Faculty Advisor

Tepper, Jeff

Area of Study

Science and Mathematics

Publication Date

Summer 2013


The Teanaway Basalt (TB) includes subaerial basalt to andesite flows, mafic to felsic tuffs, and rhyolite domes in the Central Cascades of Washington State. These volcanics overlie the extensive ~47 Ma Teanaway Dike Swarm (TDS) that cuts the underlying Swauk Formation. Samples were collected from 3 main localities: Easton Ridge (ER) in the west, Liberty Ridge (LR) in the east, and the Middle Fork of the Teanaway River (MF) in the center.

The bimodal TB consists predominantly of basaltic andesite and andesite (45.3–63.1 wt% SiO2) with subordinate rhyolite (75.9–79.4 wt% SiO2). The mafic rocks classify as primarily medium-K tholeiites (0.1–3.0 wt% K2O), but a few samples classify as alkaline. Possible tectonic settings suggested by graphs and diagrams include an arc setting, MOR, or within-plate volcanism.

ER and LR differ mainly in rare-earth element (REE) contents, and also show different concentrations of Fe2O3T, TiO2, MnO, and Na2O. Pearce element ratio plots suggest much of the variation reflects different degrees of plagioclase + clinopyroxene fractionation. With increasing stratigraphic height LR samples exhibit a general trend of decrease in SiO2 and increase inCaO, MnO, P2O5, and cycles of decrease followed by increase in Mg#.


University of Puget Sound

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