Syn-rift volcanism (ca. 670 Ma) in the lower Windermere Supergroup, southern Canadian Cordillera: New constraints on the syn- to post-rift transition for northwestern Laurentia

Abstract

The southern Canadian Cordillera is considered the type area for the Windermere Supergroup, where it had long been assumed that the syn-rift basaltic lavas of the Irene Formation were co-genetic with the ca. 720 Ma Franklin LIP. Accordingly, the inferred age of the Irene volcanics is variously cited for either the breakup of Rodinia or for the base of Cryogenian strata in the southern Canadian Cordillera, and yet they have not been directly dated until now. Reported here are geochronological data from a volcanic sample of the Irene Formation yielding a U-Pb SHRIMP zircon age of 669.6 ± 6.7 Ma, which is significantly younger than the presumptive age. The Irene Formation is overlain by basin floor turbidite facies that correlate with the Horsethief Creek Group, which represent some of the oldest post-rift deposits in the study area and are interpreted as the earliest indication of a passive continental margin setting for northwestern Laurentia. The dated sample was collected from the upper portion of the Irene Formation and so the transition from syn-rift to post-rift is younger than ca. 670 Ma, which is estimated at approximately 660–650 Ma. Additionally, these new age data place the associated diamictites at the end of the Sturtian glaciation.