Geochronological constraints on magmatism and polyphase deformation and metamorphism in the southern Omineca Belt, British Columbia

Abstract

The Omineca Belt between Nelson and Creston in southeastern British Columbia was affected by overlapping pulses of Mesozoic magmatism, metamorphism, and deformation. U–Pb geochronological data from zircon and monazite were collected by laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) to constrain the timing of these events. The Porcupine Creek stock (162.3 ± 1.3 Ma) intruded across folds and fabrics associated with the earliest phase of regional deformation and metamorphism (D1M1), restricting it to the Early–Middle Jurassic. The Jurassic structures are overprinted northwards by Early Cretaceous deformation and metamorphism (D2M2). The Baldy pluton (117.8 ± 1.2 Ma) crosscuts the regional 144–134 Ma M2 isograds, yet was pervasively affected by the D2 deformation, indicating that D2 deformation outlasted M2 metamorphism but had ceased by 111 Ma, the age of an undeformed pluton. Monazite dates from a kyanite-bearing rock in the contact aureole of the Middle Jurassic Wall stock overlap with the age of the intrusion (167 Ma), indicating a contact rather than regional origin for the kyanite. In the southeast part of the study area, three samples from the regional sillimanite zone contain monazite intergrown with sillimanite that yield dates between 80 and 69 Ma, indicating an episode of Late Cretaceous (M3) Barrovian metamorphism and deformation (D3). To the north of this domain, in an area characterized by the older D2M2 deformation, a sillimanite zone schist contains two main monazite age populations, suggestive of overlapping effects of Early Cretaceous and Late Cretaceous metamorphic episodes.