Crustal Evolution in the Southwestern Grenville Province: Petrogenesis of Frontenac Intrusive Rocks and Provenance of the Flinton Group

Abstract

The tectonostratigraphic evolution of the Flinton Group within the Mazinaw terrane remains poorly constrained, primarily due to extensive deformation and high-grade metamorphism during the Grenville orogeny. This study integrates new zircon U–Pb geochronology from clastic sedimentary and intrusive units to better constrain the provenance, maximum depositional ages (MDAs), and tectonic affinity of the Flinton Group. Our data show that the youngest unit (MDA = 1144 ± 19 Ma) has a significantly higher proportion (~75%) of pre-Grenville (≥1300 Ma) zircons. In contrast, the oldest unit (MDA = 1157 ± 9 Ma) is mainly composed of syn-Grenville detritus (~49%) with less cratonic contribution. These differences in zircon population indicate a shift over time from mainly arc-related sources to a mixture of cratonic and arc-related sources, interpreted as deposition in an extensional backarc basin. We suggest that the Flinton group records syn-orogenic sedimentation during the phase of arc magmatism before the Ottawan orogenic phase (~1090-1020 Ma), with diminishing cratonic input linked to progressive tectonic isolation of the basin from Laurentian sources during the opening of the backarc basin. Crustal contamination during magma evolution commonly involves the assimilation of pelitic sediments, which typically increases peraluminosity and δ18O of the resulting melt, leading to the widespread interpretation that high δ18O signatures reflect the assimilation of sediment. However, trace zircon elements, major element data, and stable isotopic data (δ18O, δ13C (fluid inclusion in quartz) and δ2H (fluid inclusion in quartz)) of the metaluminous intrusive rocks in the Frontenac terrane prove otherwise. We report that these rocks are characterized by elevated δ18O (~11.9 ± 2‰), depleted δ13C (-9.7 ± 4‰) and mantle-like δ2H (-56.2 ± 10‰), but these isotopic characteristics are inconsistent with pelite assimilation as previously proposed. In particular, the absence of a correlation between peraluminosity and δ18O undermines models that invoke sedimentary assimilation as the primary cause of elevated δ18O. Instead, we suggest that the high δ18O values reflect incorporation of hydrothermally altered volcanic material rather than pelitic sediment. This alternative explanation not only accounts for the isotopic data but also aligns with the tectonic setting of the arc-back-arc system during Flinton Group deposition.