Late Cretaceous Construction of the Mantle Lithosphere Beneath the Central California Coast Revealed by Crystal Knob Xenoliths

Abstract

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 (εNd) of 10.3–11.0 and 87Sr/86Sr of 0.702 are characteristic of underplated suboceanic mantle. Pyroxene exchange geothermometry shows equilibration at 950–1060 °C. Phase stability, Ca-in-olivine barometry, and 65- to 90-mW/m2 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.