ORIGIN AND DEVELOPMENT OF THE DEVIL’S BEDSTEAD PYROXENITE WITHIN THE PIONEER MOUNTAINS METAMORPHIC CORE COMPLEX, CENTRAL IDAHO

The Pioneer Mountain metamorphic core complex in central Idaho is primarily composed of granites and metamorphosed sedimentary rocks from the Belt Supergroup and has been dated to ~2.6 Ga. Much of the complex has been exposed at the surface due to activity along the Wildhorse fault. Among the numerous igneous and metamorphic intrusions in the core complex is the Devil’s Bedstead pyroxenite. This pyroxenite is unique, in that it was discovered at ~11,000 feet (>3.3 km) above sea level. Its composition and characteristics suggest that it did not form in the mantle as the majority of pyroxenites do, but rather, originated from a crustal basaltic magma chamber. Given that pyroxenites are exceedingly rare at Earth’s surface, we aim to determine the timing of formation and emplacement of this pyroxenite, and its relationship to the regional core complex geology. The pyroxenite may have emerged during uplift events that exposed rocks from the root of the mountain, or it may have formed later on during a volcanic eruption in the region called the Challis Volcanic Event (CVE). Petrological and whole rock geochemical analyses have been performed on samples collected of the Devil’s Bedstead pyroxenite and surrounding rocks, in an effort to characterize textural and compositional changes in the intrusion across its width. Initial petrological results show a slight change in mineral crystal size and compositional changes (including alteration) throughout the intrusion. Whole rock geochemical analysis will be used to trace fluid migration throughout the intrusion and across the entire region, and correlate it to other intrusions and structures within the core complex. Characterizing the fluctuation in trace elements across the entire intrusion will lead to a more comprehensive model of formation and emplacement of the pyroxenite. The formation and composition of the Devil’s Bedstead pyroxenite has also been directly compared to that of several pyroxenites found throughout the Adirondack region in New York State. Allthough the pyroxenites in New York State formed at much deeper depths and in a different manner to the Devil's Bedstead pyroxenite, this comparison will help constrain a possible formation mechanism for the Devil's Bedstead pyroxenite.