Geochemistry of mafic magmas in the Hurricane volcanic field, Utah; implications for small- and large-scale chemical variability of the lithospheric mantle.

Abstract

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 super 87 Sr/ super 86 Sr from 0.7035 to 0.7049, epsilon sub Nd from 1.6 to -7.5, and super 206 Pb/ super 204 Pb 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 super 40 Ar/ super 39 Ar 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.

59 samples from this reference