THE RELATION OF MULTI-MINERAL CRYSTAL SIZE DISTRIBUTIONS TO RATES OF TEMPERATURE CHANGE DURING METAMORPHISM
To investigate the effect of thermal history on the size and spatial distributions of garnet, staurolite and sillimanite, textures in rocks near tabular plutons in the middle crust were calculated using combined thermal/kinetic models. The thermal model provides T-P-t information around the pluton that is used as input to kinetic models of metamorphic reactions, which simulate mineral nucleation and growth at various positions around the pluton. The country rock permeability, pluton depth, pluton thickness, and geothermal gradient were varied to provide different types of thermal histories for the nucleation and growth models.
Environments that experience rapid heating and cooling (conductive heating; t<50,000yr) produce rocks with many small crystals; the garnet, staurolite and sillimanite will be approximately the same size (diameters <0.5 mm). Environments that experience slower heating and cooling (advective heating; t~100,000-500,000 yr) produce rocks with fewer and larger crystals (diameters >0.5 mm). Environments that experience two thermal peaks, one related to rapid heating & cooling due to early conductive heat transport and a second longer lived thermal event due to advective heat transport, have many small garnets produced by the conductive thermal history and larger staurolite and sillimanite, which grew during the advective portion of the thermal history.