Petrology of the Western Reykjanes Peninsula, Iceland

Abstract

The active tholeiitic volcanic zone of the Reykjanes Peninsula consists of five volcanic fissure swarms, the two westernmost of which are the subject of this petrological study. The recent (less than 12,000 years) extrusives of the swarms group morphologically and petrographically into small picrite basalt lava shields, large olivine tholeiite lava shields and tholeiite fissure lavas; formed in that chronological succession. The picrite basalts exhibit a primitive mineralogy with chromite, olivine (Fo 89) and plagioclase (An 90) as phenocrysts and may represent a primary liquid from the mantle. Simultaneous crystallization of olivine, plagioclase and augite to form glomerocrysts in the fissure lavas indicate low pressure cotectic crystallization conditions. Twenty-eight new major element chemical analyses of the lavas are presented. They are generally characterized by a low content of alkalies and high CaO. The lavas constitute two main suites, a lava shield suite and a fissure lava suite. There is a positive correlation between the volume of individual lavas and the content of incompatible elements of the lavas within each group. Likewise there is an overall chemical trend through time demonstrated, for example, by a rise in K2O from about 0.02 per cent to 0.24 per cent during the last, approximately, 12,000 years. There is an apparent chemical zoning within each volcanic swarm such that the most evolved and youngest lavas are found in the central axial area of the swarm. This central area is also characterized by graben subsidence, high magnetic anomalies and high temperature thermal areas, all indicative of shallow magma reservoir(s). In spite of indications of fractional crystallization in the evolution of the olivine tholeiites and tholeiites, some other processes must be sought to explain the volume chemistry relations. Cyclic volcanic activity is tentatively suggested to explain the observed regular temporal variations within the swarm, each cycle starting with the formation of picrite basalts.