WHOLE ROCK CHEMISTRY AND TECTONIC EVALUATION OF PERMIAN- TRIASSIC SEVEN DEVILS JUVENILE ARC, WESTERN IDAHO

The accretion of the Western Cordillera to the western edge of North America, starting at the end of Paleozoic and ending in the Cenozoic, was associated with large-scale volcanism and resulted in the formation of many mountain ranges during the accretion. The Permian to Triassic Seven Devils group represents such a mountain range located in the Wallowa terrane of western Idaho. This geologically complex region has not been thoroughly studied which raises questions about the current geologic interpretation of Seven Devils Mountains.

This study has focused on intrusions from within the Seven Devils group. The petrological and geochemical characteristics of Seven Devils rocks are a direct result of the mantle and fractional crystallization processes. Most of the samples exhibit week pervasive greenschist facies metamorphism, hence only immobile elements are used to characterize their source. Major minerals present include clinopyroxene, calcium-rich plagioclase, biotite mica, and hornblende fragments. The most common secondary minerals are quartz, albite, epidote, calcite, and chlorite. The samples show variable silica content ranging between 41 wt.% to and 64 wt.% SiO2 wt.%, and are characterized by low MgO # (0.29–0.55 wt.%), MnO (1.6 to 5.44 wt.%), and TiO2 (0.8–1.4 wt.%) content. These rocks have an average 15.8 wt. % Al2O3, and 2.11 wt.% Na2O. CaO is ranging from 3.4 to 10.8 wt.%. Two groups of rocks can be distinguished based on chondrite REE and primitive mantle-normalized diagrams. Some samples have a flat HREE segment, positively fractionated LREE segment, and a small negative Eu anomaly. Others are characterized with plagioclase- like REE pattern, and a slightly negative fractionated LREE segment. Overall, Ti negative anomaly, a clear Nb depletion, and a positive Zr–Hf anomaly due to the presence of zircon are recognized in most of the Seven Devils rocks. Tectonic discrimination diagrams suggest two distinct cycles of magmatism in Seven Devils lavas. The system was initially dominated by calc-alkaline magmatism followed by tholeiitic MORB-like magmatism. We interpret the switch in magmatism to be related to the subduction of a spreading ridge beneath the Wallowa arc.