Subduction- and exhumation-related structures preserved in metaserpentinites and associated metasediments from the Nevado-Filabride Complex (Betic Cordillera, SE Spain)
Résumé
The Cerro del Almirez massif (Nevado–Filábride Complex, Betic Cordillera, SE Spain) is composed of antigorite (Atg-) serpentinite and chlorite (Chl-) harzburgite separated by a thin reaction front formed in a palaeo-subduction setting. These ultramafic rocks preserve unique prograde structures related to the pre- and peak high-pressure event (1.6–1.9 GPa and 680–710 °C). The oldest subduction-related structures are preserved in Atg-serpentinites: a penetrative S1 foliation and associated L1 stretching lineation that formed in a non-coaxial regime with a top-to-the-W sense of shear. This planar–linear fabric is crosscut by olivine ± Ti-clinohumite veins, formed during the prograde breakdown of brucite and pre-metamorphic clinopyroxene, which form a system of veins decimetres long. They record embrittlement processes due to the release of 6 vol.% of H2O associated with brucite dehydration. The growth of prograde olivine and/or tremolite porphyroblasts is syn- to post-kinematic in relation to the S1 foliation. Further reactions at higher temperature related to the complete breakdown of the Atg (i.e. Atg-out) are post-kinematic to S1. Prograde Chl-harzburgite is crosscut by sets of conjugate zones associated with grain-size reduction of olivine grains. These grain-size reduction zones are interpreted as brittle structures generated by hydrofracturing due to overpressure fluids in a compressional setting at low-differential stresses.Structures related to the exhumation process are mainly preserved in the metasedimentary host rock, where an S2/L2 planar–linear fabric developed within a shear zone dominated by a non-coaxial regime with a top-to-the-W sense of movement in a transpressional regime. Peak metamorphic conditions deduced for the schists are similar in pressure (1.3–1.9 GPa) but lower in temperature (560–590 °C) compared to the ultramafic rocks in contact with them, suggesting a major shear zone at the base of the ultramafic massif during D2 deformation in a right-handed transpressional regime.