Abstract

The early Oligocene East Tintic volcanic field of central Utah, located near the eastern margin of the Basin and Range Province, consists of mafic to silicic volcanic (mostly intermediate-composition lava flows) and shallow intrusive rocks associated with the formation of small, nested calderas. Radiometric ages establish a minimum age for initiation (34.94 ± 0.10 Ma) and cessation (32.70 ± 0.28 Ma) of East Tintic magmatism. The igneous rocks of the field are calc-alkalic, potassic, silica-oversaturated, and met-aluminous, and can be categorized into the following three compositional groups: the shoshonite-trachyte series, the trachyandesite series, and the rhyolite series. Based on composition and phenocryst assemblage, the shoshonite-trachyte series is divided into two groups: a clinopyroxene group and a two-pyroxene group. The rhyolite series consists of three field units: the Packard Quartz Latite, the Fernow Quartz Latite, and the rhyolite of Keystone Springs. The trachyandesite series is by far the most voluminous. This series is also subdivided into a clinopyroxene group and a two-pyroxene group. Temperature and oxygen fugacity estimates indicate that shoshonite-trachyte series magmas were the hottest and least oxidizing and that two-pyroxene trachyandesite series magmas were the coolest and most oxidizing.

Clinopyroxene shoshonite-trachyte series magma evolved mainly by fractional crystallization. The high K₂O, Rb, and Al₂O₃/CaO ratios and modest SiO₂ enrichment of these rocks appear to result from extensive, high-pressure fractional crystallization of clinopyroxene (without plagioclase). Two-pyroxene shoshonite-trachyte series magma was likely produced by mixing between mafic and silicic clinopyroxene shoshonite-trachyte series magmas at low pressure. Assimilation of crustal material appears not to have been important for shoshonite-trachyte series magmas. We believe that parental clinopyroxene shoshonite-trachyte series magma originated in the mantle wedge above a Cenozoic subduction zone and then interacted with older subduction-metasomatized lithospheric mantle. Rhyolite series magma was likely the differentiate of a lower crustal partial melt. Trachyandesite series magma likely evolved by magma mixing and subsequent fractional crystallization. Trace-element compositions indicate that the mixing that produced trachyandesite series magmas was between mafic clinopyroxene shoshonite-trachyte series magma and Fernow Quartz Latite magma, at low pressure for two-pyroxene trachyandesite series magma, and at high pressure for clinopyroxene trachyandesite series magma.