Feldspar Relations in Icelandic Alkalic Rhyolites

Haraldur Sigurdsson
Dept. of Geology, The University of Durham1
1Present address: Seismic Research Unit, University of the West Indies, St. Augustine, Trinidad.

Summary: Alkalic rhyolites with peralkaline affinities, bearing quartz and potassic feldspar phenocrysts, are described from Iceland for the first time. Evidence from electron-probe analyses of feldspar phenocrysts indicates crystallization in or near the thermal valley of the system SiO2-Or-Ab. Icelandic acid volcanic rocks are subdivided into alkalic rhyolites, belonging to transitional and alkalic basalt lineages, and the mildly calc-alkaline rhyolites of tholeiite lineages.

Mineralogical Magazine; December 1971 v. 38; no. 296; p. 503-510; DOI: 10.1180/minmag.1971.038.296.13
© 1971, The Mineralogical Society
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