SIGNIFICANCE OF LOCAL PROVENANCE IN BASAL SAUK QUARTZ ARENITE IN SOUTHWEST MONTANA

Although zircon U-Pb ages of N. American Cambrian sandstones have been studied extensively, little is known about the zircons in units in the northwestern United States (Flathead Sandstone). We determined zircon U-Pb ages in the Flathead Sandstone in the W. Highland Range at Camp Creek (S.W. Montana), where it locally overlies gneisses and amphibolites, thought to be late Archean or early Proterozoic. Here the Flathead Sandstone is ~20 m thick and it is deposited on a distinct paleosol. Two Flathead samples were studied, one of the lowermost 10 cm, one of the uppermost 30 cm. Forty-nine grains from each sample were analyzed by LA-ICP-MS. The largest grouping of grains from the upper sample have ²⁰⁷Pb/²⁰⁶Pb ages of 1.9-1.7 Ga (39%). In this sample there is one concordant or nearly concordant grain of late Archean ²⁰⁷Pb/²⁰⁶Pb age. The oldest ²⁰⁷Pb/²⁰⁶Pb ages are 2 grains at ~3.18 Ga. These zircon ages compare well with zircon age populations in quartzites of the Missoula Group (Belt Supergroup) (Ross and Villeneuve, 2002). Zircons from the lower sample are strikingly different. Here 16% of the grains have ²⁰⁷Pb/²⁰⁶Pb ages greater than 3.18 Ga, ranging up to 3.46 Ga. The lower sample may reflect partial derivation from the immediately underlying basement, the zircons in the upper sample may be more representative of detritus derived from the Belt Supergroup. These results for the lower sample would tend to support the suggestion by Gunn (1991, unpublihsed, Ph.D dissertation) that gneisses in the southern highland range include middle Archean components.