Transformation of kyanite to andalusite in the Benamocarra Unit (Betic Cordillera, S. Spain). Kinetics and petrological significance

Kyanite is directly replaced by andalusite in quartz-plagioclase veins included within graphite-bearing micaschists of the Alpine Benamocarra Unit (Betic Cordillera, Spain). Electron back-scattered diffraction indicates that: i) precursory kyanite contains planar defects; andalusite growth was crystallographically controlled by the kyanite; and iii) the structure of both Al2SiO5 phases shares nearly the closest-packed oxygen array and chains of edge-sharing octahedra. The small entropy difference of the kyanite andalusite polymorphic inversion makes it difficult to overcome the energy barrier of this transformation. The driving force needed for the kyanite-to-andalusite reaction was a temperature (7) increase during a pre-Alpine tectonometamorphic evolution. The low-P/medium-T metamorphic conditions that affected the rocks studied took place in relation to a late Variscan extensional collapse. The reaction pathway proposed here corresponds to the first part of a poly-orogenic tectonometamorphic evolution, consisting of a pre-Alpine metamorphism of high thermal gradient with mainly static growth of porphyroblasts, followed by an essentially dynamic metamorphism during the Alpine orogeny.