RELATION BETWEEN MIOCENE VOLCANISM AND IRON-AXIS LACCOLITH EMPLACEMENT, SOUTHWEST UTAH

The Miocene Harmony Hills Tuff (HHT) is a voluminous (~1200 km3), regionally extensive ash-flow tuff erupted over an ~13,000 km2 area of SW Utah and SE Nevada. The HHT is an andesite to trachy-andesite, with ~50% phenocrysts of mostly plagioclase, and lesser biotite, clinopyroxene, hornblende, resorbed quartz, Fe-Ti oxides, and sanidine. Previously published isopach maps suggested that its source caldera could underlie the southern Caliente caldera complex or the southern Iron Axis (Bull Valley Mountains). However, the isopach maps had limited resolution. Geochemical data indicate that the HHT is chemically distinct from the rhyolitic composition of well-documented Caliente area tuffs. Instead, lithologic and geochronologic data of the HHT generally mimic those of the Iron Axis laccoliths to the SE, and their erupted ash-flow tuffs. New geochemical data show, in fact, that the composition of the HHT matches remarkably well with that of the Lookout Point pluton (LPP), one of the central Iron-Axis plutons in the eastern Escalante Desert. Concordant K-Ar and 40Ar/39Ar geochronologic data on the HHT and the LPP are consistent with a co-magmatic origin. Furthermore, new AMS (anisotropy of magnetic susceptibility) data from 15 HHT sites in the northern Pine Valley Mountains and Antelope Range (both SW of the Escalante Desert), yield maximum susceptibility axes (flow lineations) that plunge shallowly and trend toward the NNW (after corrections for strata dip and vertical axis rotation). The AMS orientation data strongly support SE flow of the HHT from a source located to the NNW in the Escalante Desert (LPP area). Well data indicate that the LPP extends westward beneath the Escalante Desert and are roof exposures of a large batholith (see Rowley and others, 2005, Cedar City 30'x60' quadrangle). Most of the Iron Axis laccoliths appear to have been emplaced laterally from the NW along NE trending Sevier thrust faults immediately after eruption of the HHT (22.0-21.5 Ma). We postulate that the Lookout Point batholith first erupted the HHT from a roof caldera. The tuff and pluton apparently blocked further vertical ascent of resurgent intracaldera magma that was then injected laterally along thrust faults and rapidly emplaced toward the southeast as ‘single-shot' laccoliths.