GEOCHEMICAL AND STRATIGRAPHIC ANALYSIS OF THE CHISANA FORMATION, INSULAR TERRANES, EASTERN ALASKA: EARLY CRETACEOUS MAGMATISM AND TECTONICS ALONG THE NORTHERN CORDILLERAN MARGIN

In terms of a fundamental and global tectonic process, suturing of oceanic crust onto continental crust is poorly understood compared to other processes, such as oceanic subduction and mid-ocean ridge formation. The Chisana Formation in south-central Alaska represents an ideal location in which to study this process, as it occurred just before the largest addition to the North American crust in the last 250 million years. The Chisana Formation consists of relatively unaltered Lower Cretaceous volcanic and sedimentary rocks that occur in the Nutzotin Mountains of eastern Alaska. New stratigraphic analysis indicates that, at the Bonanza Creek type section, the Chisana Formation is more than 2-km-thick. We use stratigraphic, geochemical, Sr-Nd-Pb isotope, and U-Pb age data from samples collected from various stratigraphic levels of the Chisana Formation in order to constrain its origin, and to shed light on the origin of coeval basins stationed along the inboard margin of the Insular terrane. We divide the Chisana Formation into a lower subaqueous unit, a middle transitional unit, and an upper subaerial unit. Chisana Formation lavas range from transitional to subalkaline basalts through andesites, with trace element geochemistry showing high field strength element depletions relative to large ion lithophile elements and hydrous mineral assemblages with calc-alkaline to tholeiitic chemistries, consistent with a magmatic arc origin. Chisana lavas yield geochemical compositions and isotope characteristics that overlap with magmas from volcanic suites formed within juvenile continental crust and immature island arcs. New U-Pb ages of detrital zircons combined with previously reported lava ages support an age of ~131-117 Ma for the Chisana Formation. Our results support existing tectonic models in which an east-dipping subduction zone existed beneath Wrangellia during Early Cretaceous time. The upsection shift from marine to terrestrial depositional conditions in the Chisana Formation and the overlying ~117–93 Ma Beaver Lake formation was coincident with regional shortening. Together, the geologic evidence for shortening and terrestrial deposition are interpreted to reflect accretion/suturing of the Insular terranes against inboard terranes.