Abstract

Brokeoff volcano, a High Cascade stratovolcano located in Lassen Volcanic National Park, California, is composed primarily of andesite, with subordinate amounts of basalt, basaltic andesite, dacite, rhyodacite, and rhyolite. A geochemical study was undertaken to investigate the genetic relationship between members of the basalt-andesite-dacite-rhyolite magma series.

Both major-element and trace-element abundances vary regularly with the silica content of the lavas. Relatively small increases in K, Rb, and Ba abundances preclude derivation of most of the rhyodacites by fractional crystallization. A model is proposed for the generation of andesite by partial melting of a garnet peridotite and generation of rhyodacite by partial melting of a similar source at greater depths, followed by hornblende fractionation.

A small volume of dacites and rhyodacites have distinctly higher incompatible element abundances and were probably produced by fractional crystallization of the andesite. Andesites and dacites produced by flank eruptions are characterized by complex phenocryst assemblages and were most likely formed by mixing of andesite and rhyodacite magma. It is concluded that members of the basalt-andesite-dacite-rhyolite magma series are not related by a single process, but probably by several processes, even at a single volcanic center.

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