Integrated Numerical Models of Metamorphism: Linking the Regional (km) and Thin-Section (cm) Scales in Space and Time

Abstract:

Advances over the last decade in regional numerical models of metamorphism and models of mineral nucleation and growth at the hand specimen scale allow quantitative estimates of how metamorphism behaves at the kilometer (km) and centimeter (cm) scale in both time and space. Integration of these two methodologies provides a holistic approach that allows predictions of km-scale regional thermal models to be linked to cm-scale hand specimen textures throughout the spatial domain and over the duration of a metamorphic event. To illustrate this concept, models of conductive and advective transport of heat around intrusions in the middle crust display the complex variation of thermal fields at the km scale in space and time for regional-contact metamorphic terranes. The results of these regional scale calculations show that rocks in different locations that have reached the same peak conditions (T_max,P_Tmax) attained T_max, P_Tmax at different times (t_Tmax) during the metamorphic event, cross key pelitic isograds at different times (t_{Grt in}, t_{St in}, t _{Al-sil in}) and have different rates of reaction overstepping when an isograd is crossed (dΔG_{rxn Grt in}/dt, dΔG_{rxn St in}/dt, dΔG_{rxn Al-sil in}/dt). The variation in these parameters in the 3-D spatial domain around the intrusion causes index minerals to have different nucleation and growth histories depending on their location. Linking the evolution of metamorphic textures to the specific local P-T-t path from the regional thermal models using a local equilibrium, diffusion-controlled nucleation/mineral growth model allows simulation of the evolution of rock textures at various locations as metamorphism progresses. The results of the textural modelling demonstrate that there are subtle differences in crystal size distributions and other textural features in metamorphic rocks that allow a careful petrologist to distinguish their different thermal histories, even though traditional methods using thermobarometry or pseudosections would imply thermal histories were all the same.