AGE, SETTING, AND TECTONIC SIGNIFICANCE OF THE FISH HATCHERY STOCK: LOWER RAPID RIVER CANYON, WEST-CENTRAL IDAHO

The Fish Hatchery stock [FHS] is a medium-grained biotite tonalite located 25 km west of the ocean–continent geochemical boundary [initial Sr-0.706 isopleth] in west-central Idaho. Hosted by late Mesozoic island arc rocks of the Wallowa terrane, the FHS carries local significance in terms of potentially dating ~east-west accretion-related[?] contractional deformation. Previous Ar-Ar dating produced a hornblende cooling age of ~145 Ma [Snee et al., 1995]. New LA-ICP-MS U-Pb zircon geochronology [this study] yields a weighted mean age of 130.1±1.9 Ma. Th/U values range from 0.27 to 0.9 [avg. ~0.47]. Zircon crystallization ca. 130 Ma predates early metaluminous magmatism in the Idaho batholith [~100 Ma; Gaschnig et al., 2010] and small tonalite-trondhjemite intrusions in northeastern Oregon [e.g. Cornucopia stock/Wallowa terrane, ~124–120 Ma; Johnson and Schwartz, 2009]. The FHS is closer in age and location to the Heavens Gate stock [U-Pb 136.0±1.0 Ma; Gray, 2013]. Along its northern margin, the FHS records a moderate to steep northwest-dipping penetrative fabric defined by aligned biotite and chlorite. Country rocks consist of east-tilted metavolcanic flows assigned to the Middle and Late Triassic Wild Sheep Creek Formation [Seven Devils Group]. Massive spilite flows are locally cut by northeast-dipping imbricate faults, consistent with overlying Late Triassic carbonate rocks [Martin Bridge Formation]. Most workers place a low-angle, east-dipping thrust—post-metamorphic Morrison Ridge fault; Gray and Oldow, 2005—between the Wild Sheep Creek and Martin Bridge units. However, temporal relations between stock emplacement, regional fabric development, and brittle faulting are unclear. Reconnaissance mapping suggests that low-angle sliding along the volcanic\carbonate rock contact postdates 130 Ma [stock emplacement]. In this context, intrusive rocks [FHS] in the lower plate were truncated and translated westward into higher elevations of the northeastern Seven Devils Mountains [Mt. Sampson and/or Carbonate Hill areas].