Petrography, Geochemistry, and Geochronology of Cretaceous Porphyry Intrusives near Red Lode, Montana

There are many localities of Cretaceous porphyry intrusives in and around the Beartooth Range and particularly along the Beartooth Front near Red Lodge, Montana. Although Rouse et al. (1937) carried out a study devoted to them, they have been almost ignored since then and today new methods are available in order to further our understanding of their history. In recent decades, there has been some confusion as to the age of these rocks due to their proximity to Paleogene porphyry felsic to intermediate intrusives. In contrast, Cretaceous porphyry intrusive rocks represent intrusion prior to the main Laramide thrusting of the region. These are felsic porphyries ranging in composition from andesite to dacite and all have large plagioclase phenocrysts with prominent zoning. The initial geochemical data suggests that these intrusives are the result of fractional crystallization. Geologic observations of the region have shown through relative geochronology that these rocks are late Cretaceous in age and this has been confirmed by preliminary geochronology by Barry Shaulis which provided an age of 93 Ma, and by this study which yielded an age of 96.7 +/- 1.77 Ma.