Petrogenesis and geochemistry of Pleistocene and Pliocene basalt flows of the Pine Valley Volcanic Field, Utah and their relationship to the tectonics of the Utah transition zone

The Pine Valley Volcanic Field in southwestern Utah consists of at least 24 mafic lava flows and 10 vents. Eruptions have spanned the past 2.4 m.y. The youngest eruptions at the Snow Canyon and Diamond Valley cones occurred between 1 and 20 ka. Lava flows range in composition from basalt to andesite, and include both alkaline and subalkaline rocks throughout the field. Trace element abundances for the youngest lavas show both oceanic island basalt (OIB) and mid-oceanic ridge basalt (MORB) components in their source. Sr and Nd isotopic ratios exhibit evolved values, spanning a large range from epsilon Nd of 0.54 and 87Sr/86Sr of 0.7036 to epsilon Nd of -7.83 and 87Sr/86Sr of 0.7055. The apparent conflict between geochemical evidence for an asthenospheric source (MORB and OIB-like trace element abundances) and a lithospheric source (evolved isotopic ratios) is resolved through a model that calls upon contamination of an asthenospheric melt with ancient (isotopically-evolved) mid-oceanic ridge basalt (MORB) within the lithosphere.