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.