PRELIMINARY U-PB AGES AND ATTITUDES OF INTRUSIVE ROCKS SAMPLED WEST OF THE ARC–CONTINENT BOUNDARY IN WEST-CENTRAL IDAHO

The north- to northeast-striking Jura-Cretaceous Salmon River suture zone [SRSZ] in west-central Idaho overlaps accreted island arc assemblages of the Blue Mountains province, the western margin of ancestral North America, and intrusive rocks emplaced therein. New impetus for studying this collisional orogen stems from recent advances in geochronology supporting long-lived metamorphism, magmatism, and contractional deformation in the SRSZ. We report LA-ICP-MS U-Pb zircon ages from tonalitic intrusive rocks sampled in the Rapid River canyon [Locality 1: ~130 Ma], Pollock Mountain summit area [2: ~205 Ma], Little Salmon River corridor [3, 4: ~114 Ma, ~108 Ma], and lower Hazard Creek drainage [5: ~112 Ma]. Locality 1 [Fish Hatchery stock: 145 Ma, Ar-Ar hbl; Snee et al., 1995] shows a steeply northwest-dipping, penetrative fabric defined by aligned biotite ± chlorite; host rocks include east-tilted metavolcanic flows of the Seven Devils Group. Locality 2 [Hazard Creek cpx.; Aliberti, 1988] contains a northeast-striking, moderately southeast-dipping gneissic foliation consistent with the adjacent Pollock Mountain amphibolite. Locality 3 is characterized by subvertical northeast-striking spaced foliation which is not observed at locality 4. Sample locality 5 [western Hazard Creek cpx.?] displays a moderately northeast-dipping gneissic foliation. Taken together with existing geochronology, field mapping, and contractional structures described in the Riggins region, our preliminary data suggest that [1] ca. 130 Ma intrusive rocks underlie shallow east-dipping imbricate faults, [2] ca. 205 Ma orthogneiss located above/along the Pollock Mountain fault is not part of the Hazard Creek cpx., and [3] high-strain ductile deformation extends >10 km west of the western Idaho shear zone as delineated in previous studies.