RUBY TERRANE, NORTH-CENTRAL ALASKA: COMPOSITE OF CONTINENTAL AFFINITY ROCKS

The Ruby terrane of north-central Alaska, similar to other pericratonic terranes that have undergone extensive deformation and metamorphism, suffers from an identity crisis. As originally defined, it contains quartzite, pelitic schist, and augen gneiss, with lesser amounts of calcareous schist, marble, and metabasite. We propose that the Ruby terrane is composite in nature, with the common thread being that all units were deformed during a Late Jurassic-Early Cretaceous orogeny, most likely the Brookian.

The Ruby terrane can be defined in part by what distinguishes it from adjacent pericratonic terranes. It is quite distinct from the Farewell terrane to the south: igneous ages, detrital zircon ages, and metamorphic history of the Ruby and Farewell terranes have few if any similarities. The Farewell terrane has faunal affinities with Siberia and/or the western Brooks Range and Farewell quartzites have a detrital zircon peak at around 2.0 Ga. In contrast, 3 of the 4 detrital zircon samples from a structural stack assigned to the Ruby terrane have signatures that support links to the North American craton, with SHRIMP ages primarily between 1.0 and 1.5 Ga, with lesser peaks up to 1.9 Ga. A fourth sample is dominated by two peaks between at 1.83-1.93 Ga and contains numerous grains between 2.0 and 2.8 Ga. This detrital zircon signature is typical of the Yukon-Tanana terrane and Wickersham grit, units southeast of the Ruby terrane. The most unique unit within the Ruby terrane is K-feldspar augen gneiss with U/Pb zircon ages of 380-390 Ma. The augen gneiss and the schist it intrudes share a Late Jurassic blueschist facies metamorphism. The augen gneiss protolith, emplacement age, and the blueschist metamorphism are identical to that in the southern Brooks Range, thus most authors infer the Ruby terrane is derived from the Brooks Range. The high P/T metamorphism has not been confirmed in the structural units that contain the three detrital zircon samples, thus the primary relationship between them and the augen gneiss and related units is not clear. The complex Brookian orogen, involving both the collision of an island arc with continental affinity rocks and the opening of the Canada basin, may have lead to the juxtaposition of disparate thrust sheets that now fall under the name Ruby terrane.