ORIGIN AND TECTONIC EVOLUTION OF THE ROBERTS MOUNTAINS ALLOCHTHON: NEW INSIGHTS FROM U-PB GEOCHRONOLOGY AND HF-ISOTOPE GEOCHEMISTRY OF DETRITAL ZIRCONS

U-Pb geochronology and Hf-isotope geochemistry of detrital zircons provide new insights into the origin, age, and tectonic evolution of the Roberts Mountains allochthon (RMA) of north-central Nevada. Zircon grains from six Ordovician to Devonian arenites were analyzed for U-Pb ages and Hf isotope ratios using laser-ablation inductively coupled plasma mass spectrometry. The Ordovician lower Vinini Formation yielded unique results: a 490-500 Ma age peak with εHf values of +3 to -5, a 1.10-1.12 Ga peak with with εHf = +4 to +6, a 1.42 Ga peak with with εHf = +7 to +3, a 1.66-1.80 Ga peak with with εHf = +10 to 0, and a 2.47-2.75 Ga peak with εHf = +6 to -6. The other five samples share a 1.82 – 1.96 Ga peak with εHf = +10 to -15, a 2.06-2.12 Ga peak with εHf = +3 to -6, and a 2.65-2.75 Ga age peak with εHf = +6 to -7. Comparison of our data with those of igneous basement rocks and sedimentary units indicates that the lower Vinini Formation originated in the north-central Laurentian craton. The other five units sampled had a source in the Peace River Arch region of western Canada, which is also the source of Ordovician passive margin sandstones of western Laurentia. Ten Harmony Formation arenite samples analyzed for U-Pb ages, with eight further analyzed for Hf-isotope ratios, document the two recognized petrofacies, quartzose A and feldspathic B. Three Harmony A samples have a 1.0-1.4 Ga peak with εHf = +12 to -3 and a 2.5-2.7 Ga peak with εHf = +7 to -5. These originated in the central Laurentian craton. The seven Harmony B samples have age peaks of 1.7-1.9 Ga with εHf = +10 to -20 and 2.3-2.7 Ga with εHf o=+6 to -8. These originated in eastern Alberta-western Saskatchewan, north of the Harmony A source.

Our work indicates that the RMA strata were not deposited as the offshore equivalent of the western Laurentian passive margin, nor did the strata originate in an exotic, extra-Laurentian locale. We propose that the RMA strata, exclusive of the lower Vinini and Harmony formations, were deposited near the Peace River Arch, and subsequently tectonically transported south along the Laurentian margin. The Harmony Formation strata were deposited near eastern Alberta, then tectonically interleaved with the RMA strata. The entire package was tectonically transported south along the Laurentian margin and then emplaced onto the craton during the Antler orogeny.