PETROGENESIS OF THE CASCADE HEAD BASALT SERIES, CENTRAL OREGON COAST RANGE

Late Eocene Cascade Head Basalt was erupted in the forearc of the Cascades, creating an emergent shield volcano in a shallow marine environment. Lava flows form an alkalic series, ranging in composition from picrobasalt and basalt to hawaiite to mugearite to benmoreite and trachyte; most samples are neither qz nor ne-normative, and more are ne-normative rather than qz-normative. The Cascade Head series thus contrasts with the subalkaline series of largely qz-normative rocks formed by the nearby, contemporaneous, tholeiitic Yachats Basalt series.

Our data, combined with those of previous workers, forms a data base of 48 Cascade Head whole-rock analyses. These combined with electron microprobe analyses of phenocrysts allow a detailed examination of petrogenetic processes. Sample/primitive mantle variation diagrams form patterns typical of Ocean Island Basalts, with most showing negative anomalies for Sr, P and Ti. Four samples were chosen in petrogenetic modeling to represent three steps in a fractional crystallization process: alkaline basalt to hawaiite, hawaiite to mugearite, and mugearite to qz-normative trachyte, involving about 82 weight percent crystallization of starting basalt to form 18 percent trachyte. Major element and trace element compositions are extremely well modeled in this sequence, except for Ba in the final step, which may require fractionation of alkali feldspar, not observed in our samples.

Comparison with basaltic series from the Hawaiian Islands reveals similarities to the pre-shield and post-shield alkalic phases of magmatism, suggesting that the Cascade Head parental basalt was produced by modest degrees of melting of mantle rocks.