EVIDENCE OF HYDROUS DIFFERENTIATION AND CRYSTAL ACCUMULATION IN THE LOW-MGO, HIGH-AL2O3 LAKE BASALT FROM MEDICINE LAKE VOLCANO, CALIFORNIA

The late Pleistocene Lake Basalt of Medicine Lake volcano, California is comprised of variably porphyritic basalt and basaltic andesite flow s and scoria. These eruptives are similar in composition and phenocrys t abundance to the low-MgO, high-Al2O3 mafic magmas common in converge nt margin settings. The petrogenesis of the magmas that produced the L ake Basalt has been inferred from field relations, melting experiments and subsequent major and trace element modeling. Their formation invo lved both hydrous differentiation and plagioclase accumulation and thu s the Lake Basalt can be used to constrain the relative contributions of these processes to the production of high-Al2O3 arc basalt. Phenocr yst-poor lavas of the Lake Basalt formed by hydrous differentiation; t heir compositions and observed phenocrysts were reproduced in 1 kbar, H2O-saturated melting experiments. Anorthite-rich plagioclase composit ions of the lavas of the Lake Basalt necessitate crystallization from melts with between 4 and 6 wt% dissolved H2O. Phenocryst-rich lavas of the Lake Basalt, with 18 modal% phenocrysts and greater, formed by pl a gioclase accumulation in magmas similar to the phenocryst-poor lavas . This interpretation is supported by the depleted incompatible elemen t abundances and enriched Sr/Zr ratio of the more porphyritic lavas re lative to the phenocryst-poor lavas. We model the formation of the Lak e Basalt as a two-stage process that combines a differentiation model and a plagioclase accumulation model. Stage one involved hydrous fract ionation, granitic assimilation and mixing with undifferentiated paren t magma. This process generated lavas with up to 19.2 wt% Al2O3 and 7 modal% phenocrysts. In stage two, plagioclase accumulated in these liq uids and produced more aluminous and porphyritic lavas with up to 21.8 wt% Al2O3 and 33 modal% phenocrysts.