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The AAPG/Datapages Combined Publications Database

Utah Geological Association


Central Utah: Diverse Geology of a Dynamic Landscape, 2007
Pages 163-180

Petrogenesis of the Oligocene East Tintic Volcanic Field, Utah

Daniel K. Moore, Jeffrey D. Keith, Eric H. Christiansen, Choon-Sik Kim, David G. Tingey, Stephen T. Nelson, Douglas S. Flamm


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.

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