GEOLOGICAL AND GEOCHEMICAL CONSTRAINTS ON OLIGO-MIOCENE HYPABYSSAL INTRUSIVE BODIES FROM THE WRANGELL ARC, ALASKA

The Wrangell Arc (WA) extends from southeastern Alaska into the Yukon Territory, and is the result of the Yakutat microplate subducting under North America. The Denali Fault (right-lateral strike-slip) lies northeast of the WA, and the off-branching Totschunda Fault bisects the WA. The WA provides a unique magmatic environment due to Yakutat flat-slab subduction and the region being an arc-transform junction, and presents the opportunity to study the relationship between these two settings. Continuous WA volcanism initiated at ~29 to ~26 Ma constrained by U-Pb and ⁴⁰Ar/³⁹Ar geochronology and evidence of this magmatism is limited to the Sonya Creek volcanic field and mapped intrusives that crop out across the northern and central WA for ~65-80 km. These intrusives are generally porphyritic and all have similar mineralogy dominated by feldpsar, amphibole, and oxides. They are are calc-alkaline, medium-K dacites and plot in the volcanic arc field on trace element discrimination diagrams. These rocks display characteristics that overlap with Preece and Hart (2004) trend 2b adakites and trend 2b transitional (from adakite to “normal” arc intermediates) dacites. Sr/Y ranges from 33 to 90, while La/Yb (only Yb data for five rocks currently) ranges from 4.8 to 8.7. FeO/MgO vs wt. % SiO₂ as well as Ba/La vs. wt. % MgO (coupled with Sr/Y) follow trend 2b adakite patterns. However La/Yb is more similar to the trend 2b transitional dacites. <5 Ma WA adakites from Mt. Churchill and Drum represent slab melting (Preece and Hart, 2004) and a similar interpretation was made by Berkelhammer et al. (2016) for some of the ~20-29 Ma magmatism at Sonya Creek. The geochemical data we present provides evidence that slab melting occurred along the edge of the Yakutat plate at ~20-24 Ma, but more study is needed to explain why the rocks with La/Yb <10 have low La/Yb relative to young WA adakites. It is possible that some of these magmas represent the outcome of some other melt-generation/magma evolution process (e.g., lower crustal melting and/or crustal interaction), coupled with slab-edge melting. The similarity in geochemistry of the older Oligo-Miocene hypabyssal intrusive bodies with Oligo-Miocene and younger volcanics add support to the WA being older than previously considered and more extensive.