Magmatic evolution of the Heiðarsporður ridge, NE-Iceland

The Heiðarsporður ridge, located just south of the Krafla central volcano in the northern rift-zone of Iceland, was formed in several small eruptions in the early Holocene. The rock suite of Heiðarsporður is quite different from that of Krafla and other central volcanoes in the northern rift zone. It consists of an almost continuous series of olivine tholeiites, basaltic icelandites, icelandites and a dacite in addition to small amounts of Fe-rich tholeiites which are similar to rocks from Krafla. The occurrence of silicic rocks in the Heiðarsporður ridge is rather anomalous because usually silicic rocks in Iceland are associated with central volcanoes. This paper presents the petrography and mineral chemistry of the Heiðarsporður rock suite together with major and trace element geochemistry and oxygen isotope ratios. The intermediate and silicic rocks of Heiðarsporður are characterized by relatively high concentrations of CaO and Al2O3 coupled with relatively low concentrations of FeO, compared with evolved rocks elsewhere in the rift zones of Iceland. Furthermore, ∂18O values are higher in the evolved rocks of Heiðarsporður. The olivine tholeiites are of a very common regional type and represent mantle-derived magma. The Fe-rich tholeiites which only occur in the northern end of Heiðarsporður are derived from the Krafla central volcano. The basaltic icelandites represent hybrids of olivine tholeiites and icelandites. The icelandites and the dacite are proposed to have formed by near-solidus differentiation at a deeper level in the crust than silicic rocks in the nearby Krafla central volcano. This interpretation is supported by petrogenetic modelling using the MELTS software. The melts were extracted from the source by crustal deformation, most likely related to concurrent activity in the Krafla volcanic system.