U-Th-Pb analyses of zircon suites document the presence of three lower Paleozoic alkalic plutons in east-central Idaho. The igneous bodies range in composition from alkali-feldspar syenite to alkali-feldspar granite. The epizonal Arnett Creek pluton contains a morphologically bimodal suite of anhedral and euhedral zircon, which combined gives an upper intercept age on a concordia diagram of 492 ± 39 m.y. Zircon analyses from the Deep Creek pluton plot near the 500-m.y. locus on concordia, but the pattern suggests the presence of a small component of inherited radiogenic lead. Limited Rb-Sr whole-rock data for the Arnett Creek and Deep Creek plutons are compatible with an Early Ordovician age but are too imprecise to refine interpretations. Zircon from the epizonal Beaverhead pluton, for which the enigmatic Ordovician-Silurian K-Ar and Rb-Sr mineral ages have long been debated, yields an approximate age of 483 m.y.
The tectonic setting of these intrusions is poorly known. The coincidence of early Paleozoic plutonism and independently documented syndepositional faulting with the coeval topographic high, known variously as the "Lemhi" or "Salmon River Arch," tentatively suggests a mutual tectonic control. Further geochronologic, stratigraphic, and especially petrogenetic studies, however, are required to evaluate this possibility.