Two magma types of the high-alumina basalt series of Osorno Volcano, Southern Andes(41.DEG.06'S)-plagioclase dilution effect.

Abstract

Osorno volcano is mainly composed of aphyric and porphyritic high-alumina basalts and basaltic andesites. Dacites are scarce, and not discussed in this paper. Aphyric rocks predomi. nate in the oldest units (1 to 3), which represent most of the history of the volcano (200, 000-4, 000 y). Porphyritic lavas erupted mainly during the last 4, 000 y. (unit 4, old and new parasitic cones). Compared to the porphyritic rocks, the aphyric ones are enriched in Ti, Fe, Mg, K, P, Ni, Co, V, Rb and Ba, and exhibit lower FeO*/MgO ratios. However, in spite of their age differences, both group of rocks have similar and relatively low 87Sr/86Sr ratios (average 0.70441±0.00005). The europium anomaly consistently changes with the modal percent of plagioclase phenocryst : The aphyric type enlarges negative Eu anomaly with decreasing the amount of plagioclase phenocryst and with increasing SiO2 and FeO*/MgO. This phenomenon is due to fractionation of plagioclase. The positive Eu anomaly of the porphyritic type increases with decreasing of plagioclase phenocryst but has no relation with SiO2 and FeO*/MgO. This may be ascribed to dissolution of plagioclase. The geochemical features of the porphyritic rocks, relative to the aphyric ones, suggest that their parental magma was formed by mixing a plagioclase-rich magma. Plagioclase phenocrysts in the porphyritic type are not always a cumulate phase and/or a xenocryst