Not all that's high is sediment: Conflicting evidence for sediment assimilation in intrusive rocks of the Frontenac terrane, Grenville province

Crustal contamination during magma differentiation commonly involves the assimilation of pelitic sediments. Such assimilation typically increases peraluminosity and δ18O of the resulting melt, leading to the widespread interpretation that high δ18O signatures in igneous rocks reflect the assimilation of sediment. However, in the Frontenac terrane of the Grenville Province, metaluminous intrusive rocks display δ18O values substantially higher than those of the mantle (∼8–16.2‰), despite lacking peraluminous compositions, which complicates this interpretation. Contrary to previous proposals, we hypothesize that these high δ18O values can be explained entirely through the incorporation of altered mafic igneous rocks rather than sedimentary materials, and this is supported by the lack of correlation between aluminosity and δ18O. In addition to the elevated δ18O (∼11.9 ± 2‰), we also report primary fluid inclusion isotopic signatures showing depleted δ13C (−9.7 ± 4‰) and mantle-like δ2H (−56.2 ± 10‰). These δ13C and δ2H isotopic characteristics are inconsistent with pelite assimilation but are compatible with the partial melting of altered mafic lithologies in the presence of carbon-depleted fluids, potentially sourced from serpentinized ultramafic rocks. This alternative explanation challenges the prevailing assumption that high δ18O values in granitoids necessarily reflect sediment assimilation. Our results highlight the need to integrate elemental, isotopic, and geological evidence when interpreting magmatic contamination processes.