Voluminous Eocene to Oligocene intermediate to silicic volcanic rocks related to subduction erupted throughout the Great Basin and were supplanted by bimodal eruptions of basalt and rhyolite related to extension in the Miocene. Locally, in the northern East Tintic Mountains of central Utah, this important transition is marked by a distinctive package of mafic alkaline magmas that reveal important details about the nature of this fundamental change. A late Oligocene anorthoclase-bearing shoshonite lava in the Boulter Peak quadrangle contains megacrysts of anorthoclase, with phenocrysts of olivine, clinopyroxene, magnesiohastingsite, magnetite, and apatite. The anorthoclase grains occur as glomerocrysts with irregular, resorbed edges, indicating they are not in equilibrium with the mafic phenocrysts in the shoshonite. They are interpreted to be xenocrysts incorporated into an ascending mafic magma that came into contact with a partially crystallized syenite. The mafic magma involved was probably derived by partial melting of the lithospheric mantle based on its high Mg/Fe ratios, magnesian phenocrysts, high water content, and high ratios of lithophile to high field strength elements. The syenite body likely crystallized from a highly differentiated melt. The 40Ar/39Ar age of the shoshonite is 25.35±0.04 Ma, and appears to represent the transition from subduction before the onset of extension (Christiansen et al., 2007). Other Oligocene mafic units in the area may represent different variations of the mafic alkaline endmember for the mixing process. The Gardison Ridge dike, a potassic alkaline basalt with an 40Ar/39Ar age of 26.3±0.3 Ma, contains olivine and clinopyroxene phenocrysts that are compositionally very similar to those found in the shoshonite. Other mafic dikes have even higher alkalis. All of these dikes have similar trace element patterns, with negative Nb and positive Pb anomalies, and high Ba and K concentrations. The minette of Black Rock Canyon (28.45±0.13 Ma) also contains high alkalis, particularly K, and its trace element pattern shows positive Ba and negative Nb anomalies. The clinopyroxene phenocrysts in the minette are also very similar to those found in the other alkaline rocks. The high water contents of these units are evidenced by amphibole in the shoshonite, phlogopite in the minette, and the lack of plagioclase phenocrysts in the basaltic dikes. The ages, mineral assemblages, and chemical compositions show that these late Oligocene alkaline magmas formed after a shallowly subducting oceanic slab peeled away from the overlying continental lithosphere and rolled back. Hot asthenosphere flowed in to replace the subducting plate and caused partial melting of the variably metasomatized lithospheric mantle. These alkaline magmas include the shoshonite, mafic alkaline dikes, and minette of Boulter Peak; they mark the transition from older subduction-related magmatism to Miocene magmatism caused by lithospheric extension.



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Physical and Mathematical Sciences; Geological Sciences



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anorthoclase, Boulter Peak, East Tintic Mountains, alkali basalt, syenite

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