EVALUATING THE CAUSE OF CRETACEOUS INTRAPLATE MAGMATISM IN THE SOUTHERN OMINECA BELT

Intraplate magmatism is a phenomenon where magma is produced away from an active plate margin, often linked to decompression due to extension/exhumation, or to anatexis due to crustal thickening. In the Canadian Cordillera, the Omineca crystalline belt represents substantial intraplate magmatism that occurred parallel to, but about 600 km inboard of the Cordilleran margin. Previous studies have linked Omineca magmatic rocks to regional metamorphism and crustal thickening, but the mechanisms driving magmatism remain enigmatic. This study integrates new whole rock geochemistry, together with zircon U-Pb dating and Lu-Hf isotope analysis to investigate the timing and petrogenesis of Cretaceous plutons in the southern Omineca Belt. Cretaceous granitoids are felsic (66% – 78% SiO₂), mildly to strongly peraluminous, and are more alkalic and have higher Rb/Sr than arc granitoids to the west. Trace elements show steep REE patterns with an enrichment in LREEs relative to HREEs (La/Yb > 10) and most plutons lack Eu anomalies, suggesting the stability of garnet in the magma source. Zircon U-Pb ages are complex and overdispersed, inherited Proterozoic and Paleozoic cores are common, and some zircons display bright rims in cathodoluminescence images. Age distributions reveal that magmatism in the Cretaceous occurred in two pulses, one in the mid-Cretaceous (110 – 95 Ma), and a second, minor pulse in the Late Cretaceous (79 – 75). Magmatic episodes follow Barrovian metamorphism, documented in the region at 144 – 134 Ma and 94 – 76 Ma. Initial zircon εHf values range from evolved to moderately juvenile (-16 to +7), with plutons intruding the Precambrian basement to the east being more evolved than those intruding the Paleozoic rocks of the Intermontane to the west. Whole-rock geochemical trends, zircon inheritance, and evolved εHf signatures of these plutons indicate that magmas were likely derived from crustal melting. High La/Yb and lack of Eu anomalies are consistent with crustal thickening, suggesting that Cretaceous Omineca granites formed as a result of anatexis driven by crustal thickening. Scattering of εHf values to moderately juvenile values, however, raise questions about the degree of mantle influence.