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Mesozoic Magmatism in the Ivrea-Verbano Zone and its geodynamic implications.
No full textabs. 10.1474, 01-0560.In the Southern Alps, Mesozoic magmatism is well documented in the Dolomite region and in the Lombard Pre-Alps. In the western portion of the Southern Alps, the Mesozoic magmatic activity has received much less attention.In the present study we report a review of geochronological evidences of the existence of Mesozoic magmatism in the portion of the Southern Alps west of the Lugano lake. The Mesozoic events are exclusively located: a) south of the Cremosina fault system; b) in the northeasternmost portion of the Ivrea-Verbano Zone.a) A suite of diorite-norite dykes occurring into the Baldissero mantle peridotite (Southern Ivrea-Verbano Zone) has been recently recognized. The age of dyke intrusion is currently constrained by: a) a two point mineral (plagioclase + clinopyroxene) best fit calculated from Sm-Nd isotopic data which yields a slope corresponding to an age of 18026 Ma, with a Ndi = 0.512804 and Ndi = 7.8 (Obermiller, 1984, PhD Thesis Un. Mainz, Germany); b) Re-Os model ages (Re depletion model age) between 140 and 190 Ma calculated on whole rock samples of the ambient peridotite (Mazzucchelli et al., 2004, EGU04 Geophys. Res. Abs., 6, 03966). Moreover a number of acid tuff layers from mm to several meters in thickness are present in the Mesozoic sedimentary cover of the Southern Alps from the Lugano to Biella area. They occur in the Crevacuore and Sostegno sedimentary succession, in the Villafortuna-Trecate oil field, and in the sedimentary cover of Monte San Giorgio (Ticino, Switzerland). In the Monte San Giorgio occurrence, high-resolution U-Pb zircon age gives 241±0.8 Ma (Mundil et al., 1996, Earth Planet. Sci. Letters, 141, 137-151). b) In the Basic Complex cropping out in the Finero and Val Grande area, most of the isotopic data invariably give Triassic or Early Jurassic ages. The Basic Complex in this area shows an antiformal structure, constituted by various cumulus rocks and gabbroic lithotypes (Internal Gabbro, Hornblende Peridotite and External Gabbro Units). At the core of the antiform, in tectonic contact with the rocks of the Complex, a mantle phlogopite-bearing peridotite occurs, whose metasomatic imprint was attributed to crustal components, deriving from a subducting slab. The age of metasomatism is Mesozoic [207 Ma - U/Pb on zircons from chromitites (Von Quadt et al., 1992, Ivrea-Verbano Zone Workshop, U.S. Geol. Survey Circular 1089, Abs., 20.); 226-177 Ma - Rb/Sr internal isochrons on amphibole and phlogopite pairs (Hartmann & Wedephol, 1993, Geochim. Cosmochim. Acta, 57, 1761-1782); 220 Ma - Ar/Ar on phlogopite (Hartmann & Wedephol, 1993)]. Magmatic, subeuhedral, pink crystals with oscillatory zoning in CathodoLuminescence (CL) from the External Gabbro Unit has been recently dated with SHRIMP (Peressini et al., 2005, this session). Magmatic growth of the zircons took place at 232±2 Ma and was overprinted at 214±5 Ma by a second event, dated by the rim-recrystallization ages. These ages are well in accordance with the literature data reported for the Basic Complex. In spite of the similar ages, the Basic Complex does not record any evidence of the metasomatic agent which affected the mantle peridotite. The Finero Basic Complex is in tectonic contact, marked by a high-temperature ENE shear-zone, with the Permian (Peressini et al., 2005, this session) relatively anhydrous mafic-ultramafic sequences occurring in Val Sesia and on the right side of the Val d'Ossola.Presently, the evidences of Mesozoic magmatism in the westernmost sector of the Southern Alps are confined by tectonic lineaments to the southernmost and northernmost portion, respectively. This put new constraints for the comprehension of the geodynamic reconstruction of the whole Southern Alps
Triassic U-Pb SHRIMP Ages on magmatic Zircons from the External Gabbro unit of the Finero mafic complex, Ivrea Zone, Western Italian Alps.
No full textabs. 236-38The northern part of the mafic-ultramafic Ivrea Verbano complex, the Finero region, differs from the rest of the complex in many features: petrology and geochemistry of mantle peridotites, stratigraphy, lithology and geochemistry of igneous bodies, relationships with the metamorphic sequence (Kinzigite Formation) into which the complex intruded. We provide evidence for a substantial difference also in the age of emplacement. The Mafic Complex (MC) in the Val Sesia area has been recently proved to have intruded between 283 and 289 Ma. At Finero, published Sm-Nd isochrones span 203-533 Ma, zircon ages span 208-549 Ma (Lu et al, 1997, Chem.Geol.140, 223-235, and ref. therein).We performed a zircon study on 5 samples from the External Gabbro unit (EG) of the complex. Three events are revealed by the SHRIMP U-Pb results on the 2 most representative samples, one of which has a composite population of magmatic and detrital zircons, clearly distinguished for grain-morphology, color and CL-pattern. Primary crystallization of the pink magmatic zircons was dated at 232±2 Ma; these were overprinted at 214±5 Ma by a second event, dated by the rim-recrystallization ages. A 280-to-310 Ma age peak is clearly, but poorly constrained by the colorless zircons (U<20 ppm), proving that older ages are preserved, but must be considered detrital. This is further confirmed by the ages recorded in the second sample, which yielded no magmatic zircons, but only metamorphic grains. As for the significance of the older age peak, zircon yield and degree of crustal contamination (Nd-Sr isotopic compositions) of the studied samples suggest that zircons and zirconium were inherited/digested from the country rock. This latter underwent a major event that opened and reset the U-Pb system in zircon around 300 Ma, compatible with ages in the MC of the Balmuccia sector.In the EG, though, no magmatic Carboniferous or Permian zircon was found: the EG was emplaced in Anisian-Ladinian time at 232Ma, and a separate thermal event took place in Norian time at 214 Ma (Staehle et al., SMPM 70, 1990; von Quadt et al., SMPM 73, 1993). The clearly documented Mesozoic igneous activity, distinct from the predominant Permo-Carboniferous magmatism in the Ivrea-Verbano, and the possibly Mesozoic age of mantle metasomatism at Finero (Grieco et al, 2001, J. Pet.52, 89-101) suggest revisiting the current interpretations of Mesozoic magmatism in the Southern Alps of Lombardy and Trentino regions
Triassic emplacement of the External Gabbro unit of the Finero mafic complex: U-Pb SHRIMP zircon ages and their implications for the Ivrea-Verbano Zone, Western Italian Alps.
No full textSRef-ID: 1607-7962/gra/EGU04-A-05072The Mafic Complex (MC) of the Ivrea Zone displays distinctive features in different sectors. Apart from petrology and geochemistry of the mantle peridotites (e.g. the strong metasomatism of Finero is absent in Balmuccia), the Finero region differs from the rest of the complex in stratigraphy, lithology and geochemistry of the mafic intrusives, and their relationships with the Kinzigite Formation, the metamorphic sequence into which the MC intruded. We give new evidence for a substantial difference also in the age of magmatism. The age of emplacement of the MC in the Val Sesia area has been recently proved to be homogeneous, between 283 and 289 Ma, after a thermal, upper-amphibolite event that affected the country rock at 320-to-310 Ma. At Finero, published Sm-Nd isochrones span 203-533 Ma, zircon ages span 208-549 Ma (Lu et al, 1997, Chem.Geol. 140, 223-235, Grieco et al, 2001, J. Pet. 52, 89-101). The more composite nature of the Finero sector is possibly hiding a more articulated geological evolution of the MC, as suggested by the presence of numerous high-T shear-zones (e.g., Kenkmann, 2000, J.Struc.Geol. 22, 471-487, Manckeltow etal., 2002, J.Struc.Geol. 24, 567-585, and ref. therein). We performed a detailed zircon study on samples from the External Gabbro unit of the complex, most of which have a homogeneous population of colorless zircons. The key for interpretation is one sample, bearing both magmatic, subeuhedral, pink crystals with oscillatory zoning in CL, and detrital zircons, rounded, small, colorless grains with blurred CL-patterns. The three events recorded by these zircons are clearly recognizable in CL, and were dated with SHRIMP U-Pb analyses. Magmatic growth of the zircons took place at 232+/-2 Ma and was overprinted at 214+/-5 Ma by a second event, dated by the rim-recrystallization ages. A 280-to-310 Ma age peak is clearly, though imprecisely constrained by the colorless zircons, proving that older ages are preserved, but must be considered detrital. Two problems arise: reason and significance of the older age peak. Together with the positive correlation between zircon yield and degree of crustal contamination (Nd-Sr isotopic compositions) of the studied samples, the U-Pb data allow to conclude that zircons and zirconium were inherited/digested from the country rock. The latter underwent a major event that reset the U-Pb system in zircon around 300 Ma, compatible with both magmatic and metamorphic ages in the MC of the Balmuccia sector. In the External Gabbro, though, no magmatic Carboniferous or Permian zircon was found: emplacement took place at 232 Ma, and a separate thermal event is dated at 214 Ma. The new data extend as far west as the Ivrea zone the evidence of the important Mesozoic magmatism of the central and eastern South Alpine. Mesozoic igneous activity in the Finero region is clearly distinct from the predominating Permo-Carboniferous magmatism in the Ivrea-Verbano Zone. Also, as for most gabbroic intrusions in the alpine-appenine system, ages in the Ivrea Zone cluster in two ranges, at either 285-310 Ma, or at 200-250 Ma. This distribution is thoroughly represented in the zircons from one sample only, the ages of which represent distinct episodes of heating, melting and metasomatism
Ladinian magmatism of the South Alpine: geochemical and isotopic data and their implications for the geodynamic setting of Adria
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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
The Sesia Magmatic System
No full textThe Ivrea-Verbano Zone (IVZ) and Serie dei Laghi (SdL) of northwest Italy, are two
lithostratigraphic units that constitute the deep- and the middle- to upper-crustal components,
respectively, of a tilted and exposed section through the pre-Alpine crust of northwest Italy. Both
units were significantly affected by a Permian igneous event, leading to underplating of the Mafic
Complex in the deep-crustal IVZ and intrusion of granitic bodies in the upper-crustal SdL, which is in
turn capped by volcanic, predominantly rhyolitic rocks. SHRIMP U-Pb zircon ages of volcanic rocks
(288 ± 2 to 282 ± 3 Ma), formation of granitic plutons in the SdL (289 ± 3 to 275 ± 5 Ma), and gabbro
in the IVZ (289 ± 3 to 286 ± 6 Ma) indicate that the onset of bimodal volcanism and granitic
plutonism was coincident with and probably triggered by intrusion of mantle-derived mafic melt in
the deep crust, and that volcanic activity and presence of granitic melt at depth persisted after
underplating had ceased. Collectively, all these coeval igneous rocks are grouped in the “Sesia
Magmatic System”, whose activity culminated with the collapse of a caldera at least 13 Km across. In
this paper we resume the most significant processes which affected the emplacement and igneous
evolution of the IVZ Mafic Complex, briefly discussing its relationships with the upper-crustal
igneous activity. Also, in the appendix we report a field guide for a two-days classic excursion across
the entire sequence
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