GEOCHEMICALLY ANOMALOUS IGNEOUS ROCKS OF THE OLYMPIC PENINSULA: IMPLICATIONS FOR EOCENE TECTONICS

Geochemically anomalous igneous rocks from four little studied localities on the Olympic Peninsula have been analyzed to establish their possible tectonic implications. Rocks from these sites are considered anomalous because they differ from Crescent basalts (the main Eocene unit on the Peninsula), having higher SiO₂ and/or more alkaline compositions. Petrographic and ICP-ES and ICP-MS analysis were used to characterize these samples and infer their origins.

The four localities are: (1) Makah Bay (MB) where lahar deposits contain clasts of hornblende-bearing basaltic trachyandesites and trachyandesites (50.6 - 56.1 wt.% SiO₂; 3.0 – 4.7 wt.% K₂O) . These rocks display moderate LREE enrichment (La/Yb_N = 8.7-13.3), negligible Eu anomalies, OIB-like spidergrams, and extreme Ba enrichment (>5000 ppm). (2) Point of the Arches (PA) where hornblende-bearing andesitic-to-dacitic dikes/sills (57.0 – 67.2 wt.% SiO₂; 0.9 – 2.0 wt.% K₂O) are present. These rocks are previously K-Ar dated at 59±3 Ma (Snavely et al., 1993) and show modest LREE enrichment (La/Yb_N = 5.8-12.7), negligible Eu anomalies, and arc-like spidergrams (Nb depletions). (3) Yahoo Lake (YL) where a rhyolitic tuff (74.7 – 77.0 wt.% SiO₂) is present, and (4) Happy Valley (HV) where there is a rhyolite flow.

Pronounced chemical differences between localities indicate these rocks do not share a common origin. In addition, none show adakite traits as seen in felsic volcanics elsewhere on the Olympic Peninsula (Wolfe and Tepper, 2004). MB sample compositions are consistent with a rift setting; we tentatively attribute the elevated Ba to alteration. The PA samples are unlikely to be differentiates of a more mafic parent; their compositions are more consistent with shallow melting (above the garnet field) of a basaltic source. Both rifting and the high temperatures necessary for melting in the forearc may be related to subduction of the Kula-Farallon Ridge at this time. Ongoing work will better establish the age of YL and the character and origin of the YL and HV.