Garnet compositions as recorders of P–T–t history of metamorphic rocks

On the basis of tests made in recent studies, we conclude that the simultaneous solutions of the equilibrium conditions of garnet–biotite Fe–Mg exchange and Grs = Plag + Aluminosilicate + Qtz (GASP) reactions, as formulated on the basis selected experimental data and well constrained thermodynamic mixing properties of garnet and plagioclase, offer robust estimates of P–T conditions of metapelitic assemblages that consist of the above minerals. Additional calculations are presented to show the compatibility of the retrieved P–T conditions of natural assemblages with the aluminosilicate phase diagram. We also calculate the minimum grain sizes of garnets that should be used for the calculation of the peak metamorphic P–T (Pp, Tp) conditions, using reasonable initial guessed values of minimum cooling rate and maximum Tp. To retrieve the thermal history of metapelites, we have developed a finite difference scheme for modeling multicomponent diffusion profiles in garnet, incorporating provisions for continuous nucleation and growth. This has been interlinked with a genetic algorithm that permits retrieval of the temperature vs. time path of metapelites through modeling of the zoning profiles in garnets, keeping several model parameters as floating variables. The numerical code has been applied to retrieve the T–t history of the low pressure–high temperature Royke metamorphic belt, Japan, by modeling the concentration profiles of Fe, Mn, Mg and Ca of garnets of different sizes that have been inferred to have developed by a process of continuous nucleation and growth.