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Petrological and tectono-magmatic significance of ophiolitic basalts from the Elba Island within the Alpine Corsica-Northern Apennine system
No full textTwo distinct ophiolitic units, which represent remnants of the Jurassic Ligurian-Piedmont Ocean, crop out in the Elba Island. They are the Monte Strega unit in central-eastern Elba and the Punta Polveraia-Fetovaia unit in western Elba. Ophiolitic rocks from the Monte Strega unit are commonly affected by ocean floor metamorphism, whereas those from the Punta Polveraia-Fetovaia unit are affected to various extent by thermo-metamorphism associated with the Late Miocene Monte Capanne monzogranitic intrusion. Both ophiolitic units include pillow lavas and dykes with compositions ranging from basalt to basaltic andesite, Fe-basalt, and Fe-basaltic andesite. Basaltic rocks from these distinct ophiolitic units show no chemical differences, apart those due to fractional crystallization processes. They display a clear tholeiitic nature with low Nb/Y ratios and relatively high TiO2, P2O5, Zr, and Y contents. They generally display flat N-MORB normalized high field strength element patterns, which are similar to those of N-MORB. Chondrite-normalized rare earth element patterns show light REE / middle REE (LREE/MREE) depletion and marked heavy (H-) REE fractionation with respect to MREE. This HREE/MREE depletion indicates a garnet signature of their mantle sources. Accordingly, they can be classified as garnet-influenced MORB (G-MORB), based on Th, Nb, Ce, Dy, and Yb systematics. We suggest that the Elba Island ophiolitic basalts were generated at magma starved, slow-spreading mid-ocean ridge. REE, Th, and Nb partial melting modelling show that the compositions of the relatively primitive Elba Island ophiolitic basalts are compatible with partial melting of a depleted MORB mantle (DMM) source bearing garnet-pyroxenite relics. Hygromagmatophile element ratios suggest that basalts from both ophiolitic units were originated from chemically very similar mantle sources. A comparison with basalts and metabasalts from Alpine Corsica and northern Apennine ophiolitic units shows that the composition of the inferred mantle source for the Elba Island basalts is similar to that of some Lower Schistes Lustrés metabasalts of Alpine Corsica ophiolites, and some basalts from the Internal Ligurian units of northern Apennine. In contrast, it slightly differs from those of other ophiolitic units of Alpine Corsica and northern Apennine. The chemical differences observed between basalts and metabasalts from different Ligurian-Piedmont ophiolitic units were likely associated with different partial melting degrees of either DMM source or garnet-pyroxenite relics and/or different mixing proportions of melts derived from them, as well as to different compositions of garnet-pyroxenite relics
Petrology and tectonic setting of Corsica ophiolitic basalt and comparison with those of the Northern Apennine: Implication for the geological evolution of the Corsica-Apennine orogenic system
No full textBoth Alpine Corsica and Northern Apennine represent a key area for the study of the ophiolites originated in the Piedmont-Ligurian oceanic basin. In fact, the coexistence in the Alpine Corsica of Ligurian-type ophiolitic units (characterized by absent or modest metamorphic imprinting) and Piedmont-type units (with HP/LT metamorphism) is of particular interest for the reconstruction of the genesis of this oceanic sector, as well as of its consumption during the early orogenic phases.
Many of the Corsica ophiolitic units have been object of recent structural-stratigraphic and petrologic studies. The Balagne, Nebbio, Pineto, and Rio Magno units display typical Ligurian-type features, whereas the Schistes Lustrès, the Inzecca units, and several units of the Castagniccia have typical Piedmont-type features. The S. Angelo di Tenda unit singularly display greenschistes facies metamorphism, thus differing from all the other Corsica ophiolites. This unit could be comparable to the less metamorphosed Inzecca unit.
The petrological data of Corsica ophiolitic basalts indicate a T-MORB affinity for the Balagne-Nebbio unit and N-MORB affinity for all other ophiolitic units. Ophiolitic basalts of both the Tyrrhenian area (e.g. Elba I.) and Internal Ligurides of the Northern Apennine (e.g. Vara unit) show N-MORB affinity. Ophiolites represented in olistoliths and olistostromes in turbidites from the Northern Apennine display both N-MORB (mainly in Tuscany) and T-MORB (mainly in the Liguria-Parmesan Apennine) affinities. Recent petrological studies have shown that Corsica ophiolitic basalts show geochemical differences, probably related to differences in their mantle sources. The T-MORBs derived from mantle sources variably enriched by plume components probably related to the magmatic activity associated with the continental break-up. The S. Angelo di Tenda and some Pineto basalts display a little but definite enrichment by a plume component. The Schistes Lustrès, Rio Magno, and some Pineto basalts derived from pure sub-oceanic lithosphere without any influence of plume components.
The ophiolitic successions of the Balagne are characterized by the occurrence of siliciclastic sediments and breccias, which are interlayered in all the rocks of the succession, from basalts to Cretaceous deposits (Toccone breccias), whereas the ophiolitic successions of the Schistes Lustrès (e.g., Inzecca unit) display general features, which are very similar to those of the Internal Liguride ophiolites.
These characteristics led many authors to interpret the Balagne ophiolites (in the western side) and the Liguria-Parmesan ophiolites (in the eastern side) as the initial oceanic products, and therefore as the two opposite margins of the Ligurian ocean. By contrast, the other ophiolitic units of the Corsica and those of the Vara unit (Northern Apennine) represent products formed during the steady-state oceanic phase.
The distribution of the ages of the basalts (obtained from biostratigraphical data on radiolarites), as well as of associated gabbroic and plagiogranitic rocks (obtained from absolute datings) show that the older ages of basalts are found in the eastern Ligurian ophiolites. The absolute datings on gabbros of the External Ligurides, as well as of the Voltri Group are estimated at about 180 Ma. By contrast, in the Balagne biostratigraphic data and absolute datings on plagiogranites indicate relatively younger ages (160-150 Ma).
The integration of the various stratigraphic, tectonic, and petrological data obtained from Corsica and Northern Apennine ophiolites, as well as their significance for the dynamics of oceanic opening, the spreading phases, and the spatial-temporal evolution of the Ligurian ocean will be discussed. Finally, the several geodynamic models of oceanic consumption up to day proposed in literature will be discussed in order to examine which model can better satisfies the new data from Corsica ophiolites
The Alpine Corsica ophiolites from the Inzecca Vally and the Pineto Massif
No full textThe Northern Apennines and the Alpine Corsica are characterized by well preserved examples of Jurassic ophiolite sequences and their related sedimentary cover. These ophiolite sequences are considered remnants of the Ligure-Piemontesedomain, i.e. part of the western Tethys oceanic basin, developed between the Europe/Corsica and the Adria continental margins. In these sequences, the primary features of the ophiolites can be fully seen.
These ophiolite sequences have been studied in detail since the ‘70s. Their petrological features and the geochemical signature of the magmatic sequences suggest that the Ligurian-Piedmont basin was as a narrow ocean, with a width no more than 400-500 km. Moreover, the ophiolites from Corsica and the Northern Apennines, as well as those from Calabria and the Alps, are characterized by very thin, reduced sequences, where the whole section is less than 700-800 meters thick. Consequently, these sequences are characterized by lithostratigraphic features that are very different from those of the “typical” ophiolite sequence defined by Penrose’s (1972) ophiolite field conference. This is also suggested by the evidence of exposure of mantle lherzolites before emplacement of basaltic flows at the seafloor, the lack of a sheeted dyke complex, and the occurrence of ophiolitic breccias over or below the basaltic flows. Another interesting feature is represented by the association of the ophiolites with granulites and granitoids in the sequences interpreted as representative of the ocean-continent transition at the Adria continental margin. All these features make these ophiolites unusual and representative of an oceanic basin whose origin, architecture and development is still under debate.
This field trip focuses on the ophiolite sequences from the Northern Apennines and Alpine Corsica with the aim of providing a complete picture of the architecture of the Ligure-Piemonteseoceanic basin and its transition to continental margins. The first and second days are devoted to the ophiolite sequences from the Internal Ligurid units, representative of the inner domain of the oceanic basin, whereas on the third day the features of the ophiolites from the ocean-continent transition will be examined in the External Ligurid units. On the fourth and fifth days both un- and metamorphosed ophiolite sequences from Alpine Corsica, will be examined
Comparison among the Albanian and Greek ophiolites: in search of constraints for the evolution of the Mesozoic Tethys ocean
No full textIn this paper the stratigraphical, structural, geochemical and petrological features of the Mirdita (Albania) and Pindos, Vourinos, Koziakas, Othrys and Argolis (Greece) ophiolitic nappes are summarised and then compared. These ophiolitic nappes occur as a 700 km long belt running from Albania to Greece. These ophiolitic nappes are located between the west-verging imbricate stack of thrust sheets derived from the Adria plate continental margin to the west and the Pelagonian zone to the east. Each ophiolitic nappe is represented by several end-members represented by the sub-ophiolite mélange, the ophiolite sequence(s) with their sedimentary oceanic cover and the supra-ophiolite deposits. The latter can be divided in syn- and post-emplacement deposits, the first ones are recognised only in Albania. All the described ophiolite sequences are characterised at their base by a well-developed metamorphic sole that represents a further end-member of the ophiolitic nappe. The comparison among the features of all the end-members recognised in the studied ophiolitic nappes allows providing further constraints for the geodynamic reconstructions of the Mesozoic Tethyan oceanic basin located eastwards of the Adria plate
The Alpine Corsica ophiolites from Inzecca Valley and the Pineto Massif
No full textThe Inzecca Zone is located in central-eastern Corsica, at the southern end of the "Alpine Corsica".
Along the Inzecca gorge, thanks to the incision produced by the Fium'Orbo River, a good and continuous outcrop is visible. In this area, the tectonic pile is formed by three tectonic units, superposed to each other, from East to West:
the Schistes Lustres (Inzecca series: Amaudric du Chaffaut et al., 1972; Caron and Delcey, 1979), the Parautocthonous Unit (Netelbeek, 1951) and the Variscan Basement Unit (Hercynian Corsica). These distinguished tectonic units differ from each other in lithology and petrography and for the metamorphic-deformative Alpine evolution. The tectonic contacts between the different units are subvertical, show strike about N-S and are concordant with the internal attitude in all the studied area.
- Schistes Lustres: it comprises meta-ophiolites and their meta-sedimentary cover, whose age ranges from Late Jurassic to Late Cretatceous. In the Inzecca area a complete succession is visible; it includes (from bottom to top): serpentinites, ophicalcites, ophiolitic sandstones, pillow basalts and pillow breccias with intercalations of ophiolitic sandstones, meta-radiolarites, the Erbajolo Formation made by phyillites and marbles of Early Cretaceous age (Amaudric du Chaffaut et al., 1972). They are affected by low HP-LT retrogressed in Green Schistes facies.
- Parautocthonous Unit (roches brunesAuctt.): it is constituted by Variscan garnet-biotite-muscovite-plagioclase micaschists (Netelbeek, 1951) of pre-
Carboniferous age, (Netelbeek, 1951) with a HP-LT metamorphic imprint, transformed by the Alpine metamorphic-deformative events into sericite-chlorite-pumpellyite-lawsonite schists (Fig. 7). Their texture varies from porphyroclastic to mylonitic towards East, approaching the contact with the Schistes Lustres, but also within local shear zones. The contact between these rocks and the Variscan granite was affected by polyphased transpositive deformation phenomena, which produced a tectonic alternance of the two lithologies.
- Variscan Basement Unit: it is represented by a biotite-amphibole Carboniferous monzogranite (Durand-Delga, 1978; Amaudric du Chaffaut et al., 1985). Where local shear zones occur, the rock shows a porphyroclastic to mylonitic texture. The Alpine metamorphic imprint determined the blastesis of stilpnomelane, pumpellyite and white mica and the alteration of the feldspar into sericite. The crystallization of sodic amphibole has locally been observed. The granite is here associated to Permian meta-rhyolites and meta-conglomerates with rhyolitic and granitic clasts. As a result of the Alpine transpositive phenomena, these lithologies appear as plurimetric and pluridecamentric tectonic intercalations in the granite itself
Structural transformation of Al-Fe alloys analysed by neutron diffraction and Mössbauer spectroscopy
No full textThe mechanical treatment of Al-25 at.% Fe and Al-34 at.% Fe mixtures of pure elemental powders in a high-energy mixer-mill induces solid solution formation and substantial solid state reactivity. Neutron diffraction data were collected over a wide Q-range. A quantitative phase evaluation was carried out by the Rietveld method which enabled us to overcome problems of peak overlapping between the two constituent phases. Changes of lattice parameters, as well as lattice strain and average crystallite size were also determined versus the processing time. The local magnetic environment around the iron atoms was simultaneously monitored by means of the Mossbauer spectroscopy.
The formation of an extended bcc solid solution for prolonged treatment times is confirmed even for the present Al-rich compositions.
Neutron diffraction experiments after annealing of the Al-25 at.% Fe mechanically alloyed end products revealed a polymorphous phase transformation after 85 minutes at 583 K, accounting for the metastable character of these materials. The transformation product, Al5Fe2, is orthorhombic and stoichiometrically near to the nominal composition. The short interatomic Al-Al distances, resulting from the analysis of the unit cell properties, suggests that oxygen impurities may be present in the system and may play a role for the observed reactivity.
In the case of Al-34 at.% Fe composition the products are the Al5Fe2 and FeAl intermetallic phases
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
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