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

Abstract

A low-pressure metamorphic zonation ranging from biotite to migorogenic belts, including the Variscan Belt of Southwest matite zones occurs in the Sierra Albarrana area (Variscan Belt of Europe. De Yoreo et al. (1991) presented a review of some southwestern Iberian Peninsula) in uppermost Precambrian to Lower possible tectonic settings of low-pressure metamorphism. Palaeozoic metasedimentary rocks. The principal deformation in Those workers considered low-pressure metamorphism this area is related to a major ductile shear zone whose central part to be that in which peak temperature is above 500°C is localized immediately to the southwest of the Sierra Albarrana and pressure does not exceed that of the aluminium Quartzites. The metamorphism is synchronous with respect to this silicate (Als) triple point. The following four tectonic deformation. The metamorphic zones are symmetrically distributed situations were envisaged for the low-pressure metawith respect to the Sierra Albarrana Quartzites. Pressure–temperature morphism (De Yoreo et al., 1991): (1) magmatic arcs; (P–T) conditions are ~3·5–4 kbar and range from ~400°C (2) regions of crustal extension; (3) continent–continent (biotite zone) to 500°C (staurolite–garnet zone) up to 650–700°C collision zones where thermal relaxation occurs after (migmatite zone). We have not detected pressure variations along crustal thickening; (4) regions with significant fluxes of the different metamorphic zones. Relic kyanite is observed in the aqueous fluids, especially those located above subduction form of inclusions in andalusite within veins in the lower-grade part zones. of the staurolite–andalusite zone. The low-pressure metamorphism of This paper is concerned with the description of a highthe Sierra Albarrana area arises from a two-stage history including temperature–low-pressure facies series terrane (the Sierra moderate crustal thickening followed by subsequent localization of Albarrana area, Variscan Belt, southwestern Iberian Masdeformation in a transcurrent shear zone during peak P–T conditions. sif ) (Figs 1 and 2). In this area, a metamorphic zonation Channelized fluid flow within the major ductile shear zone may from the biotite zone up to the migmatite zone developed have contributed to the heat budget of the low-pressure metamorphism. at low pressure (Garrote, 1976; Gonzalez del Tanago & Peinado, 1990). As will be shown here, the metamorphic zonation is not related to post-thickening thermal relaxation but rather exhibits a roughly symmetric pattern