INCEPTION OF THE WRANGELL ARC: GEOCHEMISTRY AND PETROGRAPHY OF THE OLIGO-MIOCENE SONYA CREEK VOLCANIC FIELD, SOUTH-CENTRAL ALASKA

The Sonya Creek volcanic field (SC), originally split by Richter et al. (1990, 2000) into the Sonya Creek shield volcano and the Rocker Creek area, is located in the ~26 Ma-modern Wrangell arc (WA) in south-central Alaska. The WA is located within a transition zone between “normal” Aleutian subduction to the west and strike-slip tectonics along the Queen Charlotte-Fairweather and Denali-Duke River fault systems to the east. The initiation/spatial migration of WA magmatism through time is the main focus of our larger study; this project is focused on the SC. The SC is located on the northern flank of the WA and extends into southwest Yukon Territory. SC units are not ice-covered thus are well suited for a detailed stratigraphic-based study of volcanism and petrogenesis. The SC includes the oldest rocks in the arc that have been identified to date, the ~26 Ma Rocker Creek lavas of Richter et al. (1990), as well as a ~23 Ma dome field, a proximal welded tuff that we interpret to have formed during caldera collapse associated with the Sonya Creek volcano, the ~20 Ma intracaldera lavas of the Sonya Creek volcano (including an obsidian unit that was an important source for tool making for historical peoples), and the overlying “Border Lavas.” Geochemical data from SC units range from basalt to high-Si rhyolite (e.g., 49-76 wt. % SiO₂) and plot as calc-alkaline on an AFM diagram. The most evolved rocks (SiO₂ > 71 wt. %; plagioclase-phyric rhyolite domes, lavas, and tuffs) are peraluminous. Unlike samples from other locations in the SC, basaltic andesite to rhyolite lavas from the Rocker Creek area show adakitic characteristics (Sr/Y 40-80, La/Yb_n 7-20) which we interpret as evidence of slab edge melt. High-Ti rocks related to local intra-arc extension are also found in the SC. Field relations give relative ages for different units, and the stratigraphic position of two mafic lavas from Rocker Creek and Sonya Creek shield volcano suggest that at least two “pulses” of mafic magma input into the SC occurred. This new data conforms with the geochemistry of the voluminous <5 Ma volcanism in the western WA, but shows little similarity to the alkaline magmatism found ~60 km to the southeast in Yukon Territory that has been linked to “leaky strike-slip” volcanism. Further work is needed to reconcile these geochemical differences over such a small distance.