NEW CONSTRAINTS ON COOLING AGES FOR PROTEROZOIC ACCRETED ARC AND METAMORPHIC CORE ZONE ROCKS OF THE BIG SKY OROGEN IN SOUTHWESTERN MONTANA

Biotite in pelitic schist and hornblende from amphibolite demonstrate ⁴⁰Ar/³⁹Ar date constraints on cooling ages from the Proterozoic Big Sky orogen of southwest Montana in the Highland Mountains, the Henrys Lake Mountains, the Little Belt Mountains, and the Ruby Range.

Biotite and some muscovite were extracted from pelitic schists and hornblende from interlayered amphibolites, which are widespread in the Highland Mountains and a less dominant lithologic unit in the Henrys Lake Mountains, Little Belt Mountains, and Ruby Range. These minerals in thin section have been put in a petrographic context. Hornblende grains in amphibolite samples from the study areas demonstrate a well-developed fabric and are texturally simple. In pelite samples, biotite may exist in more than one fabric or habit: one large and blocky and one thin and elongate, but in some places only one habit is observed. These minerals were dated by the ⁴⁰Ar/³⁹Ar method to determine the time of cooling past retention. Pelites from the Little Belt Mountains, Highland Mountains, Ruby Range, Hebgen Lake area, and Henrys Lake Mountains have differing mineral assemblages and peak metamorphic temperatures. Therefore, ⁴⁰Ar/³⁹Ar dates document varying temperature transitions in each area. The results from these age analyses provide constraints on the metamorphic history and insight into the role the rocks played at the time of cooling. The Henrys Lake Mountains are situated on the foreland side of the Big Sky orogen whereas the Little Belt Mountains are in the hinterland and are probably part of an accreted arc sequence. (Vogl et al., 2004) The Ruby Range and the Highland Mountains were most likely in the Wyoming province prior to the Big Sky orogeny but their positions within the Big Sky orogen are uncertain.