The Beja Layered Gabbroic Sequence (Ossa-Morena Zone, Southern Portugal): geochronology and geodynamic implications

Abstract

The Beja Igneous Complex (BIC) is a major geological feature in SW Iberian Variscides, consisting of three main units developed during different stages of the oblique collision between the Ossa Morena Zone (OMZ) upper plate and South Portuguese Zone (SPZ) lower plate, namely: (1) ca. 355 Ma to ca. 345 Ma Layered Gabbroic Sequence formed in early stages of collision magmatism; (2) the ca. 335-330 Ma to ca. 320 Ma Cuba-Alvito (gabbro-diorite) Complex formed throughout the late-collision magmatic event; and (3) the Baleizão Porphyry Complex corresponding to the period of post-collision magmatism, ca. 300 Ma. The new SHRIMP U-Pb age of 342±9 Ma reported here for amphibole-bearing pegmatite dykes cutting the layered gabbros is interpreted as dating the development of late fluid-rich melts in the Layered Gabbroic Sequence, synchronous with Fe-Cu-Co sulphide deposition. The close agreement between this data and available amphibole 40Ar/39Ar ages of BIC, Beja-Acebuches Ophiolite and other geological units of the OMZ southern border, may be taken as evidence for a moderate to rapid regional crustal uplift episode at ca. 340±5 Ma; this data, coupled with structural constraints, also allow to estimate the age for the transition between the D2a – D2b deformation phases of Variscan continental collision. A complex wedge system within the SW Iberian Variscides developed during this collision, involving the OMZ upper plate to the north and the SPZ passive margin in the lower plate. The Évora-Beja-Aracena Domain, located in the upper plate above the N-dipping subduction zone, is re-interpreted as a retro-wedge domain that was kinematically coupled to the SPZ pro-wedge and subduction system. Retro-wedge growth is linked to upper plate uplift (early collision) and a late-orogenic wedge thickening. The early stages of magmatism in the retro-wedge are related to asthenospheric mantle upwelling induced by the slab break-off. Regional LP-HT metamorphism and subsequent magmatic events in the retro-wedge domain were caused by long term high heat flow sustained by (1) mafic magma underplating, (2) stacking of high-heat producing upper-crustal lithologies, and reinforced (3) by (moderate to) rapid crustal uplifting. Mass advection and orogenic architecture were strongly affected by asymmetric removal towards the lower-part foreland basin and by transient mechanical properties of the wedge system associated with the anomalous thermal regime.