Petrology of the Quaternary Volcanics of the Roosevelt KGRA,and Adjoining Area,Utah.

摘要

The Roosevelt KGRA (known geothermal resource area) located near Milford,Utah,is associated with a suite of Quaternary silicic volcanics distributed for 15km along the crest and western flank of the Mineral Mountains, a large Tertiary granite batholith. The volcanics are high-silica rhyolites and occur as domes,flows and tuffs. Initial eruptions resulted in two flows of low viscosity, phenocryst-poor rhyolite. Later activity consisted of pyroclastic eruptions and the emplacement of at least 9viscous domes. In addition to abundant glass the rhyolites contain alkali feldspar,plagioclase,biotite,iron-titanium oxides and small amounts of hornblende, allanite,apatite,zircon, sphene and topaz. Magmatic temperatures were about 700C and water fugacities ranged from 350to 3450bars. Based on water fugacity calculations it is concluded that there may have been one or more shallow magma chambers in the Mineral Range at depths of about 1.6km.