GEOCHEMICAL ARCHITECTURE OF THE WRANGELL VOLCANIC FIELD, ALASKA

Substantial geochemical and petrological diversity is identified in the < 5 Ma eruptive products of the Wrangell Volcanic Field (WVF) portion of the Aleutian-Wrangell continental magmatic arc. Three volcanic rock suites are identified: a dominant, arc-wide low-Ti calc-alkaline suite, a second low-Ti calc-alkaline suite that is restricted to the northwestern- and southeastern-most volcanoes of the arc and that is characterized by adakitic amphibole andesite/dacite magmas, and a high-Ti transitional tholeiitic suite that contains the most mafic WVF eruptive products and is concentrated in the interior portions of the field. Aspects of magma genesis in all three suites involve a MORB-like mantle wedge source that has been variably modified by the addition of slab-derived components. The downgoing slab provides an additional source of partial melts at opposing ends of the WVF arc-system resulting in highly oxidized, water-rich adakitic andesites and dacites. Post magma generation open system processes including magma mixing, magma-crust interaction, and magma-mantle interaction, along with fractional crystallization of anhydrous and hydrous mineral assemblages are responsible for within-suite differentiation to more evolved compositions and for diversity within the adakitic andesites and dacites.

The observed distribution of magma types and geochemical characteristics within the WVF is linked to four major tectonic factors: (1) decrease in subduction rate ca. 100 Ka ago, (2) presence of plate margin-like interactions at the northwestern and southeastern ends of the arc system, (3) thinning of the crust from the front-side to the back-side of the western portion of the arc system, and (3) initiation of intra-arc extension ca. 1 Ma ago in the western portion of the arc system. The first two factors are critical to WVF adakitic melt generation leading to the only documented highly explosive pyroclastic eruptions in the WVF. It is likely that such eruptions have been common for the past 3 m.y. as evidenced by widespread silicic tephra beds in Alaska and the Yukon bearing adakitic signatures. Future work may uncover the source volcanoes of these older eruptions and will be guided the growing recognition that adakitic melt generation is linked to unique tectonic elements in complex arc settings.