Whole rock chemistry and tectonic evaluation of Permian-Triassic Seven Devils juvenile arc, western Idaho

The accretion of exotic terranes to the western margin 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. The Permian to Triassic Seven Devils group represents such an accretionary complex 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 is focused on analyzing intrusions within the Seven Devils group. The petrological and geochemical characteristics of Seven Devils rocks are a direct result of mantle and fractional crystallization processes. Most of the samples exhibit weak pervasive greenschist facies metamorphism; hence, only immobile elements are useful to characterize their source. Major minerals present include clinopyroxene, calcium-rich plagioclase, biotite mica, and hornblende. 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, 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, and CaO ranges from 3.4 to 10.8 wt.%. Two groups of rocks, distinguished based on chondrite-normalized and primitive mantle-normalized REE diagrams, are classified as following calc-alkaline and tholeiitic trends respectivly. Both groups have flat to slightly depleted HREE segment, positively fractionated LREE segment, and a small negative Eu anomaly. Calc-Alkaline samples 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.