Mechanisms for generating elevated zircon ¹⁸O in Archean crust: Insights from the Saglek-Hebron Complex, Canada

The Saglek-Hebron Complex (northern Labrador, Canada) is a polymetamorphic terrane preserving felsic crustal rocks as old as 3.87 Ga. Crustal rocks of this age are rare and offer opportunities to interrogate how Earth's earliest continent forming processes may have differed from the modern plate tectonic regime. Oxygen isotope ratios (delta O-18) in primary crystalline zircon are strongly affected by the incorporation of supracrustal materials during magmatism and can provide useful information about the character of early crustal reworking events. While zircon delta O-18 signatures >+6.4 %o indicate the incorporation of supracrustal material into the source of crustal anatexis, questions remain regarding the volumetric scale, sources, and processes by which supracrustal and magmatic mixing occurred during the Archean. Here, we present 271 zircon in-situ oxygen isotope analyses for a suite of seven Saglek-Hebron granitoids emplaced between 3.87 and 2.79 Ga in age, significantly expanding the temporal and lithologic diversity of the magmatic SHC zircon delta O-18 record. Collected from carefully characterized grain domains, primary zircon delta O-18 values range from 5.0 to 8.4 %o and include some of the highest igneous Archean zircon oxygen isotope ratios recorded to date. Leveraging an integrated workflow for identifying preserved zircon geochemistry, and correlating our new data with previous information from zircon Hf and whole-rock Nd-142/Nd-144 isotopes, we identify three periods of contribution from high-delta O-18 supracrustal material to crustal melts during reworking events in the SHC at 3.63, 3.33, and 2.79 Ga. Our work points towards repeated episodes of interaction between a maturing surface environment and magmatism from 3.6 Ga onwards at this locality.