LAMPROITE-LIKE VOLCANICS ALONG THE WESTERN IDAHO SHEAR ZONE

We report the occurrence of diatreme facies units within a small lamproite-like volcanic system exposed by fresh excavations in Adams County, Idaho, located approximately 6 km northwest of McCall, Idaho. It is emplaced within the western Idaho shear zone, a crustal-scale vertical shear zone that currently demarcates the boundary between accreted terranes and continental North America. Specifically, the diatreme intrudes the granitic gneisses of the Little Goose Creek Complex. The diatreme also crosscuts the 16 Ma Grande Ronde basalt of the Columbia River basalt group, and hence must be younger than 16 Ma. Freshly exposed samples are friable and bright blue in color, although the color dulls to a grey-blue after exposure to the atmosphere. Many hand samples contain white and grey xenoliths ranging from millimeters to 10 centimeters across, interpreted to be derived from the Little Goose Creek Complex and Grande Ronde Basalt, respectively. The white xenoliths are significantly more abundant than the grey xenoliths, which together typically comprise ~10-80% of the hand sample. Thin section analysis indicates that the rocks are comprised of biotite, chlorite, amphibole, ilmenite, Na-rich plagioclase feldspar, and accessory minerals of zircon and apatite. Chondrite normalized REE plots of samples of the diatreme exhibit an enrichment in light REEs consistent with other lamproites. Similar to the Middle Park lamproite in Colorado, this body is interpreted to have experienced significant hydrothermal alteration that: 1) modified concentrations of K and Na; and 2) preserved heavier trace element signatures consistent with lamproites. We interpret that the diatreme had a deep mantle source and that the trans-crustal western Idaho shear zone provides a route for material transport from the mantle to the surface.