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High-pressure rocks in the Variscan belt of Western Europe: the Malpica-Tui Complex (NW Iberian Massif)

Abstract : Pods and relicts of high-pressure (HP) rocks are the best record of the subduction of the north Gondwana margin at the onset of the Variscan collision. Across Western Europe, blueschist-facies terranes are restricted to scarce and relatively small areas, whereas eclogite-facies terranes are more abundant. The Malpica-Tui complex (MTC) is the westernmost exposure of HP rocks in the NW Iberian Massif, and in the Variscan belt of Western Europe. It comprises two tectonically juxtaposed units separated by an extensional detachment: (i) an upper unit consisting of rocks in the blueschist-facies, and (ii) a lower unit in the medium temperature eclogite-facies conditions. Assuming a northwest-directed component of subduction, in present day coordinates, the characteristics of each sequence suggest that the upper unit would occupy an oceanward position compared to the lower unit before the Variscan collision. Thus, the lower unit is interpreted as a slice of continental crust, whereas the upper unit may represent a transitional to oceanic crust of the same continental margin. The MTC preserves evidence of late Devonian HP metamorphism varying from eclogite (P~26 kbar and T~650 °C) to blueschist-facies conditions (19-22 kbar and 460-560 °C). Petrological analysis involving P-T-X pseudosections in the (Mn)NCKFMASHTO chemical system on the HP rocks reveals a P-T evolution characterised by a subisothermal decompression to ~10 kbar, 480 °C in the blueschist-facies rocks and 650 °C in the eclogites, followed by cooling to ~5 kbar at 380 °C and 500 °C, respectively. New 40Ar/39Ar data indicate a minimum age of ~370 Ma for the subduction-related HP metamorphism. Subsequent decompression to pressures of about 10 kbar started at ~360 Ma and was contemporaneous with thrust-and-fold nappe tectonics and intrusion of early Variscan granodiorites dated at ~350- 340 Ma. Final, “post-nappe”, exhumation is interpreted to have taken place from ~345-335 Ma to 320 ± 5 Ma, which is the age of the syntectonic igneous rocks emplaced in the autochthon of the MTC. These ages support the equivalence of the HP rocks from NW Iberia and their counterparts in the southern Armorican Massif. From a methodological point of view, modelling calculations of H2O and Fe2O3 on the metamorphic evolution of blueschist-facies rocks reveals trends that may have general applications in the investigation of rocks with similar composition: (i) subduction-zone metamorphism may occur in H2O-undersaturated conditions induced by the crystallization of a significant modal amount of lawsonite, although the transition from lawsonite blueschist facies to amphibolite-greenschist facies may involve significant hydration, principally as a result of lawsonite breakdown. (ii) The analysed values of Fe2O3 may not reflect the oxidation state during the main metamorphic evolutionary stage and are probably easily modified by superficial alteration, even in apparently fresh samples
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Contributor : Isabelle Dubigeon <>
Submitted on : Monday, August 17, 2015 - 1:58:04 PM
Last modification on : Tuesday, December 3, 2019 - 1:54:04 PM

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Alicia Lopez-Carmona, Jacobo Abati, Pavel Pitra, James Lee, Gabriel Gutierrez-Alonso, et al.. High-pressure rocks in the Variscan belt of Western Europe: the Malpica-Tui Complex (NW Iberian Massif). Variscan 2015, Géosciences-Rennes, Jun 2015, Rennes, France. ⟨insu-01184671⟩

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