NOT ALWAYS WHAT THEY SEEM: CONFLICTING EVIDENCE FOR SEDIMENT ASSIMILATION IN GRENVILLE GRANITOIDS

Crustal contamination often occurs from the assimilation of supracrustal (volcanic or sedimentary) materials during magma evolution. It is widely accepted that sediment assimilation increases peraluminousity and δ¹⁸O. Therefore, high δ¹⁸O magmas are generally interpreted to represent sediment-derived melts.In the Frontenac terrane of the Grenville Province, high δ¹⁸O metaluminous granitoids have been variably attributed to sediment assimilation. However, their metaluminous nature contradicts this hypothesis. An alternative source of elevated δ¹⁸O is altered basalt.We postulate that the high δ¹⁸O signatures in the alkaline granitoids of the Frontenac terrane are instead due to the incorporation of altered basalts. Our data show elevated δ¹⁸O (~11.9 ± 2‰), depleted δ¹³C (-9.7 ± 4‰), mantle-like δD (-56.2 ± 10‰) and no correlation between aluminosity and δ¹⁸O.This lack of correlation between aluminosity and δ¹⁸O suggests that the elevated δ¹⁸O is not the product of assimilation of pelitic sediment. Instead, the observed high δ¹⁸O, low δ¹³C, and mantle-like δD signatures can be explained by the melting of altered basalt accompanied by an influx of fluids with a depleted carbon isotope signature, likely derived from ultramafic listvenites. This finding challenges the conventional assumption that high δ¹⁸O values in granitoids are exclusively linked to sediment-derived melts, raising questions about the validity of previous interpretations and highlighting the need to consider alternative sources and processes. This underscores the importance of carefully evaluating the specific geological context when interpreting high δ¹⁸O signatures.