Abstract

Combined elemental and isotopic data for Quaternary (1.3 to 0.45 Ma) basaltic rocks from the western margin of the Markagunt Plateau and the Red Hills area suggest that magmas were derived from an asthenospheric mantle source(s). Basaltic rocks, near Parowan, Paragonah, and Cedar City, Utah, range in composition from basalt to trachybasalt to basaltic andesite. The rocks show strong light- to heavy-rare-earth element enrichment patterns and a wide range in trace-element and isotopic composition. Total rare-earth element abundances range from 96 to 269 ppm, and chondrite-normalized La ranges from 61 to 206. Measured ⁸⁷Sr/⁸⁶Sr isotopic ratios of the basaltic rocks range from 0.70403 to 0.70587, measured ¹⁴³Nd/¹⁴⁴Nd ratios range from 0.512165 to 0.512515, ²⁰⁶Pb/²⁰⁴Pb ratios range from 16.995 to 18.275, ²⁰⁷Pb/²⁰⁴Pb ratios range from 15.421 to 15.537, and ²⁰⁸Pb/²⁰⁴Pb ratios range from 36.683 to 38.169. These data indicate that those magmas that erupted evolved through a combination of partial melting, crystal fractionation, and lithospheric interaction.

Dated basaltic rocks indicate that volcanism migrated eastward onto the Markagunt Plateau away from the Red Hills beginning about 1.3 Ma. The age data and the presence of local vent alignments suggest that volcanism was probably related to the structural development of the margin of the Colorado Plateau. Time spent by magmas in lithospheric channelways and chambers allowed the rising magmas to differentiate through an open-system magmatic process involving fractional crystallization and lithospheric interaction. In addition to an eastward age progression, the basaltic rocks also show an eastward decrease in silica content (53.6 to 49.9 weight percent SiO₂) that probably reflects decreasing magma residence times within the lithosphere from west to east across the transition zone between the Colorado Plateau and the Basin and Range Provinces.