The Alkalic Rock Suite of Bogoslof Island, Eastern Aleutian Arc, Alaska

Abstract

The historic eruptions of Bogoslof volcano were characterized by hornblende andesite in A.D. 1796, and nepheline-normative, hornblende basalt in A.D. 1883 and 1926-1927. Rare earth element (REE) abundances for the 1796 and 1926-1927 eruptions show enrichment of the light REE relative to the heavy REE, overall higher abundances in the andesite and a slight positive Eu anomaly in the basalt. The ratios are for the andesite and for the 1927 basalt. Lead isotopic abundances are similar for the two eruptions and the rocks are more radiogenic than mid-ocean ridge basalts. Microprobe data for phenocrysts of potassio ferroan pargasite, calcic augite and plagioclase show a considerable compositional range and degree of compositional overlap. Mafic inclusions in basalt and andesite contain the same phases as the host lavas and a similar compositional range, suggesting a syngenetic origin for inclusions and host. One inclusion contains quartz and biotite which are not present in the andesite host. Isotopic, major and trace element data do not support a direct fractional crystallization relationship between basalt and andesite. Separate mantle-derived magmatic pulses are apparently involved in the eruptions.