High-pressure metamorphism in the Early Variscan subduction complex of the SW Iberian Massif

Abstract

Several units exposed in the boundary area of the Ossa Morena Zone (OMZ) and the South Portuguese Zone (SPZ) preserve petrographic and thermobarometric evidence for an early metamorphic episode (M1), developed under a high-P, low to intermediate-T gradient, related to Early Variscan subduction in the SW Iberian Massif. In the OMZ, these are the Cubito–Moura Unit (Pmin~9.2kbar and T=395±45°C), blueschists from its basal mélange (Pmin~12.4kbar and T=310±11°C), and the underlying Fuenteheridos Group (P=10.9±0.4kbar and T=449±31°C). The equivalent units in the SPZ are the La Minilla Formation (P=8.7±0.4kbar and T=388±16°C) and the lawsonite pseudomorphs-bearing Pulo do Lobo Formation. All these units formed part of an approximately NE verging orogenic wedge (present coordinates) developed by the accretion of subducted slabs of the outermost margin of Gondwana and other elements of the Rheic Ocean realm, from at least the Middle Devonian to the lowermost Tournaisian. High-pressure rocks were subsequently emplaced on more internal zones of the OMZ that only experienced a younger high-T, low to intermediate-P metamorphism (M2). This high-T event was coeval with magmatic activity from the uppermost Devonian to the Middle Mississippian, probably as a consequence of transtensional lithospheric thinning and/or delamination of the lower crust and mantle lithosphere of the Gondwana margin. Pre-Late Devonian synorogenic sedimentation in forearc and back-arc basins of the subduction complex evolved to a Late Devonian–Middle Mississippian foreland basin system in early collisional stages. Finally, a new Middle–Late Mississippian fold-and-thrust belt with opposite (SW) vergence and new foreland basins developed during late collisional stages.