~26 MILLION YEARS OF SLAB-EDGE MAGMATISM: NEW GEOCHEMICAL CONSTRAINTS ON THE WRANGELL ARC, ALASKA

Subduction and related volcanism in south-central Alaska are responsible for the Wrangell Arc (WA) which extends for >500 km through the Wrangell Mountains into Canada. WA volcanism initiated at ~26 Ma and includes volcanoes that have erupted through the Holocene, some of which are among the largest in the world. The WA developed over/adjacent to the subducting Yakutat microplate, which is under the WA at shallow slab dip angles. At the same time, major strike-slip fault systems (e.g., Denali-Totschunda-Fairweather/Queen Charlotte and associated splays) bisect/are adjacent to the WA. Thus, the WA provides a unique opportunity to study magma production and evolution along a slab-edge margin at an arc-transform junction. Our ongoing project focuses on two primary goals: [1] Determine the temporal-spatial history of magmatism of the WA and [2] Decipher the links between strike-slip faulting and slab edge melting on the geochemical evolution of the WA. New whole rock geochemical data from locations in the central WA range from basalt through high-Si rhyolite and have tholeiitic to medium-K calc-alkaline affinities. Some samples from the oldest-known WA eruptive center, which includes ~26 Ma volcanics just west of the Yukon border (close to lavas attributed to leaky strike-slip faults) have compositions that resemble adakitic Trend 2b of Preece and Hart (2004) for younger WA rocks. Hypabyssal intrusives that crop out across the central WA (some dated ~24 - ~17 Ma) also have adakitic affinity and have (Sr/P)_n = 1.2-5.7, where the samples with (Sr/P)_n > 3 have Sr/Y > 40. We interpret these data to reflect slab edge melting during the initiation of the WA. New non-adkatitic WA samples, some of which are truncated by seismically active strike-slip faults, are characterized by bulk chemistry consistent with intra-arc extension and subduction-related processes where magmatism occurred in the front and backside of the arc. We have not found any alkaline lavas like those erupted from ~18-10 Ma Wrangell magmatism in Canada linked to leaky strike-slip faults. The lack of alkaline compositions in close-proximity to strike-slip faults suggests that while some of the Canadian magmas may have erupted along strike-slip faults, the alkaline chemistry may reflect a melt-generation process not directly related to the faulting.