A tschermakite-bearing high-alumina olivine tholeiite from the southern Cascades,California

The first occurrence of amphibole-bearing basalt in the southern Cascade volcanic province has been found in the Medicine Lake area of California. Little Mount Hoffman, a late Pleistocene cinder and scoria cone, has erupted high-alumina olivine tholeiite, the petrography of which yields a paragenetic phenocryst sequence of amphibole→olivine→plagioclase. The amphibole is compositionally a tschermakite. Comparison with available experimental data indicates an initial depth of crystallization somewhere between 40 and 100 km, temperature near 1000 °C, and f _{O 2} near the Ni-NiO and QFM buffers. Contribution No. 124