GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 47-5
Presentation Time: 2:40 PM


BRUESEKE, Matthew E.1, TROP, Jeffrey M.2, BENOWITZ, Jeff3, BERKELHAMMER, Samuel E.1, DAVIS, Kailyn3, LAYER, Paul W.3 and MORTER, Bethany K.1, (1)Department of Geology, Kansas State University, 108 Thompson Hall, Manhattan, KS 66506, (2)Department of Geology and Environmental Geosciences, Bucknell University, 1 Dent Drive, Lewisburg, PA 17837, (3)Geophysical Institute and Geochronology Laboratory, University of Alaska Fairbanks, Fairbanks, AK 99775,

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.