Abstract
Andesites of the calc-alkaline volcanic series associated with the circum-Pacific orogenic zone commonly contain crystal clots consisting essentially of plagioclase, clinopyroxene, orthopyroxene, and magnetite. It is proposed that these crystal clots represent the breakdown products of an amphibole as it enters the low-pressure environment of the upper crust. The bulk chemical composition of the clots compares favorably to that of the high-Al amphibole, pargasitic hornblende. The crystal clots support the hypothesis of the formation of andesitic magma by fractionation of early formed amphibole from a basaltic magma at total pressures less than 18 kbars and temperatures less than 1000° C.
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