Decoding Zircon Geochronology of Igneous and Alteration Events Based on Chemical and Microstructural Features: a Study from the Western Superior Province, Canada

In this study we examined Meso- to Neoarchean granitoids from the North Caribou Terrane within the Western Superior Province, Canada. Petrology, whole-rock geochemistry, zircon and titanite geochronology, and zircon trace element concentrations were analyzed. U–Pb ages from zircon and titanite are between 2·62 and 3·13 Ga. Although most of the granitoids in this study appear to record a complex magmatic history, about a third contain features that we interpret to be a result of hydrothermal alteration. Notable traits in rocks that contain altered zircons include K-feldspar overgrowths on plagioclase and compositional zoning in titanite. The altered zircon material itself occurs as CL-bright resorption shadows showing distinct chemical changes, including lower Th/U values and elevated LREE concentrations. The isotopic ages of the rims on the altered zircons (2835 Ma, 2760–2678 Ma) are similar to coexisting U–Pb titanite ages and regional U–Pb titanite and zircon ages. We propose that during the hydrothermal event, the affected areas of zircon re-equilibrated with fluid, which promoted Pb loss, resetting the isotopic clock. These results suggest that zircon rims might be useful for dating hydrothermal fluid flow episodes in addition to magmatic events and that a multi-element approach is useful for distinguishing ages that are magmatic from those that have been isotopically disturbed owing to alteration.