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,1
1 Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, Aberdeen AB24 3UE, UK
2 U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, California 95060, USA and Department of Earth Sciences, University of California, 1156 High Street, Santa Cruz, California 95064, USA
3 Lamont-Doherty Earth Observatory, Columbia University, P.O. Box 1000, 61 Route 9W, Palisades, New York 10964, USA
4 Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
5 Department of Geology, Western Washington University, Bellingham, Washington 98225, USA
The Early Jurassic Talkeetna Volcanic Formation forms the upper stratigraphic level of an oceanic volcanic arc complex within the Peninsular Terrane of south-central Alaska. The section comprises a series of lavas, tuffs, and volcaniclastic debris-flow and turbidite deposits, showing significant lateral facies variability. There is a general trend toward more volcaniclastic sediment at the top of the section and more lavas and tuff breccias toward the base. Evidence for dominant submarine, mostly mid-bathyal or deeper (>500 m) emplacement is seen throughout the section, which totals 
7 km in thickness, similar to modern western Pacific arcs, and far more than any other known exposed section. Subaerial sedimentation was rare but occurred over short intervals in the middle of the section. The Talkeetna Volcanic Formation is dominantly calcalkaline and shows no clear trend to increasing SiO2 up-section. An oceanic subduction petrogenesis is shown by trace element and Nd isotope data. Rocks at the base of the section show no relative enrichment of light rare earth elements (LREEs) versus heavy rare earth elements (REEs) or in melt-incompatible versus compatible high field strength elements (HFSEs). Relative enrichment of LREEs and HFSEs increases slightly up-section. The Talkeetna Volcanic Formation is typically more REE depleted than average continental crust, although small volumes of light REE-enriched and heavy REE-depleted mafic lavas are recognized low in the stratigraphy. The Talkeetna Volcanic Formation was formed in an intraoceanic arc above a north-dipping subduction zone and contains no preserved record of its subsequent collisions with Wrangellia or North America.
Key Words: sedimentology • arc volcanism • subduction • geochemistry • isotope geology
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