MAKING MINERALS IN CONTACT METAMORPHIC AUREOLES

During metamorphism, heat and mass transport interact in a series of feedbacks that drive forces controlling the nucleation, growth, dissolution and transformation of minerals in the host rocks surrounding an igneous intrusion. As the minerals respond to the thermal, baric and chemical changes in their local rock environment, they leave evidence of these conditions in their mineral assemblages, textures and/or mineral chemistry. Consequences of such feedbacks can be ascertained using 4-D numerical models of heat and mass transport within these environments. Experiments indicate that temperature (T)-pressure (P)-fluid flux conditions can be divided into three primary episodes: (1) an early conductive heating phase with low fluid flux as temperatures rise to near peak conditions; (2) a main stage convective heating/cooling event as peak Ts are attained with maximum fluid flux; and (3) a late stage convective cooling event when fluxes and Ts return to ambient conditions. Each of these stages exerts control on mineral formation. When computed T-t paths within the aureole are combined with irreversible models of minerals nucleation and growth, specific patterns of minerals develop in response to the system’s feedbacks. For example, during stage 1 rapid heating, reaction overstepping and cooling near the intrusion produces abundant nucleation and growth of minerals that are stable within 50^oC of the T excursion resulting in rocks with a limited crystal size distribution. Almost no growth of minerals occurs from overstepped metastable reactions. During stage 2, advective heat transport produces additional nucleation and growth in minerals formed from stage 1 metastable reactions. In the far field, minerals grow on nuclei that formed from overstepping stable reactions. Stage 3 produces additional growth and/or retrograde reactions and a zone of retrograde pseudomorphs along the flow path before Ts return to steady state. Comparison of rocks with our calculations suggests that mineral textures provide evidence of the relative impact of each stage. Final textures are path dependent. Feedbacks that produce distinctive textures and mineral assemblages within these rock systems can be critical to the interpretation of the metamorphic and hydrothermal events and the mineral patterns they produce in aureoles.