PRELIMINARY U/PB ZIRCON DATA FROM THE HOUSE MOUNTAIN METAMORPHIC COMPLEX IN THE ATLANTA LOBE OF THE IDAHO BATHOLITH

The House Mountain metamorphic complex is a ~100 km² exposure of middle to upper amphibolite facies ortho- and paragneisses within the Atlanta lobe of the Idaho Batholith, south central Idaho. The complex comprises a paragneiss suite juxtaposed between tonalitic (structurally above) and a granitic gneisses (structurally below); the entire complex contains a pervasive NW-striking, E-dipping foliation, and is intruded by deformed concordant and nondeformed discordant granitic pegmatite dikes and sills, and nondeformed granite of the Idaho Batholith. As remnants of the pre-batholith country rock, these gneisses may provide unique and invaluable insight into the pre-Cretaceous tectonics and paleogeography of the region. Preliminary U/Pb zircon data for four lithologies within this gneiss complex illustrate a tectonomagmatic evolution spanning the Neoproterozoic to latest Cretaceous. Single zircons from the lower granitic orthogneiss, imaged by CL and extracted from grain mounts, define a chord with an upper intercept of 726 ± 5 Ma (igneous crystallization) and lower intercept of 82 ± 15 Ma (age of metamorphic recrystallization and overgrowth). These data confirm the presence of Precambrian crust in southwestern Idaho, specifically within the region of the central Atlanta lobe, as previously inferred by the ⁸⁷Sr/⁸⁶Sr ratios of magmatic rocks in the region. Zircons from the upper tonalitic gneiss, displaying simple oscillatory zoning in CL, yield an igneous crystallization age of ca.157 Ma, potentially linking Late Jurassic plutonism in northern Nevada and NW Idaho. Multifaceted, low-U zircons, most likely of metamorphic origin, from an ~10m thick amphibolite sill within the paragneiss stack give concordant ages of ca.122 to 125 Ma. These ages are interpreted as a first constraint on the age of amphibolite-facies metamorphism. The fourth sample analyzed is one of numerous nondeformed pegmatitic dikes intruding the lower orthogneiss. Zircons from this sample produce near concordant crystallization ages of ca. 66 Ma, defining a minimum age for development of the strong fabric in the gneisses.