Abstract

The Yukon-Tanana terrane of the northern Cordillera comprises a basement of metamorphosed continental margin sedimentary rocks of pre–Late Devonian age (Snowcap assemblage) and overlying subduction-generated Late Devonian to Permian arc and backarc facies igneous rocks. While preliminary analytical data have suggested that the Yukon-Tanana terrane originally formed as part of the western peri-Laurentian margin, its position outboard of an upper Paleozoic oceanic terrane (Slide Mountain) and the lack of information from its basement, the Snowcap assemblage, continue to raise questions about its original paleogeographic location along the margin.

We describe here the geological relationships, geochemical and Nd-Hf isotopic compositions, and the detrital zircon signature of the Snowcap assemblage. Geochemical and Nd-Hf isotopic data for most siliciclastic rocks suggest derivation from evolved, upper crustal material, with Paleoproterozoic Nd-Hf depleted mantle model ages and detrital zircon data with major peaks in age ca. 1870 Ma and ca. 2720 Ma, and secondary peaks ca. 2080 Ma and ca. 2380 Ma. Minor juvenile contributions to some metaclastic rocks are more likely related to coeval, rift-related mafic alkalic magmatism than to younger arc magmatism in the terrane, as previously suggested. The detrital zircon signature of the Snowcap assemblage confirms a northwestern Laurentian cratonic source, similar to that of the adjacent Cordilleran miogeocline, and provides a local source in the Yukon-Tanana terrane for evolved signatures and Paleoproterozoic-Archean zircons (both detrital grains and xenocrystic cores) in younger mid- to late Paleozoic rocks of the terrane, at times when the Laurentian craton was probably not available as a direct source.

Mafic alkalic rocks of the Snowcap assemblage were the products of low degree partial melting of incompatible element–enriched lithospheric mantle sources, most likely related to one of several Neoproterozoic–early Paleozoic rifting events recorded along the western margin of Laurentia. Marble and calc-silicate rocks have trace element compositions similar to modern seawater and juvenile Nd-Hf isotopic signatures similar to the mafic rocks, implying coeval carbonate sedimentation and magmatism.

The overall character and composition of the Yukon-Tanana terrane suggest that crustal recycling processes dominated its evolution. Its accretion to the western margin of North America in early Mesozoic time contributed only a limited amount of juvenile crustal material to the Cordillera.

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