IMPLICATIONS OF OPEN SYSTEM MELTING FOR ZIRCON AND MONAZITE GEOCHRONOLOGY

Ages retrieved from accessory minerals are used to define timescales of tectonic processes operating in the deep formerly suprasolidus crust. The dissolution and growth of zircon and monazite are strongly dependent on the P–T conditions of metamorphism and the chemistry and quantity of melt present. The progressive drainage of melt during prograde heating modifies the chemistry of the source reducing its fertility and affecting the quantity of melt that may be generated along the remainder of the P–T path. These changes have consequences for the growth and dissolution of zircon and monazite.

In this study, a series of calculated P–T phase diagrams for a progressively more residual source is combined with experimental data on zircon and monazite saturation in anatectic melt to evaluate the stability of these minerals during progressive melting of an average metapelite and an average passive margin greywacke. We model P–T paths comprising heating from the fluid-present solidus up to peak T followed by isothermal decompression. The calculated quantity and chemistry of anatectic melt along each P–T path is coupled with the saturation equations for zircon and monazite for a range of initial concentrations of Zr and LREE to determine the amount of zircon and monazite dissolved.

Monazite dissolution is favored along all P–T paths modeled because the concentration of LREE required to saturate the melt far exceeds bulk rock concentrations. Consequently, monazite is expected to be scarce in residues but abundant in leucosomes and granites. Since monazite is expected to grow close to the solidus, monazite ages from the source are predicted to record the final crystallization of trapped melt. Zircon behavior is more complex as the concentration of Zr required to saturate the melt is strongly temperature dependent and similar to bulk rock concentrations. At low T, bulk rock Zr concentrations are greater than those required to saturate the melt and limit zircon dissolution, whereas at high T the relationship inverts and more zircon dissolution is expected. As a result only limited zircon growth in the source is predicted during cooling to the solidus, but since growth occurs over a wider range of T this new zircon may record a range of concordant ages representing the progressive crystallization of melt trapped on grain boundaries.