Transformation of Andalusite to Kyanite in the Alpujarride Complex (Betic Cordillera, Southern Spain): Geologic Implications

The crystal growth features of andalusite and the transformation of andalusite to kyanite allow recognition of pre-Alpine and Alpine tectonometamorphic histories in the metapelites of the Alpujarride Complex. Two types of mineral segregations occur in relation to the andalusite → kyanite transformation. One type of segregation consists of a mantle of muscovite around a pre-Alpine andalusite chiastolite core that is partially transformed to fine-grained Alpine kyanite within a matrix particularly rich in biotite + quartz. The second type of segregation consists of Alpine muscovite + kyanite domains that form after dissolution of pre-Alpine andalusite + biotite domains within the matrix. The formation of these two types of mineral segregations involves similar reactions between the fluid and the local mineral assemblage. These reactions progress simultaneously. Each of them acts as source (or sink) for the ions, and intermediate mineral phases are consumed (or produced) by the other reactions, so that a combination of individual reactions produces the andalusite → kyanite net reaction. This reaction is catalyzed by the muscovite and biotite of the matrix, whose dehydration provides the chemical driving force needed to break Si-O bonds for the andalusite → kyanite transformation. Surfaces perpendicular to F-type {110} faces of the andalusite chiastolites associated with layeritic crystal growth mechanisms constitute fast reaction pathways for the andalusite → kyanite reaction. The transformation of pre-Alpine andalusite to Alpine kyanite constitutes the first solid textural evidence of the existence of a polymetamorphic history in the rocks of the Alpujarride Complex (Betic Cordillera, southern Spain).