GEOCHEMISTRY OF THE 1989-1990 ERUPTION OF REDOUBT VOLCANO .1. WHOLE-ROCK MAJOR- AND TRACE-ELEMENT CHEMISTRY

The 1989-1990 eruption of Redoubt Volcano produced medium-K calc-alkal ine andesite and dacite of limited compositional range (58.2-63.4% SiO 2) and entrained quenched andesitic inclusions (55% SiO2) which bear c hemical similarities to the rest of the ejecta. The earliest (December 15) magmas are pumiceous, often compositionally banded, and the major ity is relatively mafic (<59% SiO2). The most silicic magmas of the er uption are the late December to early January domes (up to 63.4% SiO2) . Subsequent magmas formed domes and rare pumices which converge on 60 % SiO2. Chemical variations among ejecta comprise tight, linear, two-c omponent arrays for all elements for which the analytical uncertainty is much less than the compositional range. The two-component arrays ar e interpreted as mixing arrays between unrelated magmas because severa l of the arrays are at steep angles to the normal liquid line of desce nt. Additionally, the felsic endmember cannot be easily related to the mafic endmember by normal high-temperature igneous processes (e.g., t he silicic endmember has higher Zr yet lower Hf than the mafic endmemb er). Also relative enrichments of highly incompatible elements are dra matically different across the arrays. The mixing event must have prec eded eruption by a significant, yet unspecified amount of time because groundmass glass compositions are homogeneous for all post-December s amples (Swanson et al., 1994-this volume), in spite of the whole-rock chemical diversity. This implies time for additional crystallization a fter the mixing event. Swanson et al. (1994-this volume) discuss evide nce for a potentially different mixing event recorded only in December 15 magmas. Cognate cumulate xenoliths composed of pl + cpx + opx + hb + mt + melt were recovered from January and April deposits. These blo cks differ from local batholithic country rock in their low concentrat ions of incompatible elements (e.g., Rb < 5 ppm vs 20-90 ppm, Ba < 150 ppm vs 300-2000 ppm) and low SiO2 (<50 wt.% vs >60 wt.%). They have M g, Cr, Ni, Sc, and V contents higher than the andesites, but lower tha n Redoubt basalts and basaltic andesites. Thus, they may be crystalliz ation products of andesites, but do not represent the cumulate residue of basalt fractionation. The xenoliths were probably derived from a s hallow or intermediate crustal chamber.