TECTONOMETAMORPHIC HISTORY OF THE KAPUSKASING UPLIFT: NEW PERSPECTIVES FROM TTG-SERIES ROCKS AND MAFIC MIGMATITES

The Kapuskasing Uplift in Ontario represents a rare window into the Neoarchean lower crust of the Wawa-Abitibi Terrane of the Superior Province, where rocks of the tonalite-trondhjemite-granodiorite (TTG) series structurally overlie mafic migmatites. A lower crust vs. subducted slab source for TTGs worldwide has long been debated, with partial melting in each scenario driven by different geodynamic processes. The rocks exposed in the Kapuskasing Uplift may provide evidence for TTG genesis by partial melting of mafic lower crust, which has implications for the tectonic evolution of the Wawa-Abitibi Terrane and Archean cratons in general. Here, we test this hypothesis by using phase equilibrium modeling coupled with melt trace element modeling to estimate the composition of partial melt produced by the migmatites. These results are compared to a geochemical dataset of TTGs collected from the Kapuskasing Uplift. The natural TTGs are divided into two main groups based on trends in trace elements: one characterized by high Sr/Y, low HREEs, and a positive Eu anomaly and the other with low Sr/Y, higher HREEs, and a negative Eu anomaly. Model melt with these trends is reproduced by anatexis of mafic migmatites at different P–T conditions, with a medium-T/high-P melt best matched to the first group and a high-T/medium-P melt most similar to the second group. These results indicate that the two types of TTGs were generated in different thermal regimes and therefore by different geodynamic processes. The migmatites currently exposed in the Kapuskasing Uplift may represent the source of only the high-T/medium-P group—this is consistent with geochronological data indicating that some TTGs predate granulite-facies metamorphism. U–Pb zircon geochronology is currently underway to determine the temporal distribution of the two TTG groups, which will be used to infer a sequence of tectonic events. By interrogating the magmatic and metamorphic record of the lower crust using both modeling and data from natural samples, new insights into the Neoarchean evolution of the Superior Province and TTG genesis in general will be revealed.