Detrital zircon geochronology and the provenance of the Harmony and Valmy Formations, Roberts Mountains allochthon, Nevada

A long-standing debate over the provenance of lower Palaeozoic sedimentary rocks in the Roberts Mountains allochthon, Nevada, is addressed through abraded single-grain U-Pb detrital zircon analysis of the Harmony and Valmy Formations. The data are consistent with ultimate derivation of the Valmy Formation from the north and suggest a link between the Valmy Formation and the coeval Eureka quartzite on the adjacent shelf and miogeoclinal prism. The ~700 Ma and ~1100 Ma ages (which represent >80% of the grain sampled from the Harmony Formation) have no known western North American basement source of reasonable proximity and age, suggesting derivation from an unknown western or southern source, or perhaps from rocks that presently underlie the Roberts Mountains allochthon. A growing body of U-Pb isotopic evidence indicates that 1000-1270 Ma and 670-750 Ma zircon ages are relatively common in offshelf continental margin strata. The presence of widespread mid- and late Proterozoic felsic magmatic rocks along the Cordilleran margin is not easily accommodated by popular pre-570 Ma plate reconstructions that place Nevada and the central Cordillera opposite the Transantarctic Mountains or eastern Australia. -from Authors