U-PB DETRITAL ZIRCON AGE DATA FROM UPPER TRIASSIC STRATA OF THE CHULITNA TERRANE, SOUTH-CENTRAL ALASKA: A PRELIMINARY LINK WITH THE WESTERN MARGIN OF NORTH AMERICA

The Chulitna terrane is a structurally-imbricated succession of Paleozoic–Mesozoic sedimentary and volcanic strata and mafic units that are exposed along the southern margin of the Alaska Range in south-central Alaska. Collectively, strata of the Chulitna terrane make up one of a number of isolated crustal blocks that are located between the Wrangellia composite terrane and former Mesozoic continental margin of North America in a region referred to as the Alaska Range suture zone. The basal part of the Chulitna stratigraphy consists of Devonian (?)–Permian serpentinite, gabbro, andesite, chert, and argillite. Unique to the upper part of the Chulitna stratigraphy is the occurrence of siliciclastic and volcaniclastic “redbed” units that occur with basalt and limestone in the Upper Permian–Triassic part of the succession. New U-Pb detrital zircon ages determined from samples collected from the siliciclastic parts of this stratigraphy reveal an age range from Mesozoic–Precambrian (Mz-Pz-Pc – 5%-65%-30%). The youngest grain in these samples is 196 +/- 6 Ma and oldest grain is 2940 +/- 11 Ma. The majority of ages occur between 375–300, 1300–1000, and 2000–1500 Ma with peak ages at 335, 1147, 1729, and 1976 Ma. Isolated occurrences fall between 220–200, 475–400, and 3000–2500 Ma with isolated peak ages at 212 and 441 Ma.

Similar trends in Paleozoic and Precambrian detrital zircon age populations have been reported from Upper Jurassic–Cretaceous strata of the Kahiltna assemblage in the Alaska Range suture zone. Peak Paleozoic and older ages in these strata occur at 367, 1778, and 1995 Ma. Further study is needed to better constrain stratigraphic relationships within the Chulitna terrane and the nature of the contacts between the Chulitna stratigraphy and the Kahiltna assemblage in the Alaska Range. Based on the shared occurrence of Devonian–Mississippian age detrital zircons in Upper Triassic strata of the Chulitna terrane and Upper Jurassic–Cretaceous strata of the Kahiltna assemblage we would suggest that both of these units were receiving detrital material from North American sources at some point during their history. This would imply that the Chulitna terrane was in proximity to the western margin of North America during at least part of the Triassic.