Disequilibrium Induced by Fast Decompression in St−Bt−Grt−Ky−Sil−And Metapelites from the Betic Belt (Southern Spain)

Abstract

Medium-grade metapelites from the Torrox unit (Western Alpujarrides, Betic Cordilleras) provide an example of the phase relationships to be expected from extensional collapse of thickened crust. The rocks contain assemblages formed by combinations of St−Bt−Grt±Ky±Sil−And, and are characterized by a marked state of textural and compositional disequilibrium that originated during post-peak, near-isothermal decompression at moderately high temperatures (from >10 to 2–3 kbar, at 550–650°C). Major disequilibrium features include abundant reaction textures, metastable coexistence of the three aluminium silicate (Als) polymorphs, wide heterogeneities in the composition of phases within single samples and unsystematic or abnormal Mg-Fe partitioning among biolite, garnet and staurolite. Reaction relations and compositional trends of phases can be shown to be consistent with expectations from model systems, however, indicating that disequilibrium was a consequence of reaction overstepping along the isothermal decompression path. Ensuing fast, near-isobaric cooling abruptly terminated metamorphism, and allowed for the preservation of disequilibrium features after decompression. Given that the rocks decompressed at relatively high temperature, the occurrence of reaction overstepping suggests that decompression proceeded at a very fast rate, consistent with a bulk exhumation velocity in the range of 5–10 km/Ma, as estimated from available radio-metric data for this stage.