The Snow Mountain Volcanic Complex: An On-Land Seamount in the Franciscan Terrain, California

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Abstract
The Snow Mountain Volcanic Complex in the Franciscan terrain of northern California is a 1 1/2 to 2 km thick sequence of altered submarine pillow lavas, pillow breccias, diabase, and interbedded sediments, forming a klippe that structurally overlies nearby rocks of the Franciscan assemblage, the Coast Range ophiolite, and Great Valley sequence. All of the Snow Mountain rocks bear a weak overprint of blue-schist-facies metamorphism. The volcanic rocks were deposited on top of 200-300 m of interbedded turbiditic greywacke, shale, and radiolarian chert, which are interpreted as an interfingering abyssal fan and pelagic sequence. The volcanic pile is thus supracrustal and is not an ophiolitic fragment formed at a mid-ocean spreading center; rather, it is probably the remnant of a former seamount. The lavas are mostly T-rich transitional basalts; minor associated rocks include rhyolite and peralkaline rhyolite. Relict clinopyroxenes in the basaltic rocks are all T-A-rich augites. These petrologic characteristics are typical of the rock series on those mid-ocean islands and seamounts formed near the crests of spreading ridges, suggesting that the Snow Mountain Volcano formed at a ridge crest. The presence of pahoehoe-like flows and highly scoriaceous breccias near the top of the volcanic pile indicate that the volcanic vent approached and perhaps even emerged above sea level. Emplacement of the volcanic complex onto the continental margin during Franciscan subduction may have proceeded via decoupling of the volcano from the underlying oceanic crust at the structurally-weak sedimentary horizon that separated them.