Paleomagnetic and 40Ar/39Ar geochronologic data from late Proterozoic mafic dikes and sills, Montana and Wyoming

Abstract

We report paleomagnetic and geochronologic results from two Late Proterozoic mafic dikes exposed in Archeancored uplifts from northwestern Wyoming and 40 Ar/ 39 Ar results from a gabbro sill that intrudes lower sedimentary strata of the Belt Supergroup in western Montana. The gabbro sill yields a 40 Ar/ 39 Ar plateau date of 776 Ma that records the age of sill emplacement during Late Proterozoic time. Paleomagnetic results from the mafic dike at Mount Moran, Teton Range, and the Christmas Lake dike, Beartooth Mountains, indicate magnetizations with similar directions and virtual geomagnetic poles (VGP's), which we interpret to be primary thermoremanent magnetizations acquired during dike emplacement and cooling. A hornblende from the Christmas Lake dike yields a 40 Ar/ 39 Ar plateau date of 774 Ma, which is statistically identical to that of the gabbro sill from western Montana. The similarity of the dike site-mean directions and VGP's suggest that their magnetizations are of similar age. 40 Ar/ 39 Ar results from a hornblende-pyroxene concentrate from the Mount Moran dike are discordant and contain excess 40 Ar; a precise estimate of the age of this sample is not possible given our data, but the paleomagnetic and geochronologic results suggest that it is of Late Proterozoic age, and not Middle Proterozoic as previously argued. The VGP's from the Mount Moran and Christmas Lake dikes are similar to those of identical age from southwestern Montana and northern and western Canada. These mafic dikes and sills are part of a regional magmatic event that affected the western part of the Laurentian craton at about 780 Ma.