LAUSR

Abstract Diabase exposed amongst outcrops of the Little Dal Basalts (LDB) between the Coates Lake Group and overlying the Little Dal Group in the Mackenzie Mountains of northern Canada has a geochemical signature that is characteristic of the LDB and associated Tsezotene sills. Marginally concordant magmatic zircons from a sample of the diabase yielded CA-ID-TIMS weighted mean 207 Pb/ 206 Pb and 206 Pb/ 238 U dates of 778.4 ± 1.8 Ma (2σ) and 775.10 ± 0.54 Ma (2σ), respectively. These dates provide the first direct constraint on the age of the LDB and confirm a temporal relationship to the widespread Gunbarrel magmatic event that affected western North America during the protracted breakup of the supercontinent Rodinia. Hafnium isotopes were measured on three grains and calculated values of eHf at 775 Ma are 9.5 ± 0.4, 9.7 ± 0.2 and 9.3 ± 0.2 (2σ), consistent with a mantle plume genesis. Gunbarrel magmatism was not temporally related to the final fragmentation of Rodinia and the establishment of the west Laurentian continental margin (rift-drift transition), which did not occur until after 720 Ma. However, the age and stratigraphic position of the LDB suggest the Gunbarrel event may have thermally weakened the lithosphere leading to a period of intracontinental rifting and the deposition of the Coates Lake Group rift-sediments prior to 732 Ma. The sample of diabase also yielded a single 207 Pb/ 206 Pb 1565.7 ± 1.5 Ma xenocrystic zircon with a model Th/U ratio of 0.47 indicative of a crustal origin. Although there is limited evidence for felsic magmatism, metamorphism, and orogenesis of this age in western Laurentia, a range of felsic volcanic rocks and/or intrusive suites of this age do occur in southern, central and eastern Australia, supporting existing reconstructions of the Nuna/Columbia supercontinent that place the northwest margin of Laurentia adjacent to Australia.