Hyaloclastite breccias that occur near the top of the Stonyford volcanic complex contain abundant volcanic glass and rare basaltic clasts that have not been altered significantly by hydrothermal processes or by subsequent metamorphism. They resemble hyaloclastites observed in the summit regions of young Pacific seamounts, and appear to have formed by submarine fire fountains, during which globules of the erupting lava were fragmented by thermal shock from contact with seawater. These breccias contain the first-reported occurrence of unaltered olivine (Fo86) and plagioclase (An63-80) in volcanic rocks of the Franciscan complex.
The glasses are rich in K2O, Nb, Zr, Rb, and Sr relative to both normal and enriched mid-ocean ridge basalt (MORB), and low in Y, V, and Sc. Pillow lavas intercalated with the hyaloclastites and a crystalline basalt clast have similar compositional characteristics. These magmas are similar to alkalic and transitional basalts that are common in intraplate seamounts and along oceanic spreading centers associated with large ion lithophile element (LILE)-enriched mantle plumes. Other volcanic rocks of the Stonyford complex are LILE-rich tholeiitic basalts and ferrobasalts characteristic of intraplate seamounts. A heterogeneous source region that has at least two distinct end members is suggested by the Zr-Nb systematics: depleted suboceanic asthenosphere (normal MORB-type source) and enriched asthenosphere.