PETROLOGY OF THE GRAYS RIVER VOLCANICS, SOUTHWESTERN WASHINGTON: EVIDENCE FOR LATE EOCENE RIFTING

The Grays River Volcanics (GRV) are a 50 km long SE-trending belt of Eocene basalts (40-43 Ma) located in the Coast Range of southwestern Washington. The belt includes columnar and platy jointed subaerial flows, pillow lavas, breccias, and tuffs, which are overlain by marine sediments. For this study we have gathered petrographic, geochemical, and paleomagnetic data in order to: (1) characterize and determine the tectonic setting of the GRV, and (2) make comparisons with the neighboring but older (53-45 Ma) and more voluminous Crescent Basalts (CB).

GRV lavas contain phenocrysts of plagioclase, clinopyroxene, ilmenite, and olivine (largely replaced) in a microcrystalline to hypocrystalline groundmass.  Plagioclase displays dissolution textures and normal oscillatory zoning.  ICP-OES analyses of 38 samples collected along the length of the belt reveal limited SiO₂ variation (47.5-51.2 wt. %) and low Mg #s (56-32) but a wide range of incompatible element content (Sr: 365-566 ppm; Zr: 177-357 ppm). Spidergrams show OIB characteristics. More than one process is needed to explain the chemical variation among the GRV samples. Fractional crystallization models indicate early removal of clinopyroxene but not plagioclase, suggesting differentiation at high pressure.  However, multiple sources and/or varying degrees of melting are required to explain the large incompatible element variations.

GRV samples display lower concentrations of MgO (4.89 avg. wt. %) compared to the CB with MgO (6.42 avg. wt. %).  The GRV also have higher concentration of incompatible elements, notably TiO₂ (3.14 avg. wt. %) and P₂O₅ (0.49 avg. wt. %) whereas CB contains TiO₂ (2.12 avg. wt. %) and P₂O₅ (0.23 avg. wt. %).  If the GRV and CB had a common source then the GRV would be correlated with a lesser degree of melting. The GRV tectonic setting is most consistent with rifting.  Preliminary paleomagnetic data indicate ~15º clockwise rotation, but no northward translation.  Future research will include additional sampling, attempts to better constrain relative ages of flows within the formation, and isotopic analysis to better determine the mantle source(s).