Incomplete hydration during retrograde metamorphism: Staurolite-, kyanite-, chloritoid-bearing pseudomorphs after andalusite (Cap de Creus, E Pyrenees, Spain)
Résumé
Metapelitic rocks from Cap de Creus (Eastern Pyrenees, Spain) have developed andalusite ±cordierite- bearing assemblages during the Variscan low-pressure high-temperature metamorphism. Andalusite crystals are commonly transformed to very fine-grained assemblages containing white micas with staurolite, kyanite and chloritoid, and/or biotite, chlorite and corundum. The hydrous character of the replacement minerals (pseudomorphing anhydrous andalusite) and the fact that transformation locally occurs along fractures, suggests that it was enhanced by fluid circulation. Using calculated phase diagrams (pseudosections), the equilibration of the main andalusite- bearing assemblage is constrained at < 4 kbar and 550-600°C. Various types of phase diagrams are combined with a detailed analysis of the crystallisation sequence of the replacement minerals, in order to investigate the conditions of formation of the staurolite+kyanite(±chloritoid)- bearing pseudomorphs. Staurolite and kyanite are commonly associated with medium-pressure, barrovian type metamorphism. However, calculated pseudosections suggest that the stability conditions of these minerals depend strongly on the amount of available H2O. Contrary to the intuitive perception, such assemblages do not imply a pressure increase with respect to the initial andalusite-bearing assemblage and may develop at PT conditions as low as 2-3 kbar, 350°C. It is argued that in the present case they are due to an incomplete, progressive hydration of the relatively dry andalusite-bearing assemblage at low pressures (< 4 kbar) and temperatures of the order of 400-450°C. It remains to be investigated whether this ‘retrograde’ alteration developed during the waning stages of the Variscan orogeny, or is the result of a prograde overprint during another (Pyrenean) orogenic cycle.