High-Temperature Metamorphic Reactions from the Macro-Scale to the Micro-Scale

Abstract

Metamorphic reactions are the basis of metamorphic petrology and the lens through which we interpret metamorphic rocks and processes. They serve both as tectonic indicators, revealing the pressure--temperature history of a rock, and as tectonic drivers, responsible for the production of fluids and melts that are critical to many geological processes in the crust. At high temperatures, two types of reactions that have emphasized importance are melting reactions that have implications for large-scale crustal reworking and reactions that grow accessory minerals that we use to date petrological processes.

Zircon is the most common geochronometer, but its behaviour at high temperatures is poorly understood. Zircon-forming reactions were investigated in granulite-facies meta-granitoids in the Grenville province to better understand how zircon grows during metamorphism at high temperatures. Zircon growth occurred during retrogression as a result of melt crystallization and titanomagnetite breakdown. With this information, the dates of metamorphic zircon that were measured were interpreted as cooling dates, and provided additional context that suggests that the major orogenic phase of Grenville Orogen may have begun tens of millions of years earlier than previously thought.

Zircon was also used as a proxy to investigate the kinetics of trace elements in intergranular melt during melt crystallization in a migmatite. Key trace elements including Hf, U, Th, Y, and heavy rare earth elements were analyzed in multiple metamorphic zircon rims to compare relative concentration of zircon that grew coevally in the same thin section. The significant differences observed in concentration of these elements across zircon grains suggests rates of diffusion of these key trace elements are slower than zircon growth in migmatites. Zircon growth probably occurred as a result of size-dependent interface-controlled growth, implying that Zr diffusion was relatively fast in the melt.

On the macro-scale, evidence of regionally extensive H2O-fluxed melting reactions have been observed in multiple distinct tectonic environments across the globe, yet there is no generic tectonic model that explains regional-scale H2O-fluxed melting in the crust. Regional scale H2O-fluxed melting was studied in the Muskoka domain of the Grenville province. In the Muskoka domain, H2O-fluxed melting dominated throughout the region and until now, the source and mechanism of the H2O transport into the Muskoka domain has been unclear. Multiple examples of pegmatites with amphibole and leucosome-rich reaction selvages were found throughout the domain that show how H2O may have been transported into and through the Muskoka domain. Using a two-stage melting model, it was shown that melt generated at depth through hydrate-breakdown melting contains enough H2O to readily melt the rocks in the Muskoka domain through diffusive H2O-fluxed melting, with no fluid exsolution required.

Metamorphic reactions are used to understand regional tectonics, but there are significant gaps in our understanding of these reactions on both the micro-scale and the macro-scale. The geochronological tools that are used to unravel metamorphism are based on micro-scale processes that are still poorly understood. Simultaneously, our understanding of macro-scale tectonic processes involving H2O transport in the crust, which influence our interpretation of metamorphic rocks, is limited with H2O-fluxed melting. This thesis addresses our limits of understanding and shows how understanding metamorphic reactions allows us to better understand regional tectonics.