Abstract

The intimate association of basalt, andesite, dacite, and rhyolite within a volcanic center suggests that these rocks are genetically related. Individual lava flows that show a gradation in composition may preserve maximum evidence of the magmatic processes producing this association. One such flow of rhyolite to dacite composition, Glass Mountain in northern California, was formed by contamination of rhyolite magma as it intruded the basaltic flows of the Medicine Lake Highland shield volcano. Although dacite flows and domes commonly show less variation in composition than the Glass Mountain flow, many show similar evidence of contamination by basalt by the presence of abundant basaltic inclusions and phenocrysts and phenocryst clots from those inclusions. Similarly, many andesite flows contain rhyolitic inclusions, rhyolitic bands, and phenocrysts appropriate to rhyolite. These observations indicate that andesite and dacite are hybrid rocks that are formed when rising primary basalt and rhyolite magmas either become contaminated with the glassy debris of the volcanic pile or mix with each other directly. Linear variation in bulk composition, phenocryst assemblages of intermediate rock, and frequency distribution of lava compositions in the southern Cascade Range, Chilean Andes, Taupo volcanic zone, and Tongan Islands support this hypothesis. It appears that partial melting usually produces magma of rhyolitic and basaltic compositions and that any subsequent fractional crystallization is of limited importance.

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