Petrogenesis of alkaline basalts at the tip of a propagating rift: Evidence from the Heimaey volcanic centre, south Iceland

Heimaey is the volcanic centre in the Vestmannaeyjar Volcanic System, situated at the tip of Iceland's propagating Eastern Volcanic Zone (EVZ). The Heimaey lavas are slightly alkaline, ne-normative, basalts with plagioclase, olivine and Ti-magnetite as phenocryst phases. Inversion modelling using REE suggests that a primary magma is generated by fractional melting over a depth interval of 100-65 km beneath Heimaey of a source similar to that of MORB. The primary magma fractionated 31% olivine and clinopyroxene en route to ponding at the base of the crust (30-35 km depth). Tectonic events as a result of the extensional stress exerted by the southwards propagating EVZ causes magmas to be emplaced into different levels of the crust where they evolve by fractional crystallization independently of each other (i.e. polybaric fractionation). During residence in a parental magma chamber at the mantle/crust boundary the fractionating assemblage is dominated by olivine and clinopyroxene, whereas olivine and plagioclase dominates the fractionating assemblage during residence in crustal magma chambers. The most evolved magma composition can be related to a parental Heimaey melt by 73% fractional crystallization of predominantly plagioclase, clinopyroxene and olivine. The residence times in crustal magma chambers are short as indicated by the absence of equilibrium phenocryst assemblages (e.g. lack of cpx-phenocrysts). Oxygen and radiogenic isotopes suggests that no crustal contamination occurred, and that the role of magma mixing is insignificant in the evolution of the Heimaey lavas. The polybaric evolution of the Heimaey lavas in small, isolated, magma chambers at different levels in the crust fits well with a southward propagation of the EVZ, with the rift-tip currently located beneath the Vestmannaeyjar Volcanic System.