Interplay Between Equilibrium and Kinetics in Metamorphism of Pelites in the Nelson Aureole, British Columbia

This study examines the interplay between equilibrium and kinetics in the development of metapelitic mineral assemblages in the aureole of the Nelson Batholith, southeastern British Columbia. We focus on two main issues: (1) contrasts in the degree of over-stepping of pelitic reactions as a function of reaction affinity and fluid presence, and (2) the relative unreactivity of porphyroblasts, in particular garnet and staurolite, when they are involved as reactants in prograde reactions. With respect to reaction overstepping, we contrast the formation of staurolite and andalusite from a reactive muscovite+chlorite precursor assemblage (large reaction affinity, non-porphyroblastic reactants, low overstepping of < 20 ºC) with the formation of andalusite and sillimanite from a staurolite-bearing precursor assemblage (small reaction affinity, porphyroblastic reactants, large overstepping of up to 70 ºC). Initial garnet formation appears to be an intermediate situation (overstepped by 20-30 ºC). Once garnet has nucleated, it appears to grow broadly in accordance with phase equilibrium predictions, but does not dissolve as much as predicted, showing negligible consumption both during staurolite growth and upgrade of terminal staurolite consumption. Staurolite shows different behaviour, reacting out to form andalusite sporadically across a wide interval. Reaction progress appears to depend on fluid presence to lower activation energy barriers. Major staurolite breakdown does not occur until sillimanite appears, ca.1000 m upgrade of where staurolite starts to react out. Our study suggests that (1) thermally-activated reactions can be significantly overstepped even when the overall pattern of mineral assemblage development conforms to predictions from equilibrium thermodynamics, and (2) the actual reactions involved in the formation of a stable equilibrium assemblage can be considerably different from those predicted by equilibrium modelling. The above results carry implications for the use of reaction textures and mineral zoning in inferring P-T paths.