EARLY AND VARIABLE CONTRIBUTIONS FROM SUPRACRUSTAL ROCKS IN ARCHEAN MAGMAS FROM THE SAGLEK BLOCK (LABRADOR, CANADA)

The Saglek-Hebron Complex (SHC, Labrador, Canada), is a polymetamorphic terrane preserving crustal rocks as old as ~3.9 Ga. Dominated by tonalite-trondhjemite-granodiorite (TTG) orthogneisses which host m- to km-scale enclaves of metasediments and mantle-derived rocks, the SHC records over a billion years of Archean magmatism. Zircon U-Pb data document six magmatic episodes at ca. 3.86 Ga, 3.74 Ga, 3.56 Ga, 3.33 Ga, 3.22 Ga and 2.75 Ga. The Hf isotopic compositions of zircon from these magmatic generations reveal a history of crustal re-working involving remelting of Eoarchean to Hadean mafic crust (Wasilewski et al., 2021).

When integrated with observations from radiogenic isotopes, oxygen isotopes in zircon can lend additional context for understanding early crustal histories, as δ¹⁸O from unaltered igneous domains can record the interaction of magmas and their protoliths with surface-derived material. This record, in turn, yields information about the tectonic conditions of early Earth. The limited δ¹⁸O data set for the SHC is dominated by mantle-like to slightly elevated δ¹⁸O values (~5-7 ‰), while a small number of high-δ¹⁸O values (+9 ‰) in metamorphic zircon are consistent with metamorphism at ~3.5 Ga involving mass transfer from mature supracrustal material (Vezinet et al., 2018; 2019).

Here, we present over 150 new δ¹⁸O analyses of igneous zircon for a suite of six samples, significantly expanding the temporal and lithologic diversity represented in the magmatic SHC zircon δ¹⁸O record. Our zircon δ¹⁸O dataset ranges from 5.0 to 8.8 ‰ and is spatially correlated with existing U-Pb and Lu-Hf information. We observe the highest δ¹⁸O values (7.9 – 8.8 ‰) in the 3.6 Ga TTG, which has zircon Ɛ_Hf interpreted to represent re-melting of a Hadean mafic crust (Wasilewski et al., 2021). These high δ¹⁸O values are bracketed by lower and more variable δ¹⁸O for the preceding 3.9-3.7 Ga (5.0 to 6.8 ‰) and subsequent 3.3-2.8 Ga (6.1 to 8.1 ‰) magmatic events, both of which have Ɛ_Hf suggestive of relatively more juvenile sources than the 3.6 Ga TTG. This zircon isotopic variability does not necessarily indicate the onset of widespread tectonic regime change, but rather suggests a unique regional magmatic event at ca. 3.6 Ga that involved a source with a higher relative contribution of supracrustal components.