KINETIC INFLUENCE ON PROGRADE REGIONAL METAMORPHISM OF PELITES

For over a century, metamorphic petrologists have interpreted mineral assemblages as products of an equilibrium process, in which rocks only negligibly depart from their stable state as they are buried and heated. However, this does not consider that the constituent mechanisms of recrystallization - nucleation, dissolution/growth and transport - each have energetic barriers that need to be overcome before recrystallisation can occur. For a reaction to proceed, these obstacles must be surmounted by building up sufficient energetic driving force, sometimes termed reaction affinity, that results from the overstepping in pressure or temperature of the equilibrium conditions of the reaction. Studies in contact metamorphic settings have confirmed predictions of overstepping on mineral recrystallisation. In this study, we analyse two prograde regional metamorphic sequences – a regional Buchan-type sequence in the Harpswell Neck area, SW Maine and a regional Barrovian sequence in the Whetstone Lake area, SE Ontario - in an effort to determine whether the aforementioned kinetic impediments to recrystallisation are as important in regional settings. Detailed petrographic, textural and chemical analysis has allowed determination of the sequence, spacing and nature of metamorphic reactions in each setting. In particular, the role of staurolite is assessed at three stages across these prograde sequences: (1) does staurolite develop by reactions predicted by equilibrium? (2) to what degree is staurolite involved in the development of kyanite or andalusite?, and (3) why does staurolite persist to much higher grades than predicted? Comparison of the natural observations with the predictions of phase equilibrium modelling reveals discrepancies that can be attributed to reaction kinetics, which carries implications for the assessment pressure-temperature-time paths of metamorphic rocks. Ways in which these issues can be accounted for in future modelling are explored.