Low-Pressure Metamorphism in the Sierra Albarrana Area (Variscan Belt,Iberian Massif)

Abstract A low-pressure metamorphic zonation ranging from biotite to migmatite zones occurs in the Sierra Albarrana area (Variscan Belt of southwestern Iberian Peninsula) in uppermost Precambrian to Lower Palaeozoic metasedimentary rocks. The principal deformation in this area is related to a major ductile shear zone whose central part is localized immediately to the southwest of the Sierra Albarrana Quartzites. The metamorphism is synchronous with respect to this deformation. The metamorphic zones are symmetrically distributed with respect to the Sierra Albarrana Quartzites. Pressure–temperature (P–T) conditions are ∼3.5–4 kbar and range from ∼400°C (biotite zone) to 500°C (staurolite–garnet zone) up to 650–700°C (migmatite zone). We have not detected pressure variations along the different metamorphic zones. Relic kyanite is observed in the form of inclusions in andalusite within veins in the lower-grade part of the staurolite–andalusite zone. The low-pressure metamorphism of the Sierra Albarrana area arises from a two-stage history including moderate crustal thickening followed by subsequent localization of deformation in a transcurrent shear zone during peak P–T conditions. Channelized fluid flow within the major ductile shear zone may have contributed to the heat budget of the low-pressure metamorphism.