The Owyhee Mountains of southwestern Idaho are constructed of Cretaceous granitic rocks of the Idaho batholith and thick sequences of Cenozoic volcanics. Igneous rocks in these mountains record a fundamental transition of tectonic regimes in their systematic changes of magmatic style and composition through time. Cretaceous through Oligocene age rocks were produced from predominantly subduction-related magma sources associated with convergent tectonism along the Pacific margin of North America. Miocene age tholeiitic basalts and rhyolites erupted in an extensional tectonic setting, but many of these basalts have trace element and isotopic characteristics indicating a residual subduction component in their mantle sources. The Cenozoic section consists of: (1) silicic Challis volcanics of Eocene age; (2) Oligocene age orogenic andesites and mildly-alkaline basalts of the Salmon Creek volcanics (SCV); and (3)a Miocene to Pliocene age bimodal sequence of tholeiitic basalt and rhyolite. Based on the lithologic and compositional characteristics of these rocks, volcanism in the Owyhee area apparently became more mafic (ignoring the Miocene rhyolites) and less hydrous through time. This progressive change in the composition of the volcanism parallels a decline both in the interaction between mantle-derived magmas and continental crust, and in the effects of subduction-related components in these magmas. Asthenospheric mantle dominanted the sources of the mafic and intermediate magmas until late Cenozoic time when contributions from subcontinental lithospheric mantle became significant. Compositional trends across a well-exposed section of Oligocene andesites indicate a progression of igneous processes through time at a single eruptive center. Early in the history of this eruptive center, magma mixing and interaction with crustal rocks were significant. As the system evolved, fractional crystallization emerged as the dominant process controlling the compositions of erupted lavas. Structural features in the Cenozoic volcanics follow the temporal progression of magma compositions. The Idaho batholith was emplaced into metamorphic rocks with pre-existing compressional deformation (vertical maximum compressional strain). Conjugate faulting within the Challic volcanics and the SCV indicate strike-slip conditions (vertical intermediate compressional strain). Parallel normal faults in the Miocene basalts show that the transition to an extensional regime (vertical minimum compressive strain) was essentially complete by middle Miocene time
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