Oligocene caldera complex and calc-alkaline tuffs and lavas of the Indian Peak volcanic field, Nevada and Utah

The Indian Peak volcanic field is representative of the more than 50,000 km3 of ashflow tuff and tens of calderas in the Great Basin that formed during the Oligocene-early Miocene "ignimbrite flareup" in southwestern North America. The field formed about 32 to 27 Ma in the southeastern Great Basin and consists of the centrally positioned I n d i Peak caldera complex and a surrounding blanket of related ash-flow sheets distributed over an area of about 55,000 km2. The field has a volume on the order of 10,000 km3. A cluster of two obscure source areas and four calderas comprise the -80 x 120 km caldera complex. Only minor volumes of rhyolite and two pyroxene andesite lavas were extruded episodically throughout the lifetime of the magma system that formed the field, chiefly during its youth and old age. Six ash-flow sequences alternate between rhyolite and dacite in a volume ratio of about 1:8, and a culminating seventh is trachytic. The first, fourth, and sixth tuff units are of rhyolite that contains sparse to modest amounts of phenocrysts, chiefly plagioclase and biotite, and abundant lithic and pumice lapilli; these deposits are confined within the caldera complex and form multiple and compound cooling units that are normally zoned with respect to bulk chemical composition and crystal type, content, and size. The second, third, and fifth tuff sequences are of crystal-rich dacite that forms extensive simple cooling-unit outflow sheets and partial caldera fillings of compound cooling units. Each dacite unit contains similar amounts of plagioclase, biotite, hornblende, quartz, two pyroxenes, and Fe-Ti oxides; trace amounts of sanidiine and titanite also occur in the youngest. Cognate inclusions in the dacites show only slight intraand inter-unit differences in bulk chemical composition. The seventh eruptive sequence consists of several cooling units of trachydacite tuff containing small to modest amounts of plagioclase and two pyroxenes. These dominantly high-K calc-alkaline rocks are a record of the biih, maturation, and death of a large, open, continental magma system that was probably initiated and sustained by influx of mafic magma derived from a southward-migrating locus of magma production in the mantle. The small volumes of chemically diverse andesitic rocks were derived from separately evolving magma bodies but are modified representatives of the mantle power supply. Recurrent production of very large batches (some greater than 3,000 km3) of quite uniform dacite magmas appears to have required combination of andesite magma and crustal silicic material in vigorously convecting chambers. Compositional data indicate that rhyolites are polygenetic. As the main locus of mantle magma production sh ied southward, trachydacite magma could have been produced by fractionation of andesitic magma withiin the crust.