| Summary: | Alkalic rocks are rare in Iceland compared to the tholeiitic rock suite but are identified in two tectonically different volcanic areas which both are active at present. The Snaefellsnes volcanic zone in western Iceland is intraplate and non-rifting and consists of three volcanic systems, Snaefellsjökull, Lysuskard and Ljosufjöll. The Vestmannaeyjar system, by contrast, lies at the tip of a propagating rift zone in southern Iceland. Alkalic rocks on the Snaefellsnes peninsula sit unconformably on a Tertiary tholeiitic base which was formed as part of a rift system that was active from at least 16 up to about 6-7 million years ago when it was relocated eastwards because of the NW motion of the rift axis relative to the Iceland plume. The present activity may be interpreted as a leaky transform fault or it may be the result of magma escaping through deep seated fractures which were formed in the extinct Snaefellsnes volcanic zone. The volcanic systems on the Snaefellsnes peninsula form a narrow volcanic zone which is approximately 120 km in length from west to east. The Snaefellsjökull system consists of a central volcano (Snaefellsjökull) surrounded by numerous craters and cinder cones, and it has been active for at least 840 kyrs. More than 190 kyrs elapsed before volcanic rocks with fairly evolved compositions first appeared and more than 300 kyrs before the first hawaiites were erupted. The volcanic products are grouped as postglacial lavas and tephras, interglacial lavas, subglacial rocks, subglacial domes and xenoliths. Phenocryst content of most of the samples is less than 20% of the rock by volume and on average it is less than 10%. In general plagioclase is the dominant phenocryst phase in the Snaefellsjökull rocks, followed by olivine and then clinopyroxene and opaque oxides. However, the phenocryst assemblages in the Snaefellsjökull rocks are very variable and irregular. Phenocrysts frequently show complex morphologies and zoning patterns which can be taken as evidence of disequilibrium conditions ...
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