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

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 ⁸⁷Sr/⁸⁶Sr ratios are 0.70343 ± 7 for the andesite and 0.70296 ± 7 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.