TESTING TECTONIC MODELS FOR THE ACCRETION OF THE WRANGELLIA COMPOSITE TERRANE USING DETRITAL GEOCHRONOLOGIC DATING AND HF ISOTOPES, SOUTH-CENTRAL ALASKA

The Wrangellia composite terrane (WCT) is one of the largest fragments of juvenile crust added to the North American continent since Mesozoic time, and refining its accretionary history has important implications for understanding how continents grow. New U-Pb geochronology and Hf isotopes of detrital zircons from Upper Jurassic-Upper Cretaceous strata from the forearc of the WCT are used to test two existing tectonic models for the accretion of the WCT to North America.

Our stratigraphically oldest samples from the Late Jurassic Naknek Formation have a detrital zircon U-Pb signature dominated by Early and Late Jurassic grains (198-181 Ma; 156-151 Ma). Hf compositions of these grains are juvenile (eHf_(t)=6.3-15.1). Disconformably above the Naknek Formation are two poorly understood units Ks and Kc. The Ks unit is dominated by Early to Late Jurassic grains (198-145 Ma) with a few Paleozoic grains (352-337 Ma). Hf compositions of Carboniferous-Jurassic grains are juvenile (eHf_(t)=4.0-15.4). The overlying Kc unit has Late Jurassic to Late Triassic zircons (230-148 Ma), with a large Early Jurassic signature (201-181 Ma), and an increase in Paleozoic ages (339-318 Ma). Samples from the Matanuska Formation have Late Cretaceous (96-72 Ma), Late to Early Jurassic (183-142 Ma) and Paleozoic grains (355-328 Ma). Hf compositions have a wider range from both the Late Cretaceous grains (eHf_(t)=1.5-13.0) and Paleozoic-Precambrian grains (eHf_(t)=14.1-14.3). In addition, both samples have large Late Cretaceous (96-72 Ma) age peaks.

Our results suggest an evolving provenance from Late Jurassic to Late Cretaceous time for the WCT forearc basin. Late Jurassic sediments were dominantly derived from a juvenile Jurassic igneous sediment source. During Early Cretaceous time, a slight increase in the number of Paleozoic grains in the Ks and Kc samples, in addition to the mostly positive Hf compositions, suggests exhuming Jurassic and Paleozoic juvenile igneous sediment sources. By Late Cretaceous time, our data illustrates another increase in Paleozoic grain abundances, in addition to the introduction of Precambrian grains, all with widely variable Hf compositions. We interpret this to reflect a larger sediment flux from the interior of Alaska where more evolved igneous rocks of that age are found.