Abstract

Structural mapping and U-Pb dates from mafic sills in the Perma culmination of west-central Montana constrain the timing of a major rifting event in the early history of the Belt basin. Magmatic zircon from the Plains and Paradise mafic sills within the Prichard Formation yielded U-Pb ages of 1469 ± 2.5 and 1457 ± 2 Ma, respectively. The older sill invaded unconsolidated, wet sediments at a depth of less than 1.6 km, and its emplacement is coeval with widespread soft-sediment folding, mud diapirism, and bathymetric collapse of the basin between members E and F of the Prichard Formation. The youngest sill intruded near the base of the exposed section after significant accumulation of sediment had occurred in the basin. Correlation of these data with recent geochronologic results from the Moyie sills in British Columbia and Purcell lava in Montana suggests that the rifting and subsidence event was basin-wide, and that at least two thirds of the deposition of the Belt Supergroup occurred between ~1470 and 1440 Ma. These results support a continental rift model for the formation of the Belt basin.

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