The 780 Gunbarrel Magmatic Event: U-Pb Documentation of Widespread Mafic Magmatism Along the Neoproterozoic Western of Laurentia

Precise U-Pb baddeleyite dating of mafic igneous rocks provide evidence for a widespread and synchronous magmatic event that extended for >2400 km along the western margin of the Neoproterozoic Laurentian craton. The gabbros dated in this study include three mafic sheets that intrude crystalline basement rocks of the Northwest Territories (the Gunbarrel, Calder and Faber Lake gabbros), a mafic sill exposed in the Mackenzie Mountains (Concajou Canyon sill), a mafic dike in the Muskwa Ranges of British Columbia (Muncho Lake diabase), and three northwest-trending mafic dikes in the Tobacco Root (group B dikes) and Beartooth Mountains (Christmas Lake dike) of Montana and Wyoming. U-Pb baddeleyite analyses for eight intrusions from seven localities yield ²⁰⁷Pb/²⁰⁶Pb ages that range from 775 to 782 Ma, with most analyses less than 2% discordant. The ²⁰⁷Pb/²⁰⁶Pb dates from the eight mafic dikes and sheets are statistically indistinguishable and we have combined sixteen individual baddeleyite analyses to yield a composite U-Pb Concordia age of 780.3 +/- 1.4 Ma (95% confidence level). We term this 780 Ma event the Gunbarrel magmatic event based on exposures of the Gunbarrel gabbro found along Gunbarrel Inlet, Great Bear Lake, Northwest Territories. Other mafic rocks that may be part of the gunbarrel event include some of the voluminous mafic sills that intrude the Belt/Purcell Supergroups in the U.S. and Canada and the northwest-trending dike at Mount Moran in the Teton Mountains of Wyoming. Overall, this is the geographically largest mafic event yet identified along the Neoprotoerozoic western margin of Laurentia. The origin of the mafic magmatism of the Gunbarrel event is not clear, but it may be related to mantle plume activity or the upwelling of asthenosphere leading to crustal extension accompanying initial breakup of the supercontinent Rodinia and development of the proto-Pacific Ocean. The mafic magmatism of the Gunbarrel magmatic event at 780 Ma predates the voluminous magmatism of the 723 Ma Franklin igneous event of the northwestern Canadian Shield by about 60 m.y. The precise dating of the extensive Neoproterozoic Gunbarrel and Franklin magmatic events, combined with paleomagnetic data, provide unique time-markers and piercing points that can ultimately be used for robust testing of Neoproterozoic continental reconstructions.