Petrogenetic evolution of the torfajökull volcanic complex, Iceland I. relationship between the magma types

The Torfajökull volcano, Iceland's largest silicic centre, is situated close to the junction of the active, southerly propagating Eastern Rift Zone and the South Eastern Zone, an older crustal segment. This paper provides major, trace, and some Sr isotope data on postglacial (<10000 y) rocks, i.e., tholeiitic magmas of the Eastern Rift Zone and transitional basalts, icelandites, and rhyolites of the Torfajökull centre, and assesses the relationships between the magma types in terms of the development of the Icelandic crust.Tholeiitic magmas from the Eastern Rift Zone are LILE-enriched relative to MORB. They have undergone extensive olivine-plagioclase-clinopyroxene fractionation at low pressures. Compared with the tholeiites, Torfajökull transitional basalts show LILE/HFS enrichment and higher (⁸⁷Sr/⁸⁶Sr)₁ ratios. They define several magmatic lineages and have equilibrated over a wide range of pressures. Both basalt types were derived by very small degrees of partial melting of compositionally similar mantle sources, the main difference being that the tholeiites were generated in the spinel Iherzolite, and the transitional basalts in the garnet lherzolite, stability fields, a conclusion previously reached by Meyer et al. (1985). The mantle sources may have contained LILE-enriched streaks.Low-pressure differentiation of Torfajökull transitional basalt produced an iceiandite to sub-alkaline rhyolite sequence by crystal fractionation, the rhyolites representing >90% crystallization of parental basalts. The rhyolites were emplaced as nine separate lava fields, formed during 11 eruptive episodes. The compositional range within each field is limited, and, although similar, the ranges define several magmatic lineages. Continued fractionation of plagioclase-alkali feldspar-clinopyroxene-magnetite-apatite-zircon assemblages generated peralkaline rhyolites in certain post-glacial eruptions. Chemical variations in the deposits from the Hrafntinnusker peralkaline eruption were related predominantly to alkali feldspar fractionation, and the melts were erupted from a zoned magma chamber. All postglacial volcanic rocks at Torfajokull have been mantle derived and thus represent new additions to the Icelandic crust. © 1990 Oxford University Press.