Paper No. 50-6
Presentation Time: 9:55 AM
PRELIMINARY RESULTS OF PETROGRAPHIC ANALYSIS AND GEOCHEMICAL CHARACTERIZATIONS OF THE PERMIAN- TRIASSIC SEVEN DEVILS GROUP- WESTERN IDAHO
During the Early Triassic, tectonic processes were dominated by the break up of the supercontinent Pangea. Subduction, related to the Pangea break-up, created volcanic arcs and continental fragments, which are geological features known as terranes. The Seven Devils Mountains are located in the Wallowa Terrane, which is dominated by Permian and Triassic volcanogenic rocks. The formation of most of the Seven Devils Mountains rock units was a result of volcanism and sedimentation from the volcanic arc at a convergent plate boundaries during the late Permian and Triassic periods. These rocks are among the oldest known rocks in western Idaho. Seven Devils Mountains are considered to have been an island-arc system that developed as a result of east- directed subduction of Pacific Ocean. However, there have been few studies of the Seven Devils Mountains, and no recent works. Therefore, this study attempts to analyze the evolution of deformation, petrologic characterization, trace element analysis, and the structural history of the Seven Devils Mountains with the primary goal being to determine the tectonic setting during formation of the Seven Devils Mountains.
Our preliminary study, using petrography and XRF (X-ray fluorescence) analyses suggests the majority of samples are basalts with an abundance of the minerals: olivine (35%), pyroxene (40-60%) calcium-rich plagioclase (30-50%), biotite and muscovite mica (20%), and some hornblende (15%). Trace element discriminations diagrams show that Seven Devils Mountains rocks are have two affinities: MORB and island arc basalts. However, as opposed to two distinct affinities, the series is continuous suggesting that the Seven Devils Mountains might represent either a juvenile arc - back-arc system or distinct arc and back-arc system juxtaposed due to emplacement.