GEOCHEMICAL EVIDENCE FOR ADAKITE-LIKE MAGMATISM AT THE OLIGO-MIOCENE INITIATION OF 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. Geochemical and geochronologic data from Trop et al. (2017) reveal sparse magmatism from ~34 to ~29 Ma, while continuous WA volcanism initiated at ~29 to ~26 Ma constrained by U-Pb and ⁴⁰Ar/³⁹Ar geochronology. 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. These intrusives are calc-alkaline, medium-K dacites that plot in the volcanic arc field on trace element discrimination diagrams, and display characteristics that overlap with Preece and Hart (2004) trend 2b adakites (slab-edge melts erupted at <1 Ma Mt. Churchill and Mt. Drum) and trend 2b transitional dacites. These rocks display Sr/Y from 33 to 90, with FeO/MgO vs wt. % SiO₂ as well as Ba/La vs. wt. % MgO (coupled with Sr/Y) following trend 2b adakite patterns. Most samples have La/Yb <20, which is inconsistent with the “adakitic” signature, however one sample has La/Yb >20, which coupled with other geochemical characteristics, place it within the adakite field. Similar adakite-like silicic lavas are found in the Sonya Creek volcanic field, where they have been interpreted to form via slab-edge melting or melting of lower crust, coupled with mixing a primitive magma derived from mantle wedge melting (Berkelhammer, 2017). Collectively, the intrusives from this study are geochemically similar to these other WA silicic lavas, as well as other rocks from locations (e.g., Turkey) that have been interpreted to represent either lower-crustal melting and/or slab-edge melting. The geochemical data we present, plus data from the Sonya Creek Volcanic field, provides evidence that widespread adakite-like magmatism occurred along the edge of the Yakutat microplate from ~29 to ~20 Ma (and throughout the 29 Ma WA evolution). This was likely the result of slab-edge melting and/or lower crustal melting of mafic underplate plus mixing with the mantle wedge, triggered by mantle flow around the leading or lateral edge of the slab following the initiation of subduction of the Yakutat oceanic crust.