Evolving fluid circulation within the Variscan Beja-Acebuches ophiolite complex (SE, Portugal)

The Beja-Acebuches Complex is an extremely dismembered opbiolite sequence i ncorporated in the Variscan South Iberian Suture, representing a small marg inal (back-arc) basin formed during subduction under the Iberian Terrane of normal oceanic crust to the south. Widespread textural evidence for high t emperature (800-900 degrees C) recrystallization of the basal and intermedi ate sections of the ophiolite sequence under an anisotropic stress field, s uggests that these rocks were obducted before their total consolidation. Th e later history of this Complex reflects mainly Variscan metamorphism, that peaked at amphibolite facies, and its subsequent waning stages (during whi ch the prevalent hydration of peridotites took place). The final stages of the retrograding path occurred at temperatures below 300 degrees C and are mainly ascribable to large-scale and repeated H2O, CO2 and (subordinate) Si O2 introduction into the system through WNW-ESE left-handed vertical sheer zones, reactivated under brittle conditions with a left-lateral thrusting m ovement. This large volume, extremely well focused, fluid inflow generally induced very intense carbonatization of the adjoining rocks, with almost to tal destruction of their original mineralogy and textures and deposition of ankerite + dolomite +/- siderite +/- magnesite in subsidiary distensive st ructures. When affecting serpentinized peridotites, this metasomatic proces s leads to deserpentinization with development of strongly silicified carbo nate aggregates. Later hydrothermal events are typically related to the pre cipitation of microcrystalline quartz and/or calcite in late veins and vein lets. Fluids circulating through the shear zones must have been quits pure. slightly acidic H2O-CO2 mixtures, whose very scarcity in metals promoted t he hydrolysis of primary minerals and the removal of Al as aqueous-complexe s. The origin of these fluids is believed to be related mainly to degassing of the autochthonous carbonate/schist units during Variscan metamorphism a nd the Beja Igneous Complex intrusion.