INITIATION OF THE WRANGELL ARC: A RECORD OF TECTONIC CHANGES IN AN ARC-TRANSFORM JUNCTION REVEALED BY NEW GEOCHEMISTRY AND GEOCHRONOLOGY OF THE ~29–18 MA SONYA CREEK VOLCANIC FIELD, ALASKA

The Sonya Creek volcanic field (SCVF) contains the oldest in situ magmatic products in the ~29 Ma–modern Wrangell arc (WA) in south-central Alaska. The WA is located within a transition zone between Aleutian subduction to the west and dextral strike-slip tectonics along the Queen Charlotte-Fairweather and Denali-Duke River fault systems to the east. WA magmatism is due to the shallow subduction (11–16°) of the Yakutat microplate. New ⁴⁰Ar/³⁹Ar and U-Pb geochronology of bedrock and modern river sediments shows that SCVF magmatism occurred from ~29–18 Ma. Igneous units are divided based on field mapping, physical characteristics, geochronology, and new major and trace element geochemistry. A dacite dome yields a ~29 Ma ⁴⁰Ar/³⁹Ar age and was followed by eruptions of basaltic-andesite to dacite lavas and domes (~28–23 Ma) that record hydrous, subduction-related, calc-alkaline magmatism with an adakite-like component. This was followed by a westward shift to continued subduction-related magmatism without the adakite-like component (e.g., mantle wedge melting), represented by ~23–21 Ma basaltic-andesite to dacite domes and associated diorites. These eruptions were followed by a westward shift in volcanism to anhydrous, transitional-tholeiitic, basaltic-andesite to rhyolite lavas and tuffs of the ~23–18 Ma Sonya Creek volcano, recording the influence of local intra-arc extension. The end of SCVF activity was marked by a southward shift in volcanism back to hydrous calc-alkaline lavas at ~22–19 Ma. SCVF geochemical types are similar to those from the <5 WA, and no alkaline lavas that characterize the ~18–10 Ma eastern part of the WA exposed in Yukon territory are present. Sr-Nd-Pb-Hf radiogenic isotope data suggest the SCVF array were generated by contamination of a depleted mantle wedge by ~0.2–3% subducted terrigenous sediment. Early SCVF igneous rocks are also chemically similar to hypabyssal intrusives that crop out to the west; together these ~29–20 Ma rocks imply that WA initiation occurred over an ~<100 km belt, ~50–60 km inboard from the modern WA and current loci of volcanism that extends from Mt. Drum to Mt. Churchill. Our results, combined with prior work, reveal that WA magmatism was geochemically similar during the entire 29 Ma duration of the WA, irrespective of location or thickness of subducting slab.