U-PB AND O ISOTOPE CHARACTERISTICS OF TIMANIAN- AND CALEDONIAN-AGED DETRITAL ZIRCONS FROM THE BROOKS RANGE, ARCTIC ALASKA

The connection between the Alaskan and Canadian Arctic margins and their subsequent Jurassic-Cretaceous rifting to form the Amerasia Basin, has been debated since it was initially proposed by Grantz et al. (1979) over 30 years ago. This resource-rich region once formed part of Gondwana and later part of Pangea, supercontinents associated with major orogenic cycles. Most reconstructions predict that orogens associated with these supercontinents (e.g., Timanides, Caledonides) should occur in the Brooks Range of Arctic Alaska, which was contiguous with the Canadian Arctic margin from at least Devonian time. We present the first combined zircon U-Pb and oxygen isotope study of presumed Neoproterozoic to Devonian aged metasediments from the North Fork of the Koyukuk River in the Brooks Range. Two distinct packages are recognized: 1) metasediments with Neoproterozoic maximum depositional ages dominated by Timanian-age (697 and 616 Ma) detritus, and 2) metasediments with Devonian maximum depositional ages and a more restricted input including Caledonian-age (444, 415 and 395 Ma) detritus. This, combined with metamorphic mineralogy, allows us to suggest a stratigraphic simplification that reduces these six units to two sedimentary packages. The detrital spectra compare well with published data inferring Timanian and Baltican sources and suggest a transition to Laurentian sources in Devonian time. Significant differences in oxygen isotopes support this bipartite division, documenting a higher average δ¹⁸O of 7.29‰ ± 0.45 [2s] ‰ (n=47) for Timanian aged (700-550 Ma) zircons and a lower average δ¹⁸O of 6.10 ± 0.17 [2s] ‰ (n=82) for Caledonian aged (450-400 Ma) zircons. This is consistent with a reduced crustal component associated with Ordovician-Silurian arc magmatism and infers that detrital zircons of Caledonian age in the Brooks Range are arc-related and locally derived.