THE PROVENANCE OF DETRITAL ZIRCONS FROM THE CAMBRIAN ADDY QUARTZITE, NORTHEAST WASHINGTON: CONNECTIONS BETWEEN BRITISH COLUMBIA AND CENTRAL NEVADA?

New analysis of detrital zircons from the Late Neoproterozoic – Early Cambrian Addy Quartzite in northeast Washington reveals a possible shared source between this passive margin unit and the Cambrian Hamill Group of southeastern British Columbia (BC). Further, this analysis reveals intriguing similarities between these southern BC/northeast Washington passive margin strata and the Cambrian (?) Harmony Formation of the Roberts Mountains allochthon in north-central Nevada.

The four Addy Quartzite zircon samples (n=338) are from each of the four informal members at Stensgar Mountain, west of Chewelah, Washington. Three of the samples share a distinctly unimodal peak at 1.78 Ga; the other sample has a unimodal peak at 1.72 Ga. The first three are consistent with derivation from the nearby Rimbey arc, which comprises plutons of 1.78 -1.86 Ga. The sample with a unimodal peak at 1.72 Ga is enigmatic and could indicate basement sources which are no longer exposed at the surface, or analysis process error. The unimodal peaks, the relatively fresh detrital zircon morphology, and the relative textural immaturity of the samples indicate minimal transport, deposition proximal to the source terrane, and probable first or second-cycle deposition.

The Hamill Group detrital zircon age spectra have a sharp peak at 1.78 Ga, which correlate closely with those of the Addy Quartzite; the Hamill group spectra also have minor Archean peaks from 2.4 – 2.7 Ga. The detrital zircon age spectra of both these southern BC/northeast Washington passive margin strata share peaks with the Harmony B Formation in the Roberts Mountains allochthon of central Nevada, which has a sharp peak at 1.78 Ga, a minor peak at 1.87, and lesser Archean peaks at 2.58 and 2.68 Ga. The Harmony B Formation is a feldspathic arenite and is texturally immature. These characteristics, together with the nearly unimodal detrital zircon age peak, indicate first-cycle deposition proximal to the source terrane. If the Harmony B sands share an original location and a source in the Rimbey arc with the Addy Quartzite and Hamill Group, the Harmony B would have traveled tectonically a significant distance southward along the craton margin before emplacement onto the western Laurentian craton as the uppermost nappe of the Roberts Mountains allochthon.