DSpace Collection:http://hdl.handle.net/10174/5692024-03-28T18:21:58Z2024-03-28T18:21:58ZOfiolitos variscos e o metamorfismo de alta pressão associado, no ramo sul da Cadeia Varisca Ibérica / Variscan ophiolites and related high-pressure metamorphism in Southern Iberian Variscan foldbeltFonseca, PauloAraújo, AlexandreMunhá, JoséPedro, Jorgehttp://hdl.handle.net/10174/23762010-12-28T10:33:23Z2004-01-01T00:00:00ZTitle: Ofiolitos variscos e o metamorfismo de alta pressão associado, no ramo sul da Cadeia Varisca Ibérica / Variscan ophiolites and related high-pressure metamorphism in Southern Iberian Variscan foldbelt
Authors: Fonseca, Paulo; Araújo, Alexandre; Munhá, José; Pedro, Jorge
Abstract: The southern Iberia, Variscan ophiolites occur both as a thin belt along the boundary between the Ossa-Morena and South Portuguese Zones — Beja- Acebuches ophiolite — and as dismembered, scattered allochthonous klippen on top of lower Palaeozoic sequences within the internal areas of the Ossa-
Morena Zone.
The Beja-Acebuches ophiolite corresponds to a thin amphibolite-serpentinite belt displaying internal lithological organisation including, from bottom to top: metaperidotites (harzburgitic/dunitic) and cumulate pyroxenites, flaser gabbros with trondhjemitic intrusions, amphibolites (locally derived from a sheeted dike complex) and fine grain greenschists (locally preserving pillowed structures). Deformation structures result from three main deformation phases: D1 (early Devonian) corresponds to high-temperature ophiolite obduction towards N-NE, D2 (middle Devonian) is related to retrogression during transpressive sinistral shearing to WNW, and finally, D3 is a more brittle event, and involved sinistral south-westwards thrusting reactivating D2 structures. The ophiolite is bounded to the north by a thrust that brought Ossa-Morena
Zone infra-crustal rocks over the ophiolitic sequence; towards the south the ophiolitic complex has been thrust over the South Portuguese Zone units and is unconformably overlained by a late Devonian flysch sequence. The Beja-Acebuches amphibolites were originally tholeiitic gabbros/dolerites/basalts
displaying considerable geochemical variations that range from MORB-type to those transitional to arc tholeiites, suggesting derivation from a back-arc basin oceanic crust. The internal ophiolitic klippen were emplaced contemporaneously with the obduction of the Beja-Acebuches ophiolite. They comprise small, dismembered tectonic slices that were imbricated within a high-pressure (eclogite/blueschist), early Palaeozoic passive continental margin sequence, and then thrust onto the Ossa-Morena Zone. The high-pressure metamorphism was polyphase; early (pre- to syn-D1) eclogite recrystallization is interpreted as reflecting type-A subduction and initial D1-thrusting; late blueschist facies overprinting corresponds to tectonic imbrication related to the nappe emplacement. Ophiolite geochemistry display wide variations in incompatible element fractionation, ranging from N-MORB type LREE-depleted to LREE-enriched T/P-MORB; contrasting with similar lithologies from the Beja-Acebuches ophiolite, the orogenic (island arc-like) characteristics were not detected in these internal ophiolitic occurrences. The contrasting characteristics of the Ossa-Morena ophiolite types are reminiscent of those already described from other ophiolite belts and suggest that they probably represent different oceanic basins.2004-01-01T00:00:00ZSequências Ofiolíticas Internas da Zona de Ossa-Morena: implicações geodinâmicas na evolução da Cadeia Varisca IbéricaPedro, JorgeAraújo, AlexandreFonseca, PauloMunhá, Joséhttp://hdl.handle.net/10174/23732010-12-28T10:29:19Z2005-01-01T00:00:00ZTitle: Sequências Ofiolíticas Internas da Zona de Ossa-Morena: implicações geodinâmicas na evolução da Cadeia Varisca Ibérica
Authors: Pedro, Jorge; Araújo, Alexandre; Fonseca, Paulo; Munhá, José
Abstract: The Internal Ophiolitic Sequences correspond to allochthonous oceanic crust fragments that outcrop near the SW boundary of the Ossa-Morena Zone (Evora-Beja Domain), in an internal position relative to the Beja-Acebuches Ophiolitic Complex. The Internal Ophiolitic Sequences occur as tectonic imbrications or klippen, in the Moura Phyllonitic Complex. Despite deformation and metamorphism (greenschist/amphibolite facies), they still preserve a typical ophiolitic internal stratigraphy identical to the one described for many LOT (“Lherzolitic Ophiolite Type”) ophiolites. Geochemical data indicates that the Internal Ophiolitic Sequences have variable tholeiitic chemistry, transitional between N-MORB and E-MORB. Petrogenetic modelling suggests that the observed geochemical
variations reflect both mantle source heterogeneity and partial melting processes. The Internal Ophiolitic Sequences geochemistry indicates that their igneous protholites formed in an anorogenic tectonomagmatic environment, like the ocean ridge basins, without any influence of orogenic components related to subduction mechanisms. These features contrast markedly with those reported for the Beja-Acebuches Ophiolitic Complex, which have been interpreted as representing oceanic crust generated in a “back-arc” setting,.
Thus, supporting the existence of two distinct oceanic basins (ocean ridge and back-arc) during the geodynamical evolution of the SW Iberian Variscan Chain. Therefore, the Internal Ophiolitic Sequences in the Ossa-Morena Zone reflect the early oceanization events during the Variscan Orogeny.2005-01-01T00:00:00Z