THE PROVENANCE CHALLENGE OF THE HARMONY FORMATION IN CENTRAL NEVADA: AN ENIGMATIC PIECE IN THE CORDILLERAN JIGSAW PUZZLE

The Harmony Formation is a poorly understood unit within the Roberts Mountains allochthon (RMA) of northern Nevada: it is structurally highest in a stack of nappes derived from the west, its age is equivocal (Cambrian or possibly Devonian?), and it comprises two distinct petrofacies with different detrital zircon (DZ) age spectra. Our new samples and mapping highlight the enigmatic nature of the Harmony Formation and place some limits on its origins.

The two petrofacies record very different provenance. Harmony A is a quartzose wacke with DZ ages mainly in four groups: 980 Ma-1.2 Ga, 1.3-1.5 Ga, 1.6-1.8 Ga, and 2.4-2.5 Ga. These are consistent with derivation from the Grenville orogeny, the midcontinent region, the Mazatzal Province, Yavapai Province, and the Archaean craton, respectively. The DZ age spectra of the Harmony A are dissimilar from those of any known RMA or western Laurentian passive margin unit, but are similar to those in Laurentian Cambrian passive margin strata of the (present-day) Arctic. Harmony B is an immature arkosic wacke with DZ ages mainly in two groups: 1.7-1.9 Ga and 2.5-2.6 Ga. These are consistent with derivation from the Yavapai Province, Trans-Hudson Orogen, and the Archean craton, respectively. Although its arkosic composition is distinctive, the DZ age spectra of Harmony B are similar to those of coeval western Laurentian passive margin units, as well as Laurentian Cambrian passive margin strata of the (present-day) Arctic. Our mapping indicates the two units are conformable, and Harmony B is much more widespread.

Several alterative origins for the Harmony Formation are possible, including derivation on the Arctic Laurentian margin during Cambrian time. This would require subsequent tectonic transport to a position outboard of the RMA strata, with all then thrust onto the Laurentian craton.