Preservation of Garnet Growth Zoning and the Duration of Prograde Metamorphism

Chemically zoned garnet growth and coeval modification of this zoning through diffusion are calculated during prograde metamorphic heating to temperatures of up to 850 degrees C. This permits quantification of how the preservation or elimination of zoning profiles in garnet crystals of a given size is sensitive to the specific burial and heating (P-T) path followed, and the integrated duration spent at high temperature (dT/dt). Slow major element diffusion in garnet at T < 600 degrees C means that centimetre-scale zoning profiles may remain for > 30 Myr at amphibolite-grade conditions, but small-scale (tens of micrometres) zoning features will be lost early in the prograde stage unless this is 'rapid' (<<5 Myr for rocks reaching c. 600 degrees C). Calculations indicate that preservation of unmodified growth compositions in even relatively large (up to 3 mm diameter) pelitic garnet crystals requires prograde and exhumational events to be < 10 Myr for rocks reaching c. 600 degrees C. This timescale can be <<5 Myr for garnet in rocks reaching 650 degrees C or hotter. It is likely, therefore, that most natural prograde-zoned crystals record compositions already partially re-equilibrated between the time of crystal growth and of reaching maximum temperature. However, a large T-t window exists within which crystals begin to lose their growth compositions but retain evidence of crystal-scale zoning trends that may still be useful for thermobarometry purposes. The upper limit of this window for 500 mu m diameter crystals can be as much as several tens of millions of years of heating to c. 700 degrees C. Absolute re-equilibration timescales can be significantly different for garnet growing in different rock compositions, with examples of a granodiorite and a pelite given.