In Washington County, southwestern Utah, a thick sequence of volcanic rocks ranges in age from latest Oligocene to Holocene. The Tertiary units consist predominantly of ash flows of intermediate calc-alkalic composition, whereas the youngest volcanic rocks are mostly basaltic andesites. Thermochemical calculations indicate that some of the mafic lavas could be derived from melting of spinel peridotite within the upper part of the low-velocity zone at depths of 50 to 65 km. Some basaltic andesites contain megacrysts of quartz; if the quartz is cognate, the magmas may have formed by partial fusion of quartz eclogite at depths of approximately 120 to 150 km. We suggest that this quartz eclogite is a remnant of the Farallon plate, which underwent subduction at the time of calc-alkalic Tertiary volcanism.

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