Age and emplacement depth of intrusions in the southeastern Canadian Cordillera: Implications for Jurassic to Paleocene exhumation

Abstract

Synorogenic exhumation is important for understanding the evolution of mountain belts but is often difficult to discern because traditional thermochronology methods mostly date the final stages of exhumation. Intrusions emplaced during orogenesis provide an alternate method to elucidate these exhumation patterns since they preserve the timing and depth of their emplacement. We apply this concept the southern Omineca belt of the Canadian Cordillera by combining new and existing zircon UPb and mica Ar/Ar ages from 43 intrusions as well as new and existing estimates of emplacement depth from 23 contact aureoles. Seventeen new zircon UPb laser ablation inductively coupled plasma mass spectrometry results are presented in the paper, spanning the existing range of known ages for four episodes of phanerozoic magmatism in this region: Middle Jurassic (174–154 Ma), mid-Cretaceous (118–95 Ma), a newly identified Late Cretaceous (78–76 Ma), and Paleocene (66–60 Ma). This new work has resolved two new distinct periods of mid-Cretaceous magmatism at ~100 and ~ 76 Ma from what was previously considered to be a single mid-Cretaceous event (Bayonne magmatic suite). Furthermore, the Fry Creek batholith, previously considered to be emblematic of the Bayonne suite, the newly dated ~60 Ma Nelson stock and Shoreline stock, and the previously dated 55–62 Ma Ladybird magmatic suite form part of a newly identified phase of 66–55 Ma Paleocene magmatism in the region. Compiled intrusion ages were then combined with estimates of emplacement depth determined from metamorphic mineral assemblages in contact aureoles developed in pelitic host rocks. Jurassic intrusions typically have higher pressure staurolite + andalusite-bearing contact aureoles (3.3–4.0 kbar) corresponding to depths of 12.5–15 km, whereas Cretaceous and Paleocene intrusions mainly have lower pressure andalusite + cordierite-bearing contact aureoles (2.3–2.9 kbar) corresponding to depths of 8.5–11 km. These data indicate regional exhumation of 2–6 km occurred between the Middle Jurassic (180–160) and mid-Cretaceous to Paleocene (120–60 Ma) periods of Cordilleran orogenesis. In contrast, pluton emplacement depths changed by <2 km, suggesting minimal exhumation occurred in the southern Canadian Cordillera between the mid-Cretaceous and Paleocene. This extended lull in exhumation may be related to the partitioning of compressional deformation east into the foreland along a basal decollement beneath the study area during this time.