Ar‐Ar and U‐Pb Geochronology of a Late Paleoproterozoic Rift Basin: Support for a Genetic Link with Hudsonian Orogenesis, Western Churchill Province, Nunavut, Canada

Abstract

The Baker Lake Group (Baker Sequence) represents the record of the formative stage of Baker Lake Basin, a series of generally elongate, northeast‐striking, half‐graben, and fault‐bounded troughs filled with continental redbeds and coeval voluminous ultrapotassic volcanic rocks. An estimate for the time of basin initiation is given by a U‐Pb (zircon) age of $$1833\pm 3$$ Ma, obtained from a basal volcanic flow at the western end of the basin, which is in agreement with a less precise 40Ar/39Ar (phlogopite) step‐heating plateau age of $$1837\pm 8$$ Ma from a flow located at a similar stratigraphic level in the eastern Baker Lake Basin. 40Ar/39Ar analysis of phlogopite phenocrysts in a syenite that intrudes the lower part of the Baker Sequence yielded a plateau age of $$1811\pm 12$$ Ma. The syenite also intrudes sandstones containing detrital zircons with xenotime (YPO4) overgrowths, known to form during burial diagenesis. In situ U/Pb SHRIMP analysis of these overgrowths yields an upper intercept age of $$1838\pm 27$$ Ma, which is within analytical uncertainty of the ages obtained from the volcanic flows. Alluvial conglomerates near the top of the Baker Sequence contain discontinuous layers of laminar carbonate cements interpreted as geothermal travertine. Specific calcite layers within the travertine have very high 238U/204Pb values and yield a Pb‐Pb isochron age of $$1785\pm 3$$ Ma, considered to represent a minimum age for deposition of the Baker Sequence. Our data suggest that the Baker Sequence was deposited over an interval of approximately 55 m.yr. (1840–1785 Ma), within error of the predicted periodicity of a second‐order sequence. This interval coincides with collisional and postcollisional deformation and magmatism in the Trans‐Hudson orogen and thus supports interpretations that the Baker Lake Basin formed in response to related far‐field extension.