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Preliminary structural and geochemical data on the Rio Magno Unit (Corsica): Implications for the Alpine Corsica-Northern Apennine geodynamic evolution
No full textTectono-magmatic significance of Triassic MORBs from the Argolis Peninsula (Greece): Implication for the origin of the Pindo ocean
No full textThe Triassic age for the beginning of spreading of the Neo-Tethyan-Pindos Ocean, although proposed by some authors, is poorly constrained because it is generally based on limited data on Triassic radiolarites, which are tentatively associated with volcanic rocks represented by basaltic sequences mainly showing within-plate (alkaline) affinity or, subordinately, ranging from transitional WPB to transitional MORB compositions. The Middle Unit of the central-northern Argolis (eastern Peloponnesus, Greece) consists of a composite tectonic association of various types of thrust sheets, some of which include coherent sequences of basalts topped by radiolarian cherts previously attributed to the Middle-Late Jurassic (Baumgartner, 1985). However, recent biostratigraphic data (Bortolotti et al., 2001) indicate that several thrust sheets are Middle-Late Triassic in age. The Jurassic volcanics are represented by MOR basalts. By contrast, the nature of the Triassic basalts is still unknown, and ma..
Structural and geochemical data on the Rio Magno Unit: Evidences for a new "Apenninic" ophiolitic Unit in Alpine Corsica and its geodynamic implications
No full textThe Rio Magno Unit (RMU) tectonically overlies the Schistes Lustrés units in south-eastern Alpine Corsica. It is represented by an ophiolitic sequence, showing remarkable differences with respect to the commonly recognized Corsican ophiolites. This unit can be distinguished from the Schistes Lustrés by the lack of HP-LT metamorphism, reflecting different geodynamic setting, although both were involved early in the same tectonic events. Similarly, the RMU can be distinguished from the Balagne Unit by the presence of normal-MORB basalts and the scarcity of continent-derived sedimentary input, testifying a different oceanic paleo-geographic setting. Moreover, the petrochemical, and stratigraphic features of the RMU ophiolitic sequence show close analogies with the Internal Ligurides of Northern Apennine. The RMU represents the first record of a non-metamorphic “Apenninic”-type ophiolitic unit in Alpine Corsica, supporting the hypothesis that the Alpine Corsica - Northern Apennines system represent a double-vergent accretionary wedge
Preliminary data on the Pineto gabbroic massif and Nebbio Basalts: progress toward the geochemical characterization of Alpine Corsica
No full textThe Pineto Massif and Nebbio ophiolitic sequences represent two main examples of Corsican ophiolites that, similarly to the well-known Balagne Nappe, have not undergone high pressure-low temperature alpine metamorphism. For this reason these sequences have been so far referred to the Balagne-type ophiolites.
The Pineto Massif is composed of a cumulitic intrusive sequence consisting of layered troctolites, dunites, gabbros, and anorthosites cross cut by basaltic dykes. The chemical composition of both cumulates and dykes reveal an oceanic tholeiitic affinity. In particular, the Pineto Massif basaltic dykes have REE patterns similar to those of N-MORBs with slight LREE/HREE depletion, as also evidenced by low CeN/YbN (0.9-1.2) ratios.
The Nebbio ophiolitic sequence consists of pillow and massive lava flow basalts capped by cherts, Calpionella limestones, and Cretaceous siliciclastic formations. Chemically, the Nebbio basalts are similar to ocean-floor basalts. They are characterized by LREE enrichment with respect to HREE, as evidenced by the CeN/YbN (1.2-1.9) ratios. Their REE patterns are similar to those shown by T-MORBs.
Geochemical data show that the Pineto Massif ophiolites are different from the Balagne Nappe T-MOR basalts, suggesting that any correlation between the Pineto Massif ophiolites and the Balagne Nappe should be ruled out. By contrast, they highlight close analogies between the Pineto Massif and the Internal Ligurides ophiolites from the Northern Apennine, suggesting that both these ophiolites represent fragments of the Piedmont-Ligurian oceanic lithosphere located in an internal oceanic position, relatively far from the continental margins.
Conversely, the Nebbio basalts show striking geochemical similarities to those of the Balagne, further supporting the hypothesis that both the Balagne and Nebbio ophiolites could represent fragments of oceanic crust generated in the early stages of the Piedmont-Ligurian basin oceanization
Triassic mid-ocean ridge basalts from the Argolis Peninsula (Greece) tectono-magmatic implications for the origin of the Pindos ocean,
No full textGeochemistry, petrogenesis and tectono-magmatic significance of volcanic and subvolcanic rocks from the Koziakas Mélange (Western Thessaly, Greece)
No full textThe Koziakas ophiolites have previously been interpreted as a Jurassic complex generated at a mid-ocean spreading ridge, and have been subdivided into a lower volcanic unit and an upper ultramafic unit.
The data presented in this paper demonstrate that the lower volcanic unit is, in fact, a tectonic mélange consisting of a polygenetic, multiple packet of stacked thrust-bound slices, representing distinct volcanic sequences and radiolarian chert successions.
Volcanic sequences include pillowed and massive lava varieties, which are frequently crosscut by dykes of various nature, including boninitic dykes. Both lava varieties and dykes are predominantly aphyric, though a few samples display slightly porphyritic textures, where phenocryst assemblages include olivine and/or plagioclase in pillow and massive lavas, clinopyroxene, plagioclase and sanidine in trachytic dykes, clinopyroxene in boninitic dykes.
Geochemical data point out the presence of three compositionally distinct groups of lavas: (1) transitional to alkaline basalts, trachyandesites, trachytes; (2) tholeiitic basalts; (3) very low-Ti (boninitic) basaltic andesites and andesites.
The transitional to alkaline rocks displays high Nb/Y ratios and enriched incompatible element characteristics, similar to those of many intraplate oceanic island basalts (OIB).
The tholeiitic group display lower abundances of incompatible elements and includes rocks resembling normal mid-ocean ridge basalts (N-MORB) with light REE (LREE) depletion (LaN/SmN = 0.29) and very low Ce/Y, Ta/Hf, Th/Yb ratios, as well as rocks resembling enriched mid-ocean ridge basalts (E-MORB) showing moderate LREE enrichment (LaN/SmN = 1.26-1.52) and Ce/Y, Ta/Hf, Th/Yb ratios higher than those of N-MORBs.
The very low-Ti group displays greatly depleted incompatible element abundances and the U-shaped REE patterns typical of boninites generated in supra-subduction settings.
The Koziakas mélange appears to have formed due to tectonic dismemberment and accretion of material generated in an oceanic environment (MORBs and OIB-type rocks), possibly in an intra-oceanic forearc setting. After their accretion these rocks were affected by widespread boninitic dykes generated by partial melting of depleted peridotites in the fore-arc setting.
The record of different lava types in the Koziakas mélange is in accordance with the general geological evolution of the Neo-Tethyan Pindos oceanic basin, from the Permo-Triassic rifting to the Middle-Late Jurassic intra-oceanic convergence phase
The Rio Magno Unit (Alpine Corsica): a review of its structural, stratigraphical and geochemical features and their geodynamic implications
No full textThe Rio Magno Unit (eastern Corsica) represents an Apenninic-type ophiolitic unit, quite similar to the Internal Ligurides; its ophiolitic sequence includes N-MORB basalts, Early Cretaceous Palombini-bearing limestones, and mid-Cretaceous flysch, and it is not affected by HP-LT alpine metamorphism. These features further support the hypothesis of close geodynamic relationships between the Corsican and Northern Apenninic ophiolites. We suggest that, in the Jurassic Piedmont-Ligurian oceanic basin, the Rio Magno and Internal Ligurides ophiolitic sequences represent a continuous, internal, oceanic paleo-domain. Subsequently, during the Late Cretaceous-Late Eocene, they underwent a similar orogenic evolution, escaping deep involvement during the subduction beneath the European continental margin
Triassic mid-ocean ridge basalts from the Argolis Peninsula (Greece): new constraints for the early oceanization phases of the Neo-Tethyan Pindos basin
No full textThe Middle Unit of the central-northern Argolis Peninsula, in northeastern Peloponnesus (Greece), is composed of several tectonic slices, locally including intact sequences of mafic volcanics topped by radiolarian cherts. Although some of these sequences are Jurassic in age, many of them display a Triassic age based on biostratigraphical evidence.
The petrological studies presented in this paper indicate that the Triassic volcanic rocks were generated in a mid-ocean ridge setting, and that they represent the oldest remnants of the Pindos oceanic crust so far recognized in the Subpelagonian zone. On the basis of immobile trace element analyses, two chemically distinct groups of Triassic lavas can be recognized in the various volcanic sequences. One group is represented by transitional-type MOR basalts displaying moderate LREE enrichment, and incompatible element abundances very similar to those observed in present-day T-MORBs. The other group exhibits a range of characteristics typical of many normal-type MOR basalts: that is, variable LREE depletion and flat N-MORB normalized patterns of incompatible element abundance. Moreover, many geochemical characteristics indicate that the different N-MORB type volcanic sequences originated from chemically distinct (heterogeneous) sub-oceanic mantle sources.
Analogous to similar basalts from ophiolitic mélanges of the Dinaride-Hellenide belt, the T-MORBs from the Argolis Middle Unit are interpreted as having originated from a primitive mantle source variably enriched by an OIB-type component. By contrast, the contemporaneous occurrence of N-MORBs implies that, during the Middle-Late Triassic, oceanic spreading of the Pindos basin had already reached, at least in some sectors, a quasi-steady state involving only sub-oceanic mantle sources and their partial melt derivatives. Our model for the Triassic opening of the Pindos oceanic basin and its related tectono-magmatic evolution is largely supported by the actualistic comparison with the Red Sea embryonic ocean
N-MORB and T-MORB affinities of the "Nappe Superieurs" Corsican ophiolitic Units (Alpine Corsica, France)
No full textDeformation history of the Coast belt ophiolitic unit (Cordillera de la Costa, Northern Venezuela).
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