Coarsely porphyritic granite and augen gneiss in east-central Idaho have been dated by the U-Th-Pb zircon method. When plotted on concordia diagrams, the results give nonlinear data arrays that indicate the presence of at least one component of inherited radiogenic lead, as well as postcrystallization, probably recent, episodic lead-loss.Nevertheless, a systematic relationship between zircon size and convergence of the 207Pb/206Pb ages for the +100- and -100+150-mesh fractions from several samples are interpreted to provide a valid age of about 1,370 m.y. for these plutons. A single sample which is free of inheritance for all size fractions confirms this conclusion. Rb-Sr whole-rock samples are highly enriched in radiogenic strontium but do not define an isochron, probably because the system did not remain closed following crystallization of the plutons.

Intrusive contacts of the granitic rocks indicate that deformation and regional metamorphism of the host Yellowjacket Formation occurred prior to 1370 Ma, rather than prior to 1500 Ma as previously suggested by others. This refinement of the geochronology (1) removes the necessity that the Yellowjacket Formation and overlying Proterozoic metasedimentary rocks of central and east-central Idaho be pre-Belt in age; (2) permits the traditional age correlation of the east-central Idaho metasedimentary sequence (Yellowjacket through Lawson Creek Formations) with the Belt Supergroup; and (3) suggests that a re-evaluation is in order for paleogeographic models of central Idaho, especially the proposed topographic high known as the Salmon River Arch.

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