Variscan ophiolite belts in the Ossa-Morena Zone (Southwest Iberia): Geological characterization and geodynamic significance

Abstract

Abstract At SW Iberia, the boundary between Iberian and South-Portuguese Terranes is traced by a suture recording the closure of Rheic and related oceans through N-NE subduction. The western segment of the suture comprises imbrications of Iberian Terrane Relative Autochthon (Neoproterozoic - Lower Palaeozoic) and Allochthonous Complexes. The Iberian Terrane Relative Autochthon is topped by Lower-Middle Devonian metasediments and an unconformable cover of Lower Carboniferous (calc-alkaline) Volcano-sedimentary Complexes, both preserving evidence of very-low grade metamorphism. The Allochthonous Complexes include disrupted slivers of eclogite (ca. 370 Ma), klippen and slices of an internal ophiolite sequence (IOMZOS), a basal tectonic mélange (Moura Phyllonitic Complex) and an external ophiolite (Beja-Acebuches Complex, BAOC). A large layered gabbroic sequence (LGS), part of the Beja Igneous Complex, intruded (ca. 350-340 Ma) the SW suture domains. The IOMZOS represent obducted remnants of the Rheic Ocean, for which the available SHRIMP U-Pb data indicate and age of 4795.1 Ma; this ocean, separating Iberia from a promontory of Gondwana and Avalonia (now represented by the South-Portuguese Terrane), opened near the Cambrian-Ordovician transition through a “rift-jump” from an intra-cratonic setting (within the Iberian Terrane) to an intra-oceanic rifting. The BAOC displays geochemical features compatible with derivation from a [Lower-Middle Devonian (?); TDM (Sm-Nd)  380-440 Ma] short-lived back-arc basin. The Variscan ophiolite belts preserved along the SW Variscides were emplaced by antithetic obduction in two stages, both with shearing sense to N-NE. The first stage involved IOMZOS cold obduction, exhumated HP-rocks (ca. 370 Ma) and created a foreland bulge towards the Iberian Terrane SW flank; this mechanism controlled the development of the volcanic related carbonate shelves and the contemporaneous (NE) Terena flysch trough. The second stage corresponds to BAOC hot obduction, carrying the IOMZOS klippen in piggy-back style and affecting the southern margin of LGS at ca. 350-340 Ma.