PETROGRAPHY, GEOCHEMISTRY, AND GEOCHRONOLOGY OF CRETACEOUS PORPHYRY INTRUSIVES NEAR RED LODGE, 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. Except for a study by Rouse et al. (1937), they have been almost ignored since then. In recent decades, there has been some confusion as to the age of these rocks due to their proximity to both Cretaceous and Paleogene porphyry felsic to intermediate intrusives. The timing of these are important as Cretaceous porphyry intrusive rocks represent intrusion prior to the main Laramide thrusting of the region. In contrast, if they are Paleogene they intruded during and after Laramide thrusting. In this study we performed petrographic, geochemical, and geochronological analyses of these intrusives to better understand the nature of these rocks and help understand the regional tectonic history. These felsic porphyries range in composition from andesite to dacite and all have large plagioclase phenocrysts with prominent zoning. Our 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; this has been confirmed by preliminary geochronology on two occurrences with ages of 93 Ma in a sample that cross-cuts Cambrian sediments and of 96.7 +/- 1.77 Ma from a sample that cross-cuts the Archean basement. This confirms that these are part of the Cretaceous series of porphyry intrusives which formed prior to major thrusting in the area. This extends the Cretaceous continental arc and calls into question the time frame of flat slab subduction underneath Montana.