Notes on the chemistry of ferromagnesian phenocrysts from selected volcanic rocks, Central Volcanic Region

Abstract

Thirty-three new partial or complete chemical analyses are presented for pyroxene, amphibole, and biotite phenocrysts separated from three andesitic, three dacitic and thirteen rhyolitic volcanic rocks. The orthopyroxenes are mostly of hypersthene composition, with the rhyolitic hypersthenes generally being slightly more Fe-rich compared to the andesitic types. No detailed correlation, however, is evident between the Fe/Mg ratio of the rhyolitic orthopyroxenes and their parent rocks or co-existing groundmasses. A correlation between Ca content and the type of co-existing ferromagnesian assemblage was found. The clinopyroxenes are augites, and occur most commonly in the andesites and dacites. Two amphiboles are found, a calcic amphibole and cummingtonite. The former range from a tschermakitic hornblende (andesitic) to magnesio-hornblende in the rhyolites. One dacitic ferro-hornblende was found. The data illustrate the increasing substitution of Al and increasing Na in the andesitic hornblende. A close correlation of the Niggli mg ratio, and also MnO, exists between the analysed co-existing orthopyroxenes and hornblendes. Again, however, no detailed correlation between Fe/Mg ratio of hornblende and parent rock (or co-existing groundmass) was found to exist.