Mafic volcanism in the Colorado Plateau/Basin-and-Range Transition Zone, Hurricane, Utah

The Hurricane volcanic field (HVF) is a small-volume (0.48 km{dollar}\sp3{dollar}) mafic volcanic field in the Colorado Plateau/Basin & Range Transition Zone located in the eastern part of the St. George basin in southwestern Utah. Strombolian-Hawaiian style eruptions produced thin (10 m) a\u27a lava flows and cinder (scoria) cones composed of vesicular basalt, bombs and agglutinate. Radiometric dating and geologic relationships demonstrate that the HVF formed over a period of at least 100,000 years. In the upper crust, magma probably rose along joints in sedimentary rocks because chains of volcanic vents follow joint orientation maxima in sedimentary rocks; Three rock groups, low-silica basanite ({dollar}{dollar}46% SiO{dollar}\sb2{dollar}), originated from the partial melting of four isotopically distinct garnet-free mantle sources. Limited mixing between two of the four types of magmas may explain intra-element variation of basanites and some alkali basalts. HVF mafic lavas have relatively high La/Ba, La/Nb and {dollar}\sp{87}\rm Sr/\sp{86}Sr{dollar} and lower {dollar}\varepsilon\sb{\rm Nd}{dollar} values compared to Basin-and-Range basalts less than 5 m.y. old indicating that HVF magmas originated in the lithospheric mantle and interacted with lower crustal component(s) in one or two steps. With distance from the Colorado Plateau and time HVF magmas, like those in other areas of the Transition Zone, become more like ocean island basalt (OIB). The contamination by a lower crustal component of the Transition Zone basalts reflects the thick lithosphere beneath the Transition Zone and Colorado Plateau when compared with the Basin-and-Range basalts which lack this component. The transition of HVF lavas toward an OIB composition with time may reflect the thinning of the lithosphere during extension. (Abstract shortened by UMI.)