gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1971.038.296.13 Feldspar Relations in Icelandic Alkalic Rhyolites Sigurdsson Haraldur Ph.D. Dept. of Geology, The University of Durham1

Present address: Seismic Research Unit, University of the West Indies, St. Augustine, Trinidad.

 12 1971 38 296 503 510 1 9 1970 Copyright © 1971, The Mineralogical Society 1971 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.