GEOCHEMICAL CONSTRAINTS ON ~30 MILLION YEARS OF DIACHRONOUS MAGMATISM ALONG AN ARC-TRANSFORM JUNCTION, WRANGELL ARC, SOUTHERN ALASKA

The Oligocene to Present Wrangell Volcanic Belt (WVB) extends for ~500 km across south-central Alaska (U.S.A.) into Canada and includes some of the largest continental volcanoes. The WVB formed in a complex volcanic arc-transform junction and provides a long-term geologic record of subduction, transform slip, and magmatism. New ⁴⁰Ar/³⁹Ar ages from bedrock, coupled with U-Pb geochronology of detrital zircons and ⁴⁰Ar/³⁹Ar geochronology of igneous clasts from modern rivers encircling the arc, document continuous magmatism from ~30 Ma to present. Together with bulk rock geochemistry from the same igneous bedrock and modern river clast samples (and prior published work) we are able to provide a unique and comprehensive spatiotemporal record of WVB magmatism including arc initiation. WVB magmatism was initially limited to the Sonya Creek volcanic field and mapped intrusives (~30-18 Ma), which crop out for ~65-80 km in Alaska. These igneous products span a range of compositions from basalt through high-Si rhyolite and have adakitic, calc-alkaline, and tholeiitic geochemistry. Additionally, the leaky transform alkaline magmatism that characterizes ~18-10 WVB magmatism in Canada along the Duke River fault has not been identified in this Alaskan arc initiation stage or during later times. We interpret the new geochemical data (in conjunction with Sr-Nd-Pb isotope data) to reflect processes including slab-edge melting (e.g., adakites), mantle wedge-melting leading to calc-alkaline magmatism, and tholeiite eruptions due to intra-arc extension. The same processes that gave rise to previously documented <12 Ma WVB volcanism also characterized ~18-30 Ma WVB magmatism in Alaska. Our results, in conjunction with prior geological, geophysical, and geochemical studies show that Alaskan WVB magmatism occurred chiefly due to subduction and should be considered a volcanic arc (e.g., the Wrangell Arc). Slab-edge upwelling and faults acting as magma conduits were likely responsible for the large volcano sizes and high eruption rates of the Wrangell Arc.