Abstract

A long-standing debate over the provenance of lower Paleozoic sedimentary rocks in the Roberts Mountains allochthon, Nevada, is addressed through abraded single-grain U-Pb detrital zircon analysis of the Harmony and Vahny Formations. Twenty-seven zircon grains from mature quartz arenite of the Middle Ordovician Valmy Formation yielded concordant to a few percent discordant ages of ∼1050 Ma (n = 2),1830-1845 Ma (n = 4),1910-1960 Ma (n = 5),2270-2340 Ma (n = 2), 2650-2750 Ma (n = 8), and ∼2900 Ma (n =1), and moderately discordant ages of ∼2070 Ma (n =1) and ∼3240 Ma (n =1). These ages are consistent with ultimate derivation from the north (Slave craton and Medicine Hat province and central/northern Alberta) and suggest a link between the Valmy Formation and the coeval Eureka Quartzite on the adjacent shelf and miogeoclinal prism. Twenty-seven analyses of zircon from immature quartzo-feldspathic wacke of the Upper Cambrian Harmony Formation include concordant analyses in the range of 695-710 Ma (n = 9), 1015-1225 Ma (n = 12), ∼1330 Ma (n = 2), ∼1745 Ma (n = 1), ∼1915 Ma (n = 1), and 2570 Ma (n = 1). The ∼700 Ma and ∼1100 Ma ages (which represent >80 %of the grains) 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 latter set of ages is also represented by orthogneiss that intrudes or is overlain by continental margin strata, necessitating the widespread exposure of felsic plutonic rocks of these ages along the Cordilleran margin during early Paleozoic time. Subsequently, such rocks may have been overlain by younger continental margin strata, structurally overlain by allochthonous units, or tectonically removed from the region by younger rifting events. 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.

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