EVOLVING FLUID CIRCULATION WITHIN THE VARISCAN BEJA-ACEBUCHES OPHIOLITE COMPLEX (SE, PORTUGAL)

Abstract

The Beja-Acebuches Complex is an extremely dismembered ophiolite sequence incorporated in the Variscan South Iberian Suture, representing a small marginal (back-arc) basin formed during subduction under the Iberian Terrane of normal oceanic crust to the south. Widespread textural evidence for hightemperature (800-900 oC) recrystallization of the basal and intermediate sections of the ophiolite sequence under an anisotropic stress field, suggests that these rocks were obducted before their total consolidation. The later history of this Complex reflects mainly Variscan metamorphism, that peaked at amphibolite facies, and its subsequent waning stages (during which the prevalent hydration of peridotites took place). The final stages of the retrograding path occurred at temperatures below 300 oC and are mainly ascribable to large-scale and repeated H2O, CO2 and (subordinate) SiO2 introduction into the system through WNW-ESE left-handed vertical shear zones, reactivated under brittle conditions with a left-lateral thrusting movement. This large volume, extremely well focused, fluid inflow generally induced very intense carbonatization of the adjoining rocks, with almost total destruction of their original mineralogy and textures and deposition of ankerite + dolomite ± siderite ± magnesite in subsidiary distensive structures. When affecting serpentinized peridotites, this metasomatic process leads to deserpentinization with development of strongly silicified carbonate aggregates. Later hydrothermal events are typically related to the precipitation of microcrystalline quartz and/or calcite in late veins and veinlets. Fluids circulating through the shear zones must have been quite pure, slightly acidic H2O-CO2 mixtures, whose very scarcity in metals promoted the hydrolysis of primary minerals and the removal of Al as aqueous-complexes. The origin of these fluids is believed to be related mainly to degassing of the autochthonous carbonate/schist units during Variscan metamorphism and the Beja Igneous Complex intrusion.