Contributions to Mineralogy and Petrology

, Volume 158, Issue 2, pp 223–244 | Cite as

Magma chamber processes in central volcanic systems of Iceland: constraints from layered gabbro of the Austurhorn intrusive complex

  • Sigurjon B. Thorarinsson
  • Christian Tegner
Original Paper


New field work and petrological investigations of the largest gabbro outcrop in Iceland, the Hvalnesfjall gabbro of the 6–7 Ma Austurhorn intrusive complex, have established a stratigraphic sequence exceeding 800 m composed of at least 8 macrorhythmic units. The bases of the macrorhythmic units are composed of 2–10 m thick melanocratic layers rich in clinopyroxene and sometimes olivine, relative to the thicker overlying leucocratic oxide gabbros. While the overall compositional variation is limited (Mg# clinopyroxene 72–84; An% plagioclase 56–85), the melanocratic bases display spikes in Mg# and Cr2O3 of clinopyroxene and magnetite indicative of magma replenishment. Some macrorhythmic units show mineral trends indicative of up-section fractional crystallisation over up to 100 m, whereas others show little variation. Two populations of plagioclase crystals (large, An-rich and small, less An-rich) indicate that the recharge magma carried plagioclase xenocrysts (high An-type). The lack of evolved gabbros suggests formation in a dynamic magma chamber with frequent recharge, tapping and fractionation. Modelling of these compositional trends shows that the parent magma was similar to known transitional olivine basalts from Iceland that had undergone about 20% crystallisation of olivine, plagioclase and clinopyroxene and that the macrorhythmic units formed from thin magma layers not exceeding 200–300 m. Such a “mushy” magma chamber is akin to volcanic plumbing systems in settings of high magma supply rate including the mid-ocean ridges and present-day magma chambers over the Iceland mantle plume. The Austurhorn central volcano likely formed in an off-rift flank zone proximal to the Iceland mantle plume during a major rift relocation.


Iceland Austurhorn Layered gabbro Central volcano Magma chamber RTF processes 



This work was financed by the Science Faculty, University of Aarhus, and the Nordic Volcanological Center, University of Iceland. Additional financial support was provided by grants from the Danish Natural Science Research Council (to CT). Erik Sturkell, Niels Óskarsson and Karl Grönvold are thanked for constructive discussions and helpful input. Sidsel Grundvig, Ingrid Aaes and Jette Villesen are thanked for help with the electron microprobe and laboratory work. Peter L. Thomadsen, Peter Momme, Jakob K. Jakobsen, Martin B. Klausen and Anna Maria Nielsen are thanked for assistance and company in the field. An early version of the manuscript was considerably improved as a result of constructive review and comments by J.R. Wilson. Two anonymous reviewers are greatly thanked for their constructive criticism.

Supplementary material

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© Springer-Verlag 2009

Authors and Affiliations

  1. 1.Department of Earth SciencesUniversity of AarhusÅrhus CDenmark
  2. 2.Nordic Volcanological CenterUniversity of IcelandReykjavikIceland
  3. 3.Department of Geography and GeologyUniversity of CopenhagenKøbenhavn KDenmark

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