PETROLOGY OF THE TEANAWAY DIKE SWARM, CENTRAL CASCADES, WASHINGTON

The Eocene Teanaway Group is the third largest basalt province in Washington behind the Columbia River Basalts (CRB) and the Crescent Basalts of the Olympic Peninsula. It crops out over an area of about 220 km² in the east-central WA Cascades and consists of: (1) a swarm of thousands of basalt dikes that intrude the Swauk Formation and (2) a sequence of mostly basaltic lava flows deposited on top of the Swauk Formation. The Teanaway dikes (TD) are generally presumed to be feeders for the overlying flows although evidence of this link has been inconclusive. For the present study we have sampled 32 dikes along three transects chosen to provide broad geographic coverage. The primary goals of the project are: (1) to establish whether there is a chemical “fingerprint” that can be used in distinguish TD from CRB dikes, (2) to ascertain whether there are geographic patterns in the chemical composition of TD, and (3) to determine the tectonic setting in which these basalts formed.

Based on analysis of 46 samples the TD are medium-K tholeiitic basalts and basaltic andesites (51.5 – 62.6 wt% SiO2). Low Mg #'s (20-48) and low compatible trace element contents (Ni < 40 ppm) indicate none of the TD are primitive. REE patterns show slight LREE enrichment (La/Yb_N = 1.1 – 3.4) and generally small negative Eu anomalies (Eu/Eu* = 0.7 – 1.2). The coherent behavior of many incompatible trace elements (e.g., Ba vs. Zr, Th vs. Pb) suggests the dikes have a common source and there is no evidence of systematic geographic patterns in magma composition. Relative to the CRB, TD contain consistently lower Zr, Nb, and REE contents, differences which should make it possible to distinguish these basalt provinces. Based on LILE enrichments and HFSE depletions on spidergrams, we tentatively conclude that the Teanaway formed in a subduction zone setting.