A GARNET POPULATION IN MICA SCHIST FROM THE GRENVILLE PROVINCE OF SE ONTARIO: INFERENCES ABOUT ITS CRYSTALLIZATION KINETICS AND METAMORPHIC P-T HISTORY

The conditions and rates of reactions among minerals dictate the physicochemical properties of metamorphic rocks. Numerous models have been proposed to encompass these parameters in order to assist in the quantitative understanding of metamorphic crystallization. These models can be grouped into two end-member concepts that differ in terms of the processes assumed to control the overall reaction rates: interface- and transport-controlled crystallization.

This paper investigates the chemical composition of an entire garnet population and the microstructure it forms in a mica schist sample from the Grenville Province of southeastern Ontario. Existing models of metamorphic crystallization are discussed based on the statistical analysis of the shape, size and spatial distribution of garnet in this rock, and the chemistry of central garnet core sections.

Multiple lines of evidence, including a random distribution of garnet and a chemical zoning pattern characteristic of interface-controlled garnet growth, suggest that garnet grew with rates controlled by chemical attachment and detachment processes at its surface. Results of numerical simulations of garnet crystallization indicate that garnet nucleation, growth and resorption occurred concomitantly during Barrovian metamorphism over a T-range of ca. 115°C. Based on the simulation of intracrystalline diffusion in garnet, radial garnet growth rates that range between 40 and 60 µm/Ma are obtained. Estimates for the departure from equilibrium required for nucleation of garnet in this rock are comparable to the uncertainties inherent in commonly used thermodynamic data. This indicates that interfacial energies associated with garnet nucleation were exceedingly small or that the release of strain energy may have significantly assisted in overcoming barriers to metamorphic crystallization.