Feldspar relations in Icelandic alkalic rhyolites

SUMMARY. Alkalic rhyolites with peralkaline affinities, bearing quartz and potassic feldspar pheno-crysts, 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 nd alkalic basalt lineages, and the mildly calc-alkaline rhyolites of tholeiite lineages. PUBLISHED chemical and mineralogical data on some acid rocks from eastern Iceland have provided good documentation f a suite of acid volcanic rocks typified by a high silica/alkalis ratio (greater than 9), an affinity with calc-alkaline acid rocks, and the presence of plagioclase phenocrysts as the salic, and often the only pheno-cryst phase (Carmichael, 1962, 1963). Their association with tholeiitic basalt lavas has been pointed out by Carmichael (I963) who suggested that these one-feldspar rhyolites were typical products of the fractionation of a tholeiitic magma. For some time now, these rhyolites have come to be regarded as the typical Icelandic acid magma, but the discovery of alkalic acid lavas in western and southern Iceland has shown the compositional range of Icelandic acid volcanics to be much greater tha