DETRITAL ZIRCON GEOCHRONOLOGY AND HF ISOTOPE GEOCHEMISTRY OF ROBERTS MOUNTAINS ALLOCHTHON STRATA IN NEVADA: CONSTRAINTS ON PROVENANCE AND TERRANE ORIGIN

New U-Pb geochronologic and Hf isotopic analyses of the Cambrian – Devonian strata of the Roberts Mountains Allochthon (RMA) in Nevada constrain their origin and provenance. The RMA is a complex of thrust sheets of ocean sediments emplaced onto western Laurentia in the Late Devonian – Early Mississippian Antler orogeny. Our new data reveal differences between coeval units, and similarities among units that persist from Ordovician through Devonian time.

Analyses of the Cambrian (?) Harmony Formation confirm earlier findings that this unit contains two distinct age spectra. Three samples of the Harmony A (n=443) share a major peak at 1.1 Ga and minor peaks at 1.2 and 1.4 Ga, consistent with origin in the Grenville orogen. Five samples of the Harmony B (n=596) share a major peak at 1.78 Ga and minor peaks at 1.85 and 2.6 Ga, consistent with derivation from the Peace River Arch region.

The Ordovician Vinini Formation also contains two distinct age spectra. The two samples of the Lower Vinini (n=294) have age peaks at 490 Ma and 1.72 Ga, consistent with derivation from central Laurentia, except for the 490 Ma zircons, which have no known source in central or western Laurentia. The Upper Vinini sample (n=194) has a major peak at 1.84 Ga and minor peaks at 1.92 and 2.1 Ga, similar to other RMA strata, and consistent with origin in the Peace River Arch.

Other RMA strata show a stable pattern of detrital zircon ages that persisted from Ordovician through Devonian. The Ordovician Snow Canyon (n=190), Upper Vinini (n-194), and Valmy formations (n=105) and McAfee Quartzite (n=194), the Silurian Elder Sandstone (n=192), and the Devonian Slaven Chert (n=192), are similar, with a major peak at 1.84 Ga and minor peaks at 1.92 and 2.1 Ga, consistent with origin in the Peace River Arch region.

Hafnium isotope analyses of these grains confirm the distinct patterns shown in the U-Pb data. The Harmony A grains are juvenile to moderately juvenile, while the Harmony B grains are mostly from evolved crustal material. The Lower Vinini grains are juvenile to moderately juvenile, while the Upper Vinini and other RMA grains are from evolved crust.

From our new work, we conclude that these RMA strata are Laurentian. Our new data also show that there are four distinct spectra, with four distinct source areas: 1) Harmony A; 2) Harmony B; 3) Lower Vinini; and 4) the rest of the RMA strata.