Peridotites are infrequently seen at the earth’s surface, making the glimpses of mantle processes they reveal invaluable. An unusual metaperidotite outcrop in Domenigoni Valley, CA offers insight into subduction zone processes associated with the accretion of western North America. These serpentinized dunitic peridotites, exposed at the northernmost tip of the Peninsular Ranges Batholith arc, display a melt-depleted pyroxene-poor mineralogical composition. High forsterite contents of olivines and high chromium number in chrome spinels indicate that these highly melt-depleted samples are of forearc origin (Arai, 1994), further substantiated by low concentrations of incompatible elements. Serpentinization fluids appear to have refertilized light rare earth elements (LREEs) in these samples without affecting the concentrations of major and minor elements. Calculations of melt depletion based on heavy rare earth element (HREE) concentrations of olivines and whole rock glasses indicate melt depletion of up to 25%. Fostering a better understanding of mantle behavior associated with arc accretionary processes may help to illuminate the subtleties of continental crust formation.