THE STRAWBERRY VOLCANICS: A NEW UNIT OF THE COLUMBIA RIVER BASALT GROUP

The Strawberry Volcanics (SV) of NE Oregon plays an important role in our understanding of the intra-continental mid-Miocene volcanic history of the Pacific NW. Because of the proximity, coeval eruptions, and the similarity in the geochemistry including major, trace, and radiogenic isotopic ratios of the SV to the Columbia River Basalt Group (CRBG) we argue that the SV be considered a new subunit of the CRBG.

Ar/Ar age dates of the SV indicate that the eruptions began with rhyolite at 16.2 Ma and continued to 14.6 Ma, intermediate and mafic eruptions followed ranging from 15.6-12.5 Ma. The most mafic magmas of the SV (≤54 SiO₂ wt. %) share geochemical signatures with lavas of the CRBG therefore strongly suggesting CRBG lavas and SV have a similar petrogenetic origin. Specifically, the SV overlap CRBG lavas in major elements including MgO, CaO, and TiO₂ but diverge in Al₂O₃, FeO* and P₂O₅. Normalized REE and incompatible trace-element diagrams indicate that the greatest overlap of lavas of the SV is with samples of Steens and Imnaha Basalt as all samples of the SV are within their ranges and have similar patterns to these groups, although REE patterns are slightly steeper than those of main CRBG members.

Furthermore, incompatible trace elements ratios such as; Ba/Zr, La/Y, U/Nb, also indicate that the SV overlap with units of the CRBG specifically with Steens Basalt. Whole rock radiogenic isotopic data for samples of the SV provide final evidence for similarity among the SV and CRBG units. Isotopic ratios for SV are the following: ⁸⁷Sr/⁸⁶Sr = 0.7052-0.7037, ¹⁴³Nd/¹⁴⁴Nd = 0.5129-0.5125, ²⁰⁸Pb/²⁰⁴Pb =38.89-38.47, ²⁰⁷Pb/²⁰⁴Pb = 15.61-15.49, and ²⁰⁶Pb/²⁰⁴Pb = 18.89-18.55. These values fall within the range or overlap with units of the CRBG with the greatest similarities to Steens, Imnaha and Grande Ronde Basalt. Hence, these data suggest that the SV are part of the regional basalt (as well as rhyolite) magmatism of the CRBG province and should be intergraded as a subunit.