PETROLOGICAL AND GEOCHEMICAL ANALYSIS OF THE CHALLIS FORMATION VOLCANIC SUITE (IDAHO)

The Challis Formation is a suite of volcanic deposits in the Copper Basin near Mackay, Idaho. This geologic feature is mapped by USGS as a single deposit, however, during recent field examination at North Fork Salmon River variation within the formation was seen, with at least four different units identified. The volcanic deposits found at this locality (43ᵒ 55’ N, 114ᵒ 07’W) are of varying chemical composition and petrological characteristics. The goal of this undergraduate independent study was to further our understanding of magma genesis and its migration toward the Earth surface using petrography and geochemistry. Our first objective was to characterize the magmas of the units involved in order to better constrain their petrogenesis and evolution. The units were first mapped and classified in the field based on appearance and stratigraphic sequence, identifying 4 units. In the laboratory a closer analysis was made using petrographic microscopy and geochemistry. Mineral compositions were obtained using an electron probe micro analyzer and then interpreted using classification diagrams. The major minerals found throughout the units were plagioclase feldspar, clinopyroxene, and enstatite. Based on mineral compositions, it appears that the units derive from a single magma source with chemical differentiation over time. Additional whole-rock geochemistry should provide further details on the petrogenesis of the melts as well as this area’s interaction with the Snake River Plain and the Yellowstone hot spot complex.