Petrogenesis of Tertiary Andesite Lava Flows Interlayered with Large-Volume FelsicAsh-Flow Tuffs of the Western USA

Abstract

The San Juan volcanic field (Colorado), the Indian Peak volcanic field (Utah–Nevada) and the central Nevada volcanic field formed during Oligocene and Miocene times. Each field is characterized by >3000 km3 of rhyolitic and dacitic ash-flow tuff sheets and smaller volumes (<300 km3) of interlayered andesitic lavas. In each field, andesite lavas erupted from vents within and peripheral to calderas formed by approximately contemporaneous felsic ash-flow eruptions. Olivine andesites occur only peripheral to calderas, whereas hornblende andesites occur only within calderas. Pyroxene andesites occur at both locations. The parental magmas of the andesites formed by partial melting of mantle material, as shown by the presence of olivine or Cr-spinel in some units. Compositional evolution of andesites appears to have been controlled in part by crystal fractionation at 0.2–0.8 GPa, based on compositional and mineralogic comparisons with liquid lines of multiple saturation from experiments involving data for andesitic compositions. Modeling of bulk compositional variations suggests crystal fractionation may have been accompanied by mixing with dacitic or rhyolitic magmas. At locations beneath present-day calderas, upward-migrating basaltic to andesitic magmas encountered dacitic or rhyolitic magmas. The more felsic magmas gravitationally trapped the rising mafic magmas. These trapped magmas evolved from basaltic or olivine andesitic compositions to hornblende andesitic compositions by crystal fractionation combined with mixing with dacitic and rhyolitic magmas. Subsequent eruption or crystallization of dacitic and rhyolitic magmas removed the density contrasts and allowed hornblende andesites to erupt within calderas. In contrast, parental mafic magmas at locations peripheral to present-day calderas ascended without encountering dacitic or rhyolitic magmas, and these erupted as olivine andesites. Thus andesites initially evolved separately from the region's voluminous dacite and rhyolite magmas, but later mixing with the felsic magmas influenced compositions of many andesites.