GEOCHEMICAL ANALYSIS OF THE TEANAWAY FORMATION, CENTRAL CASCADES, WASHINGTON. EVIDENCE TO SUPPORT SLAB BREAKOFF AFTER ACCRETION OF SILETZIA

The Teanaway Formation is a bimodal Eocene (ca. 49-46 Ma) volcanic sequence located in the central Cascades, Washington State. The Teanaway Formation uncomfortably overlies the Swauk Formation and conformably underlies the Roslyn Formation. The age of the Teanaway Formation overlaps a time of extension in the central Cascades of Washington. This formation consists of a diverse assemblage of volcanic and minor sedimentary rocks, primarily basalt, basaltic andesite, and andesite, with small amounts of rhyolite and dacite. The Teanaway Formation features an extensive swarm of basalt and diabase dikes that intrude the underlying Swauk Formation. It is interpreted that the dikes are feeders for the overlying lava flows. This study utilized whole-rock geochemical analysis from 11 Teanaway Formation samples located near the east bank of Cle Elum Lake. The aim of this study is to interpret the magmatic affinities and volcanic development of the Teanaway Formation. Major elements from the Teanaway Formation indicate a primarily intermediate, subalkaline, tholeiitic, and medium-K compositions. Samples are primarily basaltic andesites. Ba/Nb ratios range between 33 and 111. K₂O/Nb ratios range between 0.06 and 0.23. Sr/Nb ratios range between 17 and 79. These samples predominantly plot in fields defined by mid-ocean ridge basalts (MORB) on several tectonic discrimination diagrams. These new findings support previous results from three different localities in the Teanaway Formation. The high Ba/Nb ratio and medium-K affinities are similar to modern arc settings. The tholeiitic, K₂O/Nb , and Sr/Nb ratios are indicative of transitional arc and MORB settings. The major and trace element data – as well as samples plotting predominantly in MORB fields on tectonic discrimination diagrams – suggests magma generation from both decompression melting and subduction. This supports the generation of the Teanaway Formation as a slab breakoff after the accretion of Siletzia. Forthcoming trace element data will further refine the interpretation.