Paper No. 6
Presentation Time: 10:00 AM
NEW GEOCHEMICAL AND AGE CONSTRAINTS ON THE INITIATION OF THE LOST ARC, WRANGELL VOLCANIC BELT, ALASKA
The Wrangell Volcanic Belt (WVB) extends > 500 km across the Alaska-Canada border. Slab edge volcanic belts like the WVB that develop at arc-transform junctions often host magmatism ranging from “typical” calc-alkaline arc affinities to alkaline compositions associated with upwelling asthenosphere along slab edges/slab windows and leaky strike-slip faults. In the WVB, well-preserved magmatic products provide a view of the entire ~25 Ma of arc-transform junction tectonomagmatic processes and allow for the determination of the full spatial and temporal extent of subduction-related (arc) and leaky strike-slip magmatism, and whether time-space patterns in magmatism relate to the progressive insertion of the Yakutat flat slab, translation along regional strike-slip faults, and/or previously unrecognized events (e.g., slab rollback, slab tear, forearc subduction erosion?). New 40Ar/39Ar whole rock results from hypabyssal intrusives near the town of Nabesna yield ages of ~24 Ma, similar to new 40Ar/39Ar and U-Pb ages of detrital volcanic grains and zircons from the Nabesna and Chisana rivers. At the mouth of the Chisana Glacier an ~2.4 Ma age was obtained from Euchre Butte, a potential leaky transform volcano that lies directly adjacent to the Totschunda Fault. New whole rock geochemistry from the older samples illustrate that they are medium-K, calc-alkaline dacite, characterized by (Sr/P)n = 2.5-6.5, Ba/Nb = ~80-250, and other trace element characteristics indicative of magmas sourced from subduction-affected fluid-rich sources. These also define an array from ~30 to ~75 Sr/Y and have other geochemical characteristics of adakite suggesting they are similar to the trend 2b of Preece and Hart (2004) for <5 Ma WVB volcanism. Given these results and the geological context of the older rocks, it is likely that these intrusives record slab-edge melting in the northern portion of the central/eastern WVB at 24 Ma. Euchre Butte has similar bulk chemical characteristics, though is not adakitic. Thus far, geochemical evidence for leaky transform magmatism is identified only in the Canadian part of the WVB. Overall, these results are consistent with a more complex history for the initiation of the WVB than previously recognized and a much longer history of central WVB magmatism.