The origin of composition gaps at South Sister volcano, central Oregon: implications for fractional crystallization processes beneath active calc-alkaline volcanoes

South Sister is an active, calc-alkaline strato-volcano located in west-cen tral Oregon in the high Cascades. South Sister contains lavas that range fr om basalt (50% SiO2) to rhyolite (75% SiO2). Within this compositional spec trum the lavas are restricted to three distinct populations: basalt/basalti c-andesite, andesite and rhyolite. Compositional, textural and mineralogic criteria indicate that the basaltic-andesites are hybrid mixtures of basalt and andesite magma. This implies that the system is characterized by basal t, andesite and rhyolite magma populations that are separated from one anot her by composition gaps. Major, minor and trace element modeling suggests t hat the three magma populations are related to one another primarily throug h fractional crystallization. We propose a standard fractionation model for generating compositional diversity amongst erupted lavas wherein in situ f ractional crystallization occurs within a downward descending solidificatio n front. Extraction and eruption of the interstitial fractionated liquid, n ow separated from the parental liquid by a composition gap, occurs only aft er the percentage of crystallization within the solidification front has ex ceeded the appropriate critical crystallinity value for the given parental magma. We envision a magmatic plumbing system characterized by multiple, in dependently solidifying bodies of magma. Thus, at any given time, discrete bodies of basalt, andesite and rhyolite magma may exist. Interaction and mi xing among these bodies could generate the continuous range of lava composi tions that characterize most calc-alkaline volcanic complexes. (C) 2000 Els evier Science B.V. All rights reserved.