KINETIC CONTROLS ON STAUROLITE-AL2SIO5 MINERAL ASSEMBLAGES, WITH IMPLICATIONS FOR BARROVIAN AND BUCHAN METAMORPHISM (Invited Presentation)

Staurolite-Al₂SiO₅ mineral assemblages are of key importance to the interpretation of the P-T conditions of Barrovian and Buchan metamorphic terrains. Increasing evidence points to petrologically-significant kinetic control on their development. Barrovian terrains are characterized by mineral assemblages containing kyanite, or co-existing staurolite and kyanite. When interpreted in current equilibrium phase diagrams, they imply surprisingly restricted and rather high-pressure conditions of formation (>6 kbar = >20 km depth) in which, for any reasonable P-T path, kyanite is predicted to form upgrade, and at the expense, of staurolite. Whereas in some localities there is evidence of kyanite having formed from reaction of staurolite, at least as commonly there is no evidence of such a reaction relationship; rather, the two minerals appear to coexist stably, or to be intergrown. For these, an alternative interpretation is that both minerals crystallized in response to chlorite breakdown reactions. This possibility is favoured by the similarity of the two minerals’ crystallographic structures, the similar P-T conditions of the St- and (metastable) Ky-producing, Chl-consuming reactions (<10 °C separation), and the slightly larger entropy change of the Ky-producing reaction that allows it to approach and then overtake the St-producing reaction in terms of reaction affinity as overstepping increases. If so, considerably lower pressures (as low as ~5 kbar) may be possible for the formation of (some? many?) St-Ky assemblages than assuming equilibrium. In the Nelson aureole, BC, staurolite and andalusite have been shown to co-crystallize in response to chlorite-consuming reactions like described above in the range 3-4 kbar. Taken together, the pressure difference between Barrovian (St-Ky) and higher-P Buchan (St-And) sequences may be less than widely assumed. This interpretation would solve the present conundrum, assuming equilibrium, of the rarity of prograde sequences in which staurolite yields upgrade to sillimanite with neither kyanite nor andalusite in the sequence. If these interpretations are correct, they demonstrate the need to consider kinetically-controlled processes in the interpretation of mineral assemblages and microtextures in regional as well as contact metamorphic settings.