Geochemistry of Mafic Magmas in the Hurricane Volcanic Field, Utah: Implications for Small‐ and Large‐Scale Chemical Variability of the Lithospheric Mantle

Low‐silica basanite, basanite, and alkali basalt lava flows and cinder cones make up the late Quaternary Hurricane volcanic field (HVF) in the Colorado Plateau/Basin and Range transition zone of southwestern Utah. Strombolian‐ and Hawaiian‐style eruptions produced thin (10 m) a'a lava flows and 10 cinder and scoria cones that group geographically into five clusters. The five clusters can be further divided into four isotopic magma types that vary in 87Sr/86Sr from 0.7035 to 0.7049, &egr;Nd from 1.6 to −7.5, and 206Pb/204Pb from 17.4 to 18.7. Except for the Radio Towers and Volcano Mountain cone clusters, each volcano had a different parent magma and evolved by fractional crystallization of different amounts and proportions of olivine and clinopyroxene. Parent magmas of each isotope group formed by 0.5%–7% partial melting of lithospheric mantle composed of fertile lherzolite varying in garnet content from 1 to 4 wt %. New 40Ar/39Ar dates indicate that the HVF formed over a period of at least 100 ka during the late Quaternary. Along a transect from the Basin and Range to the Colorado Plateau, the source for Pliocene–late Quaternary alkali basalt magmas changes from asthenosphere in the Basin and Range to lithospheric mantle on the Colorado Plateau. The melting of a heterogeneous lithospheric mantle is the most viable mechanism for producing the observed chemical variability in the transition zone–Colorado Plateau part of the transect. Furthermore, chemical differences across the transect may reflect a major lithospheric boundary originally defined on the basis of Nd and Pb isotopes that is older and perhaps more fundamental than the present structural and physiographic boundary between the Basin and Range and Colorado Plateau.