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The Pleistocene (1.65 Ma) Crystal Knob volcanic neck in the California Coast Ranges is an olivine-plagioclase phyric basalt containing dunite and spinel peridotite xenoliths. Crystal Knob erupted through the Nacimiento belt of the Franciscan complex and adjacent to Salinian crystalline nappes. Its xenoliths sample the mantle lithosphere beneath the outboard exhumed remnants of the southern California Cretaceous subducting margin. This sample set augments previously studied xenolith suites in the Mojave Desert and Sierra Nevada, which linked the mantle lithosphere architecture and crustal structure of the western Cordillera. We examine six peridotite samples ranging from fertile lherzolites to harzburgite residues. Time-corrected (epsilon(Nd)) of 10.3-11.0 and Sr-87/Sr-86 of 0.702 are characteristic of underplated suboceanic mantle. Pyroxene exchange geothermometry shows equilibration at 950-1060 degrees C. Phase stability, Ca-in-olivine barometry, and 65- to 90-mW/m(2) regional geotherms suggest entrainment at 45- to 75-km depth. The samples were variably depleted by partial melting, and re-enrichment of the hottest samples suggests deep melt-rock interaction. We test the Crystal Knob temperature depth array against model geotherms matching potential sources for the mantle lithosphere beneath the Coast Ranges: (A) a shallow Mendocino slab window, (B) a young Monterey plate stalled slab, and (C) Farallon plate mantle nappes, underplated during the Cretaceous and reheated at depth by the Miocene slab window. Models B and C fit xenolith thermobarometry, but only model C fits the tectonic and geodynamic evolution of southern California. We conclude that the mantle lithosphere beneath the central California coast was emplaced after Cretaceous flat slab subduction and records a thermal signature of Neogene subduction of the Pacific-Farallon ridge.

Plain Language Summary The Crystal Knob volcanic neck is a small, deeply sourced lava that erupted near in the California Coast Ranges 1.65 million years ago. It carried xenoliths, hand-sized fragments of the mantle, which can help us understand how the western edge of the North American continent was formed. We focused on six samples from Crystal Knob with different mineralogical characteristics, all of which originated in the mantle. Their chemistry suggests that they were not originally part of North America. Some xenoliths had been partially melted, and many had additional material added, changing their composition from pristine mantle. Mineral phases record their temperature (950-1060 degrees C) and depth (45-75km) prior to eruption. This rare direct record of the temperature of the upper mantle allows us to test several options for the formation of the under pinnings of the central California coast. The most viable option is that the mantle beneath the edge of North America was tectonically pushed under the continent similar to 75 million years ago. It heated from below similar to 24 million years ago at the end of Farallon plate subduction. This history fits with extensive evidence that much of the deep architecture of the California coast is inherited from the Cretaceous period.