QUANTITATIVE MODELS OF BIMODAL GARNET POPULATIONS IN METAPELITIC ROCKS

Bimodal size distributions in garnet populations can be explained by two different generations of garnet growth along a metamorphic pressure-temperature-time (PTt) path. Large garnets are produced when nucleation and transport rates allow significant garnet growth to occur after only a few nuclei have formed in the rock. This process can be due to: 1) slow overstepping of the garnet forming reactions, which produce low nucleation rates, and relatively long periods of time to transport constituents to the growing garnet by diffusion along grain boundaries; 2) the presence of relatively few sites that have low activation energies for nucleation; or 3) enhanced grain boundary transport due to fluid flow, presence of melt, or high temperatures. Abundant small garnets are produced when nucleation and transport rates allow many nuclei to form before significant garnet growth occurs. These conditions are caused by rapid overstepping of a garnet forming reaction or suppression of transport rates due to dry grain boundaries.

Scenarios that can produce bimodal garnet size distributions include: 1) multiple thermal events due to two or more orogenies or several metamorphic phases during the same orogeny; and 2) multiple garnet forming reactions during a single heating and cooling episode. In this study, size and spatial distribution of garnets produced by these scenarios are investigated using the quantitative modeling techniques of Foster and Dutrow (2003) to provide insight into the origins of these types of textures in metapelites.