PAIRED SEAMOUNTS IN THE NORTHERN CALIFORNIA COAST RANGES

Two fault slices of Ti-rich ocean-island-type submarine basalts (pillow lavas and hyaloclastites) lie within the northern Calif. Coast Ranges east of Ukiah, near the boundary between the Franciscan terrain and the Great Valley Group homocline. One slice is a complex klippe comprising Snow Mtn. and nearby St. John Mtn; the other forms a synformal sheet just to the SE, near the small town of Stonyford. Early studies focused on one or the other of these volcanic masses, and tended to regard them as part of a single large thrust sheet. However, geochemical and metamorphic mineralogy data demonstrate that they are distinct and represent two seamounts, possibly the remnants of a chain.

The Snow Mountain/St. John Mountain sequence consists exclusively of titaniferous, light-rare-earth-element- (LREE) enriched basalts and rare silicic differentiates (including comendites). All the rocks are highly altered greenstones; calcic pyroxene and chromite are the chief relict phases. High P/T metamorphic minerals, including lawsonite, crossite, aragonite, and near-end-member jadeite (from analcime), are ubiquitous but weakly developed.

The Stonyford sequence is more diverse and includes light-REE depleted tholeites as well as titaniferous light-REE enriched alkalic basalts; extreme differentiates have not been found. Although altered, many Stonyford rocks are unexpectedly well preserved: even primary calcic plagioclase is widespread. No “blueschist” mineralogy has been found; pumpellyite and prehnite are common. Moreover, these rocks underlie Great Valley Group sedimentary rocks and are arguably now part of the Coast Range ophiolite.

Although separated by only ~1.5 km lateral distance, the two sequences clearly do not represent a single large complex. Yet their geographic proximity can hardly be coincidental. We interpret them to be two separate seamounts or abyssal hills, likely members of a chain. One, the Snow Mountain seamount, was subducted. The Stonyford seamount somehow escaped that fate and was accreted directly to the Coast Range forearc.