Thermobarometric modelling of zircon and monazite growth in melt-bearing systems: examples using model metapelitic and metapsammitic granulites

ABSTRACT

U–Pb age data collected from zircon and monazite are used to draw fundamental inferences about tectonic processes in the Earth. Despite the emphasis placed on zircon and monazite ages, the understanding of how to relate the timing of growth of zircon and monazite to an evolving rock system remains in its infancy. In addition, few studies have presented large datasets of geochronological data from zircon and monazite occurring in the same metamorphic rock sample. Such information is crucial for understanding the growth of zircon relative to monazite in a systematic and predictive manner, as per this study. The data that exist support the generally held conception that zircon ages tend to be older than monazite ages within the same rock. Here experimental data for zircon and monazite saturation in melt-bearing rocks are integrated with phase diagram calculations. The calculations constrain the dissolution and growth behaviour of zircon and monazite with respect to evolving pressure, temperature and silicate mineral assemblages in high-grade, melt-bearing, metasedimentary rocks. Several key results emerge from this modelling: first, that in aluminous metapelitic rocks (i.e. garnet + cordierite + sillimanite assemblages), zircon ages are older than monazite ages in the same rock; second, that the growth rate of accessory minerals is nonlinear and much higher at and near saturation than at lower temperatures; and third, that the difference in zircon and monazite ages from the same rock may be ascribed to differences in the temperature(s) at which zircon and monazite grow rather than differences in closure temperature systematics. Using our methodology the cooling rate of granulites from the Reynolds Range, central Australia, have been constrained at ∼4 °C Myr⁻¹. This study serves as a first-pass template on which further research in applying the technique to a field study can be based.