedimentation. Our preorogenic reconstruction is significant in an eclogitized collisional orogenic belt in which chaotic rock units may be confused with the exclusive product of subduction-related tectonics, thus obscuring the record of an important preorogenic history. © 2017 Geological Society of America
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