Origin of alkali basalts in Iceland: A plate tectonic model
Authors: Steinthorsson Sigurdur , Oskarsson Niels , Sigvaldason Gudmundur E. 
Published: April 2008

Alkalic rocks appear in Iceland in two off‐rift areas, the Vestmann Islands (VE) off South Iceland and the Snaefellsnes (SNS) volcanic belt in West Iceland. In South Iceland a transitional rock suite forms a bridge, both chemically and geographically, between the alkalic rocks in the south and the rift zone tholeiites to the north. The two alkalic suites are similar in many ways; however, the SNS suite has a greater compositional range, it is slightly more potassic, and it has higher 87Sr/86Sr and Ce/Sm ratios. Both the alkalic suites and the tholeiites of the rift zones appear to have evolved in equilibrium with olivine, plagioclase, and augite at upper lithospheric and crustal pressures; augite is a common phenocryst in the SNS rocks and in the transitional basalts of South Iceland but not in the VE basalts. The distinguishing features of the alkalic suites are the evolved minor and trace element chemistry of late liquid fractions and, in the case of Snaefellsnes, relatively high Sr isotopic ratios. The combined evidence from mineralogical, chemical, isotopic, and volcano‐tectonic criteria points toward the existence of a crustal ne‐normative magma component giving rise to the alkalic suites in Iceland, in contrast to the generally accepted mode of origin for oceanic alkaline volcanism. The Icelandic, and oceanic, crust is shown to be mineralogically and chemically differentiated. Interaction is favored by the thickness and tectonics of the Icelandic crust between mantle‐derived magmas and the deepest crustal layers which are a potential source of ne‐normative liquids derived from the breakdown of alkalic amphibole of crustal origin. Conversely, the upper crustal layers are the source of anomalously high Sr isotope ratios owing to high Rb/Sr ratios in silicic volcanic centers. Resulting from the westerly drift of the North Atlantic plate system relative to the Iceland hotspot, the rift system has been successively relocated to the east during the geologic history of Iceland. While developing, the new rifts ‘propagate’ through older, chemically and mineralogically stratified crust, giving rise to alkaline volcanism at the leading tip, followed by FeTi basalts and later by tholeiitic rocks. This distinctive temporal and spatial spectrum of compositions results from the interaction of primary mantle‐derived ocean tholeiite melt with the stratified crust. Conversely, the Snaefellsnes belt constitutes a volcanic outlier in the western plate, a dying remnant of the relocated rift system. The volcanism is of deep crustal origin: deep as shown by the lack of hydrothermal activity in the area and crustal as shown by Sr isotopes. A reinterpretation along similar lines for Hawaii and the Canary Islands shows the initial alkaline phase of intraplate plume activity (Loihi, Lanzarote) to be technically and petrochemically closely resemblant of the tip of a propagating rift, whereas the final transitional to alkaline rock suites of these islands resemble the plate‐trapped volcanic centers of Snaefellsnes. Oceanic alkaline volcanism in general is thus interpreted in terms of the interaction of primary mantle‐derived tholeiitic magmas and the oceanic crust.