Detrital zircon geochronology of the Adams Argillite and Nation River Formation, east-central Alaska, USA

The Cambrian Adams Argillite and the Devonian Nation River Formation are tw o sandstone-bearing units within a remarkably complete Paleozoic stratigrap hic section in east-central Alaska. These strata, now foreshortened and fau lt-bounded, were originally contiguous with miogeoclinal strata to the east that formed as a passive-margin sequence along the northwestern margin of the North American continent. Seventy-five detrital zircon grains from the Adams Argillite and the Nation River Formation were analyzed in an effort t o provide constraints on the original sources of the grains, and to generat e a detrital zircon reference for miogeoclinal strata in the northern Cordi llera. Thirty-five single zircon grains from a quartzite in the Adams Argil lite yield dominant age clusters of 1047-1094 (n = 6), 1801-1868 (n = 10), and 2564-2687 (n = 5) Ma. Forty zircons extracted from a sandstone in the N ation River Formation yield clusters primarily of 424-434 (n = 6), 1815-183 8 (n = 6), 1874-1921 (n = 7), and 2653-2771 (n = 4) Ma. The Early Proterozo ic and Archean grains in both units probably originated in basement rocks i n a broad region of the Canadian Shield. In contrast, the original igneous sources for mid-Proterozoic grains in the Adams Argillite and similar to 43 0 Ma grains in the Nation River Formation are more difficult to identify. P ossible original sources for the mid-Proterozoic grains include: (1) the Gr enville Province of eastern Laurentia, (2) the Pearya terrane along the Arc tic margin, and (3) mid-Proterozoic igneous rocks that may have been widesp read along or outboard of the Cordilleran margin. The similar to 430 Ma gra ins may have originated in: (1) are-type sources along the Cordilleran marg in, (2) the Caledonian orogen, or (3) a landmass, such as Pearya, Siberia, or crustal fragments now in northern Asia, that resided outboard of the Inn uitian orogen during mid-Paleozoic time.