Paleoproterozoic Metamorphism in the Northern Wyoming Province: Implications for the Assembly of Laurentia

Abstract

U‐Pb ages measured on zircons from the Tobacco Root Mountains and monazite from the Highland Mountains indicate that the northwestern Wyoming province experienced an episode of high‐grade metamorphism at ∼1.77 Ga. Leucosome emplaced in Archean gneisses from the Tobacco Root Mountains contains a distinctive population of zircons with an age of 1.77 Ga but also contains zircons to ∼3.5 Ga; it is interpreted to have been derived primarily by anatexis of nearby Archean schist. A granulite facies mafic dike that cuts across Archean gneissic banding in the Tobacco Root Mountains contains two distinct populations of zircons. A group of small (<50 μm) nonprismatic grains is interpreted to be metamorphic and yields an age of 1.76 Ga; a group of slightly larger prismatic grains yields an age of 2.06 Ga, which is interpreted to be the time of crystallization of the dike. Monazite from a leucogranite from the Highland Mountains yields a well‐defined age of 1.77 Ga, which is interpreted as the time of partial melting and emplacement of the leucogranite. These results suggest that the northwestern Wyoming province, which largely lies within the western part of the Great Falls tectonic zone, experienced a metamorphic maximum at 1.77 Ga. This age is ∼100 m.yr. younger than the proposed time of Wyoming‐Hearne collision in the central Great Falls tectonic zone (1.86 Ga) and suggests that the northwestern Wyoming province may have been involved in a separate, younger collisional event at ∼1.77 Ga. An event at this time is essentially coeval with collisions proposed for the eastern and southeastern margins of the province and suggests a multiepisodic model for the incorporation of the Wyoming craton into Laurentia.