INTEGRATED FABRIC ANALYSIS, U-PB ZIRCON AGES, AND ZIRCON HF DATA FROM THE HAZARD CREEK COMPLEX, WESTERN IDAHO

The Hazard Creek complex is a series of variably deformed, dominantly tonalitic igneous bodies located within the Salmon River Suture zone in western Idaho. U/Pb zircon analyses suggest that the Hazard Creek complex consists of multiple plutons with crystallization ages spanning some 70 m.y. in the Jurassic and Early Cretaceous. Ages include 180 Ma (No Business pluton), 162 Ma (gneissic units near the Little Salmon River), and multiple intrusions with ages between 128-109 Ma. The zircon epsilon Hf isotope compositions range from +12.1 in westernmost samples to +4.7 in easternmost samples, with a systematic decrease in the radiogenic signature from west to east. This data is consistent with intrusion of these magmas through oceanic lithosphere in the west and increasing involvement of continental material to the east. Jurassic U/Pb zircon ages and zircon Hf data are broadly consistent with accretion of the Blue Mountains terranes to the western edge of Laurentia during latest Jurassic time, although the 162 Ma pluton with epsilon Hf of +6.6 suggests earlier involvement of continental material. Early Cretaceous plutons were emplaced after accretion of the Blue Mountain terranes, indicating development of east-directed subduction under Laurentia by this time. On the eastern edge of the Hazard Creek complex, strong foliations strike north-south and dip steeply east; lineations are downdip. These solid-state fabrics are attributed to deformation associated with the western Idaho shear zone, and dextral transpressional deformation appears to increase eastward. Weaker foliation and lineation are observed to the west, and fabric orientation is variable. Because of the tendency for granitic bodies to preferentially record fabric while in the magmatic state, the east-west gradient suggests that Early Cretaceous fabrics pre-date dextral transpressional deformation associated with the western Idaho shear zone.