Zr budgets for metamorphic reactions, and the formation of zircon from garnet breakdown

The construction of zirconium (Zr) budgets for metamorphic reactions in high-grade rocks provides new insight into zircon growth during metamorphism. In this study we target reactions involving garnet, as they enable zircon growth to be related to known pressure and temperature conditions. Two reactions involving the breakdown of Zr-bearing garnet from Rogaland, SW Norway have been investigated in detail, showing contrasting behaviour of Zr, with zircon formation being subject to the solubility of Zr in product phases. In the decompression reaction garnet + sillimanite + quartz --> cordierite, Zr released during garnet breakdown cannot be incorporated into the cordierite structure, resulting in zircon nucleation and growth. In contrast, for the reaction garnet + biotite + sillimanite + quartz --> osumilite + orthopyroxene + spinel + magnetite, no new zircon growth takes place, despite the garnet involved containing more than double the Zr concentration of the former reaction. In the latter case, all the Zr released by garnet breakdown can be detected in the product phases osumilite and orthopyroxene, thereby preventing growth of new metamorphic zircon. This study highlights the potential for high resolution geochronology in metamorphic rocks by relating zircon growth to specific metamorphic reactions.