SPINEL PERIDOTITE XENOLITHS FROM CRYSTAL KNOB AND THE STRUCTURE OF THE MANTLE LITHOSPHERE BENEATH COASTAL CALIFORNIA

The Crystal Knob volcanic neck in the Santa Lucia Range, California, was erupted during the Pleistocene (1.65 Ma from Ar geochronology) through the Salinia terrane of coastal central California. The neck consists of an olivine-plagioclase phyric basalt containing spinel peridotite and cumulate dunite xenoliths. The peridotites sample the mantle lithosphere beneath Salinia and constrain its lithology and geotherm at depth. They are useful markers of the remnant tectonic architecture of the sub-Mojave convergent plate boundary (of which Salinia, translated ~310 km dextrally along the San Andreas Fault, is a part).

Six spinel peridotite samples were analyzed, ranging from fertile lherzolites to harzburgites with modal clinopyroxene from 2-13%. They have a depleted mantle isotopic signature and were likely underplated beneath the Mojave region during flat shallow subduction of the Farallon plate. Pyroxene Ca-exchange geothermometry (Taylor, 1998 etc.) shows equilibration temperatures between 950 and 1060 ºC. Rare-earth temperatures (Liang, et al., 2013) are broadly similar. The samples fall into two groups based on temperature of equilibration, trace-element abundances, Cr content in spinels, and Ca in olivine. The modes of variation suggest that the samples were entrained along a depth gradient. Source depths are likely between 40 and 80 km, based on broad limits imposed by phase stability and temperature. The higher-temperature samples show general REE depletion but sharp enrichment in LREEs, potentially showing a history of melt extraction followed by off-axis refertilization.

Forward modeling of thermal evolution is used to test a range of scenarios for the age of the depleted mantle underplated beneath Salinia. Scenarios ranging from a young “stalled slab” (Pisker et al., 2012) to the rollback imbrication of Farallon lithosphere in the Cretaceous to Paleocene (Luffi et al., 2009) imply relatively cool modern geotherms approaching steady-state. These scenarios generally predict colder conditions at depth than are recorded by xenoliths or surface heat-flow (Erkan and Blackwell, 2009). This suggests additional involvement of mantle upwelling in the construction of the sub-Salinia mantle lithosphere, most plausibly during the opening of the Pacific-Farallon slab gap as subduction terminated.