A 10-m-thick lava flow of olivine analcimite occurs in drill core near latitude 45 ¿N in the Cascade Range in Oregon. The analcimite contains phenocrysts of calcic clinopyroxene (~23 wt % CaO) and olivine (Fo88-84 with Cr-Al spinel inclusions) in a fine-grained groundmass containing analcime, but no nepheline. Chemically and mineralogically similar rocks are usually found on ocean islands and in intracratonic rifts, both within-plate tectonic settings. The high Ti, Nb, and Ta contents of the analcimite are typical of within-plate and some back arc magmas. The analcimite is unlike many other arc rocks described as alkaline but which are enriched in K and depleted in Ti, Nb, and Ta. The analcimite is K-poor (K2O=0.68 wt %) even for a within-plate magma. Pb and Sr isotope ratios (208Pb/204Pb=38.915; 207Pb/204Pb=15.6; 206Pb/204Pb=19.543; 87Sr/86Sr=0.702848) for the analcimite are similar to ratios found in within-plate alkaline rocks from throughout the Pacific Northwest and NE Pacific Ocean.

This uniformity is consistent with the suggestion that similar mantle underlies the accreted terranes of Oregon, Washington, and British Columbia. The analcimite Pb is much nore radiogenic than Pb in other rocks from the Cascade Range. The analcimite is approximately 4 Ma and was erupted at about the time of intraarc graben formation in the northern portion of the Cascade Range. It is suggested that the analcimite was generated by a very small degree of partial melting in a shallow (~45-50 km) subarc metasomatized mantle containing amphibole and apatite. Similar small-degree partial melts are presumably present in many arcs at the margins of the large-degree partial melt zone that generates arc basalts. In the absence of structural disruption, however, small-degree melts fail to reach the surface. The metasomatism which led to the growth of amphibole and apatite in the analcimite mantle source is probably not related to subduction but rather to earlier melting events that generated oceanic lithospheres before its accretion to ¿ American Geophysical Union 1990