Generation and equilibration of olivine tholeiites in the northern rift zone of Iceland. A petrogenetic study of the Blلfjall table mountain

The Bláfjall table mountain is located in the active northern rift zone of Iceland. It is part of an olivine tholeiite complex, erupted mostly during Weichselian times (75-10 ka). The compositions and volumes of the magmas, as well as the time span and spatial proximity of the eruptive units, imply the contemporaneous existence of large magma reservoirs at different upper mantle/lower crustal levels beneath the area. distinct geochemical and petrographical tholeiite types can be identified: (1) A small unit of ca. 0.1 km3 consists of some of the most primitive tholeiites reported from Iceland. The rocks are nearly aphyric with olivine as the only minor phenocryst phase. (2) The bulk of the complex consists of large (up to 6 km3) units of average icelandic tholeiites with olivine and occasionally plagioclase phenocrysts. (3) The youngest unit amounts to ca. 3 km3, forming the top of the Bláfjall table mountain. This is the only unit that has crystallized a pseudo-invariant assemblage of olivine, plagioclase and clinopyroxene. rock compositions of all units indicate equilibrium with mantle phases at 7–10 kbar where the largest extent of melting takes place. Liquids derived by melting over a wide range of pressure in the upper mantle accumulated and mixed in a primary reservoir located at around 4 kbar. This reservoir subsequently fed most of the eruptions in the area. Glass geochemistry implies that a secondary magma reservoir developed at 2–3 kbar, crystallizing a gabbroic three-silicate-phase assemblage of crustal affinity. Extraction from this magma reservoir was triggered by a very large eruption (6 km3) from the primary reservoir.