AGE AND GEOCHEMISTRY OF SILLIMANITE GNEISS FROM A POSSIBLE CORE COMPLEX NEAR CHENEY, WA

Northeast of Cheney, Washington a series of hills display various rock types from garnet-bearing granitic rocks to sillimanite bearing gneiss. Surrounding these high-grade metamorphic rocks are fine to medium grained sandstone and quartzites mapped as the Ravalli Group of the Belt Supergroup. The lower-grade metamorphosed Belt rocks are cross cut by pegmatite dikes and multiple normal faults intruded by foliated granitic dikes, which are generally 49 Ma. The contact between the metamorphic grades seems to be gradational as faulted rocks, grade to folding and crenulation, and then to mylonitic textures and gneiss. U/Pb ages of separated zircon grains from the gneiss are older than 1.1 Ma, generally agree with expected detrital zircon ages from the Belt Supergroup and are comparable to samples from the proximal Priest River core complex. This area has been generally referred to as the Needham Hill core complex, but is it a stand-alone structure, part of a larger core complex that has been masked by younger basalt and megaflood deposits, or a remnant from the uplift of the Priest River core complex? New U/Pb ages, geochemistry, and structural analyses are presented to evaluate this unique geologic feature.