DETRITAL ZIRCONS FROM QUARTZITES CONSTRAIN THE WESTERN MARGIN OF LAURENTIA IN SOUTH-WESTERNMOST IDAHO

The Owyhee Mountains of southwestern Idaho expose pre-Mesozoic rocks in structural windows amid Cenozoic volcanic and sedimentary cover. Near South Mountain, a screen of metamorphosed sediments–quartzites, schists, and marbles–are observed and are presumed to be Paleozoic in age. We determined U-Pb detrital zircon ages from the quartzite screens to constrain the maximum age of deposition. All concordant zircon analyses (n=82) from the quartzite have Precambrian ages. The distribution of detrital zircon ²⁰⁷Pb-²⁰⁶Pb ages ranges from 3.4 to 1.0 Ga, with clusters of ages at 2.69 to 2.55, 2.1-1.8, and 1.45-1.02 Ga. We interpret the broad 1.45-1.02 Ga group as being sourced from the Grenville and/or mid-continent granites of the Mesoproterozoic. The peaks at ~1.8 Ga are attributed to a Yavapai terrane source. The 2.69-2.55 Ga group has ages consistent with derivation from the underlying Grouse Creek Block. The provenance of the 2.1 Ga peak is unclear. In total, the detrital age distribution resembles that of the relatively nearby Elba Quartzite of the Raft River-Albion ranges, which were interpreted to have formed in a depositional setting in the middle Neoproterozoic (Cryogenian) at the onset of Laurentian rifting. The Elba Quartzite, however, lacks the 2.1 Ga peak of detrital zircon ages that are observed near South Mountain. We do, however, observe some ~1.45-1.36 Ga grains, which may be sourced from rift-related 1.38 Ga magmatism in Idaho (as also recorded by detrital zircons in the Lemhi sub-basin). The results indicate that the North American (Laurentian) continental margin extended at least as far as westernmost Idaho. Similar to the Elba Quartzite, we tentatively interpret the metamorphosed sediments of South Mountain as forming during Laurentian rifting in the Neoproterozoic.