U-Pb Geochronology and Geochemistry of Intrusive Rocks from the Cougar Creek Complex, Wallowa Arc Terrane, Blue Mountains Province, Oregon-Idaho

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The Cougar Creek Complex is a compositionally diverse intrusive suite constructed of variably deformed and nondeformed dikes and plutons interpreted as the roots of the Wallowa arc terrane in the Blue Mountains Province, Oregon-Idaho. New high-precision U-Pb zircon ages, geochemical data, and structural analysis define two compositionally and temporally distinct cycles of magmatism, and two episodes of contractional deformation in the Cougar Creek Complex. From Middle Permian to Early Triassic time (265.4 ± 0.2 Ma to 248.8 ± 0.1 Ma), the Wallowa arc was dominated by silicic arc magmatism. During the Early to Middle Triassic (248.8 ± 0.1 Ma to 229.4 ± 0.1 Ma), an apparent hiatus in intrusive activity suggests a cessation or lull in arc magmatism, or a shift of the arc axis. This lull in volcanic activity coincides with D1 deformation, uplift, and erosion of the Wallowa arc. In the Late Triassic (229.4 ± 0.1 Ma to 229.1 ± 0.5 Ma), voluminous mafic to intermediate magmatism, exhibiting trace-element characteristics of a subduction-modified, strongly depleted mantle source, was initiated, and this represents a change in “normal” arc magmatism. We interpret the sequence of D1 deformation, an apparent gap in igneous activity, exhumation of the Wallowa arc, and a switch from “normal” silicic arc magmatism to depleted, mid-ocean-ridge basalt–like mafic volcanism as a response to spreading-ridge subduction and subsequent slab window magmatism beneath the overriding Wallowa arc. Late Triassic D2 synmagmatic left-lateral mylonitic shear zones in the Cougar Creek Complex illustrate strain partitioning in an intrusive, transpressional arc axis environment undergoing sinistral-oblique subduction. Synchronous U-Pb crystallization ages, 40Ar/39Ar cooling ages, the abundance of plutonic clasts within conglomerate units, and unimodal Late Triassic detrital zircon ages for volcanic sandstone units suggest uplift and erosion of the Wallowa arc during the Late Triassic; this is inferred to be related to the upwelling of buoyant asthenosphere through an opening slab window beneath the Wallowa arc. We propose that Late Triassic mafic igneous rocks from the Wallowa terrane and Wrangellia resulted from the subduction of a spreading ridge and slab window magmatism beneath a late Paleozoic arc, thus linking the Late Triassic geologic evolution of two fundamental components of the Cordilleran orogen.