NEW FINDINGS ON EARLY CRETACEOUS VOLCANISM WITHIN THE ALLOCHTHONOUS WRANGELLIA TERRANE, SOUTH-CENTRAL ALASKA: STRATIGRAPHIC, GEOCHRONOLOGIC, AND GEOCHEMICAL DATA FROM THE CHISANA FORMATION, NUTZOTIN MOUNTAINS

Lower Cretaceous volcanic rocks in south-central Alaska provide insight on the accretionary tectonic history of the allochthonous Wrangellia terrane. The Chisana Formation (CF) consists of a 3-km-thick sequence of dominantly volcanic rocks that conformably overlie Jurassic-Cretaceous sedimentary strata (Nutzotin Mtns. sequence) and older strata of Wrangellia. New data from a measured section through the lower CF at Bonanza Creek provide insight on the timing, nature, and tectonic significance of Early Cretaceous volcanism. The lower 1170 m of the CF is characterized by amalgamated volcanic-lithic breccia, basalt and andesite lavas, lithic tuff, and mudstone deposited by effusive and pyroclastic eruptions and sediment gravity flows. Deposition was typically proximal to volcanic vents on subaqueous marine slopes based on the distribution of pillow lavas, marine fossils, and volcanic breccia with blocks up to several meters long. The timing of volcanism was previously constrained by Valanginian-Barremian bivalves from the lowermost CF. ⁴⁰Ar/³⁹Ar dating on lava flows sampled 940-1150 m above the basal contact yield ages of 116-113 Ma. The new ages demonstrate that volcanism persisted into late Aptian time, ~8 my longer than previously recognized. Lower CF basalts, basaltic andesites, and andesites exhibit geochemical traits, particularly initial isotopic compositions (εNd, 8.8-9.1; ⁸⁷Sr/⁸⁶Sr, 0.70292-0.70327) consistent with a subduction petrogenesis in which recycling of continental crust was minimal. Stratigraphically correlated elemental variations suggest that multiple vents likely produced the sampled section; spatially associated granitic intrusions may be co-genetic based on overlap with the CF in age and isotopic composition. Comparison of our new data from south-central Alaska with previous studies of the Gravina belt in southeastern Alaska demonstrates close overlap in the age range and isotopic composition of volcanic strata spanning a >1000-km-long segment of Wrangellia. Within the context of existing tectonic models, regional late Early Cretaceous volcanism occurred within a proto-continental arc that developed after Jurassic-Early Cretaceous accretion of Wrangellia to the former continental margin or an intraoceanic arc prior to Late Cretaceous accretion.