Kyanite in thermal aureoles

Different models are discussed to interpret the presence of kyanite as a stable phase in contact aureoles especially in the andalusite-, sillimanitebearing aureoles. In such aureoles the polymorphic sequence kyanite → andalusite → sillimanite can be explained in terms of an essentially isobaric path during which kyanite initially crystallises from the breakdown of pyrophyllite or more likely muscovite + chlorite beneath the pressure of the triple point (3-4 kbar 10-14 km depth), followed by andalusite and sillimanite crystallisation. The sequence kyanite-sillimanite and/or fibrolite can be seen in thermal aureoles subjected to isobaric metamorphism above the triple Al2SiO5 point (5-8 kbar 17-28 km depth). The absence of kyanite in some thermal aureoles can be explained on the basis of either compositional constraint or overstepping of lowtemperature reactions due to high heating rates.