Abstract

Eocene volcanic flow and dike rocks from the Beringian margin have arc characteristics, implying a convergent history for this region during the early Tertiary. The extrusive rocks are basalt, basaltic andesite, andesite, and minor dacite and rhyolite. The intrusive sample is from a quartz diorite dike intruding serpentinized peridotite. Major-element oxide contents, particularly FeO*/MgO versus SiO2, identify both tholeiitic and calc-alkalic basalt; more silicic lavas have calc-alkalic affinities. Consistent with volcanic-arc compositions, spidergrams show pronounced Nb–Ta depletion and alkali enrichment relative to light-rare-earth-element (LREE) abundance. Chondrite-normalized REE plots show relatively flat patterns, with only slight LREE enrichment for tholeiitic compositions and greater LREE enrichment and lower heavy-rare-earth-element (HREE) abundance for calc-alkalic compositions. The samples, particularly those with calc-alkalic compositions, are rich in plagioclase that is strongly zoned; the more silicic samples contain orthopyroxene, clinopyroxene, and primary amphibole. The quartz diorite dike contains iron-rich almandine phenocrysts that appear to be magmatic, suggesting emplacement at great depth near the base of the crust or upper mantle.Chemical and mineralogical compositions are similar to those of modern Aleutian-arc lavas. They also resemble volcanic-arc compositions from western mainland Alaska, although greater chemical diversity and a stronger continental influence are observed in the Alaskan mainland rocks.Early Eocene ages of 54.4–50.2 Ma for the Beringian samples are well constrained by conventional K–Ar ages of nine plagioclase separates and by concordant 40Ar/39Ar incremental heating and total-fusion experiments. A concordant U–Pb zircon age of 53 Ma for the quartz-diorite dike is in good agreement with the K–Ar data.Plate motion studies of the North Pacific Ocean indicate more northerly directed subduction prior to the Tertiary and a continuous belt of arc-type volcanism extending from Siberia, along the Beringian margin, into mainland Alaska. Around 56 Ma (chron 25–24), subduction changed to a more westerly direction and subduction-related volcanism ceased for most of mainland Alaska. The increasingly oblique angle of convergence should have ended subduction along the Beringian margin as well. However, consistent ages of 54–50 Ma indicate a final pulse in arc-type magmatism during this period of plate adjustment, which may be explained by three different models: (1) The northern and central part of the Beringian margin maintained a higher angle of convergence, allowing a final pulse of arc-type magmatism. (2) The rocks erupted in an early, or proto, Aleutian arc and were rafted against the continental margin along transform faults. (3) The rocks erupted along a leaky transform fault, analogous to calc-alkalic volcanism in the southern California borderland.

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