1.8 billion years of fluid–crust interaction: A zircon oxygen isotope record for the lower crust, western Churchill Province, Canadian Shield

Abstract

The western Churchill Province of the Canadian Shield experienced a prolonged and complex formation history (ca. 4.04 to 1.70Ga), with evidence for multiple episodes of orogenesis and regional magmatic activity. Here we report on the oxygen isotopic compositions of garnet and zircon recovered from lower crustal xenoliths, which have U–Pb ages between ca. 3.5 and 1.7Ga. Overall, zircon from four metabasite xenoliths from the Rankin Inlet sample suite have δ18O values ranging from +5.5 to +8.6‰. Zircon from three metatonalite/anorthosite xenoliths and five metabasite xenoliths from the Repulse Bay sample suite have δ18O values of +5.6 to +8.3‰. High δ18O values (>+6.0‰) for the oldest igneous zircon cores (ca. 3.5Ga and 3.0–2.6Ga) indicate that their metatonalite/anorthosite protolith magmas were generated from, or had assimilated, supracrustal rocks that interacted previously with surface-derived fluids. Igneous zircon cores (ca. 2.9–2.6Ga) from one metabasite xenolith have δ18O values of +5.6 to +6.4‰, which suggests a formation from a mantle-derived basaltic/gabbroic magma. Metamorphic zircon cores (ca. 2.0–1.9Ga) from one metabasite xenolith commonly have δ18O values between +6.0 and +6.3‰, which is indicative of a basalt/gabbro protolith and localized reworking of the lower crust caused by regional-scale plate convergence. The wide range of δ18O values (+5.5 to +8.3‰) for ca. 1.75–1.70Ga metamorphic zircon rims (identified in all xenoliths) indicates regional transient heating and reworking of mantle- and supracrustal-derived crust, induced by magmatic underplating along the crust–mantle boundary.