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

Crustal contamination often results from the assimilation of supracrustal (volcanic or sedimentary) materials during the evolution of magma. It is generally accepted that sediment assimilation often results in increasing peraluminousity and δ¹⁸O whereas the assimilation of altered mafic volcanics would increase δ¹⁸O but not peraluminosity.

The high δ¹⁸O metaluminous granitoids of the Frontenac terrane of the Grenville Province have variably been explained by sediment assimilation however the metaluminous signature argues against this hypothesis. Another potential source of elevated δ¹⁸O are carbonate lithologies. An influx of devolatilized high δ¹⁸O carbonate-derived fluids could also raise the δ¹⁸O of the magma, but not change the peraluminousity. We postulate that the high δ¹⁸O signatures observed in the alkaline granitoids of the Frontenac terrane can be attributed to the incorporation of carbonate-derived fluids. Our data show elevated δ¹⁸O (~12±4‰), biogenic δ¹³C (-17±8), and low δD (-63±15). This is consistent with marbles found in proximity of the plutons.

Importantly, the granitoids of the Frontenac terrane lack a correlation between aluminosity and δ¹⁸O implying the elevated δ¹⁸O is not the product of progressive pelite assimilation. Continuous assimilation of both pelite and altered oceanic crust would result in an elevated δ¹⁸O but would result in either a peraluminous melt (pelite-derived) or a non-biogenic δ¹³C signature (altered ocean crust-derived).

To test this hypothesis further, we will perform analyses of ⁸⁷Sr/⁸⁶Sr. These data will provide a better understanding of the controls on δ¹⁸O enrichment in granitoids as well as the recycling of sedimentary materials in magmatic systems.