Late-orogenic retrograde zircon growth

Abstract

Metamorphic zircon is commonly assumed to record growth during prograde or peak metamorphic conditions. However, numerical models of zircon predict growth during cooling. Linking the relative timing of zircon growth to a metamorphic evolution requires determining the potential zircon forming reactions during protracted metamorphic cycles. Meta-granitoids from the Grenville Province in Ontario contain a high proportion of igneous zircon with metamorphic rims that provide a rare opportunity to study zircon forming reactions in situ; acquiring U–Pb dates while maintaining their petrographic context and textural setting in thin section. Textures and trace element composition of major minerals indicate that metamorphic zircon grew as a result of melt crystallization and breakdown of titanomagnetite. Phase equilibrium modelling shows that these reactions occurred during retrograde metamorphism. Measured U–Pb dates of these retrograde metamorphic zircons are between ca. 1100 and 1070 Ma. This contradicts previous interpretations of regional geology that suggest prograde and peak metamorphism occurred between 1080 and 1050 Ma. These results highlight the need to carefully analyze the metamorphic textures of zircon to provide the necessary context to assess the zircon-forming reaction and its link to the pressure–temperature history of the rock. Without this context, pressure–temperature–time paths determined with zircon dates should be treated with caution and could be incorrectly linked to disparate stages of orogenic cycles.