An assessment of the age and tectonic setting of volcanics near the base of the Windermere Supergroup in northeastern Washington: implications for latest Proterozoic – earliest Cambrian continental separation

Mafic metavolcanics of the Huckleberry Formation in northeastern Washington occur near the base of the Windermere Supergroup, a sequence of immature clastic rocks thought to have been deposited during a rift event associated with the establishment of the early Paleozoic miogeocline of the North American Cordillera. Previously reported K–Ar data from the Huckleberry volcanics yielded a wide scatter of ages, with a preferred age of extrusion of between 827 and 918 Ma, in apparent agreement with the Late Proterozoic (800–900 Ma) age assigned to this rift event based on geologic relations and ages from the youngest rocks unconformably underlying the Windermere Supergroup. Quantitative subsidence analyses that have been recently applied to the post-rift strata of the miogeocline yield ages for the final phases of rifting, which led directly to continental separation and the onset of sea-floor spreading, of 555–600 Ma. A significant problem arises between the older ages for rifting and the younger ages for breakup, since it implies that a rift phase preceding breakup could have spanned more than 200 Ma.This paper presents new geochemical and isotopic data from the same exposures of Huckleberry volcanics analyzed by K–Ar techniques in an attempt to assess their tectonic setting and age of extrusion. Major- and trace-element compositions are found to be consistent with the rift setting previously interpreted for the Windermere Supergroup. Isotopic analyses, although inconclusive with respect to the true age of the Huckleberry volcanics, indicate that the Rb–Sr system has been disturbed subsequent to extrusion of the volcanics, and therefore the K–Ar ages should be considered suspect. Isotopic data that are beginning to emerge from the northern Canadian Cordillera indicate that the Windermere Supergroup may in fact be younger, in closer agreement with the ages for breakup indicated by the subsidence analyses.