Abstract
True rhyolites are found in two contrasting occurrences in Central America: in recent obsidian domes in the basalt-rhyolite association of southeastern Guatemala, and in late Tertiary ignimbrites in Honduras and adjacent parts of Nicaragua. Both are on the inner side of the main volcanic axis in a region that is visibly underlain by older metamorphic and plutonic rocks. They have not been found in southern Central America where the basement series consists only of older volcanic rocks, eugeosynclinal sediments, and peridotite.
Rhyolite obsidian has erupted alternately with basalts in the Obrajuelo Complex of southeastern Guatemala. Cumulate rocks and moderate compositional variations in the basalts provide evidence of limited crystal fractionation, but abundant venocrysts and partially fused xenoliths indicate that partial melting of the plutonic and metamorphic basement may also have been important.
Extremely siliceous rhyolite ignimbrites with a volume of several thousands of cubic kilometers closely resemble the rhyolite obsidians in their major element compositions, but in contrast to the totally glassy obsidians they contain abundant phenocrysts of quartz and alkali feldspar. A few ignimbrites contain inclusions of partially melted basement rocks. The basalts with which they are associated have a limited compositional range.
Melting experiments performed on rocks of the basement series, together with chemical, isotopic, and volumetric relations, indicate that the ignimbrites are products of partial fusion of the basement series, while obsidians of the Obrajuelo Complex have differentiated from more basic magmas by a process other than crystal fractionation.
Similar content being viewed by others
References
Carmichael, I. S. E., 1963 —The crystallization of feldspar in volcanic acid liquids. Quart. Jour., Geol. Soc. London, vol. 119, p. 95–131.
Williams, Howel, A. R. McBirney, andG. Dengo, 1964 —Geologic reconnaissance of southeastern Guatemala. Univ. of Calif., Publ. Geol. Sci., vol. 50, p. 1–57.
Williams, Howel andA. R. McBirney, in press,Volcanic history of Honduras. Univ. of Calif. Publ. Geol. Sci.
Author information
Authors and Affiliations
Additional information
Paper read at the IAV International Symposium on Volcanology (New Zealand), scientific session of Nov. 27, 1965.
Rights and permissions
About this article
Cite this article
McBirney, A.R., Weill, D.F. Rhyolite magmas of central America. Bull Volcanol 29, 435–446 (1966). https://doi.org/10.1007/BF02597167
Issue Date:
DOI: https://doi.org/10.1007/BF02597167