Crystal clots in calc-alkaline andesites as breakdown products of high-Al amphiboles

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. The origin of these clots has previously been attributed to random accumulation of phenocrysts. Some features of clot-bearing andesites from Crater Lake, Oregon, U.S.A., cannot be explained by this mechanism. First, in some andesites, certain minerals occur as phenocrysts but are not constituents of the clots, and conversely, certain minerals occurring as accessories in the clots are rarely found as phenocrysts. Second, the minerals comprising the clots occur in a fixed ratio that is significantly different than the ratio of the same minerals as phenocrysts. Crystal clots may form up to 10% by volume of the andesite, imparting a glomeroporphyritic texture to the rock. Crystal clots can be distinguished from xenoliths of similar mineralogy by the presence in the latter of abundant glass, both as interstitial material and as inclusions in the plagioclase grains, giving the plagioclase a “spongy” appearance.