TIME-TRANSIENT MINERAL NUCLEATION AND GROWTH CONTROLLED BY 4D THERMAL FIELDS DURING REGIONAL-CONTACT METAMORPHISM

Thermal fields around intrusions in the middle crust that are responsible for regional-contact metamorphism are complex functions of space and time. Such systems can produce fundamentally different P-T-t paths in rocks that experience the same peak metamorphic conditions. Heat and mass transport calculations for multiple plutons intruded into host rocks with spatially variable, anisotropic permeability provide insights into the time-dependent thermal evolution of the metamorphic rocks throughout the 3-D domain.

The results of transient 3-D thermal models with geometries similar to intrusions responsible for the classic regional-contact metamorphism in northwest Maine 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_{G in}, t_{St in}, t _{Al-sil in}) and have different rates of reaction overstepping when an isograd is crossed (dΔG_{rxn G in}/dt, dΔG_{rxn St in}/dt, dΔG_{rxn Al-sil in}/dt). We have coupled the 4-D thermal models from NW Maine with diffusion controlled nucleation/growth models of metamorphic textures under local equilibrium conditions to demonstrate how rocks with similar peak conditions, but different early thermal histories due to their spatial position, evolve as metamorphism progresses.

Traditional methods in metamorphic petrology such as thermobarometry or analysis of mineral assemblages/modes using petrogentic grids or pseudosections are typically reliable tools for determining peak metamorphic conditions (T_max,P_Tmax), but they provide little insight into the timing of peak conditions or the P-T history that preceded peak conditions. However, the mineral nucleation/ growth models from this study indicate that insight into the rates that isograds are overstepped is recorded by the size and spatial distributions of index minerals in metamorphic rocks, providing important rate information at several discrete times in a rock’s thermal history.