PAIRED SEAMOUNTS IN THE NORTHERN CALIFORNIA COAST RANGES
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.