Geochronology of Archean Gneissic rocks in the Southern Bighorn Mountains, USA

The Bighorn Mountains are the dominant topographic feature in north-central Wyoming. They were uplifted during the Laramide Orogeny and include rocks that range in age from Archean to Tertiary. The Archean rocks exposed at the core of the Bighorn Mountains range in age from 2.96-2.85 Ga. The southern tracts of the Archean massif consist mainly of >2.9 Ga deformed and strongly foliated ortho- and paragneiss of tonalitic and granodioritic composition. These rocks are intruded by weakly to unfoliated granitoids, and lesser mafic and ultramafic rocks. Here we present new geochronologic data (zircon U-Pb) for nine strongly foliated orthogneisses, one meta-granite, and the tonalite at the top of Cloud Peak, to further constrain the geochronological framework of the Bighorn Mountains. We interpret that these ages reflect a magmatic protolith. Analyses were conducted at the University of Arizona Laserchron Center and the University of Arkansas. The oldest gneisses are >2.96 Ga and are present at Powder River Pass and along the Middle Fork of the Powder River. Gneisses that range from 2.95-2.96 Ga occur near Hazelton and along the North Fork of Crazy Woman Creek. Gneiss north of Dartons Bluff are ~2.93 Ga and are ~2.91 Ga near Circle Park. The meta-granite is ~2.89 Ga and the tonalite at the top of Cloud Peak has an age of 2.85 Ga, which is the youngest known quartzofeldspathic rock in the southern Bighorn Mountains. The 2.96 Ga gneisses are among the oldest rocks known in the Bighorn Mountains and are similar in age to granodiorite bodies northeast of Geneva Pass. The 2.91 Ga gneiss near Circle Park is the youngest orthogneiss in the southern Bighorn Mountains. Our data indicate a general younging of gneissic rocks from south to north.