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

摘要

New field work and petrological investigations of the largest gabbro outcrop in Iceland, the Hvalnesfjall gabbro of the 6-7 Ma Austurhom 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 lim-ited (Mg# clinopyroxene 72-84; An% plagioclase 56-85), the melanocratic bases display spikes in Mg# and Cr_2O_3 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 plagio-clase xenocrysts (high An-type). The lack of evolved gabbros suggests formation in a dynamic magma chamber with frequent recharge, tapping and fractionation. Model-ling 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 Austurhom central volcano likely formed in an off-rift flank zone proximal to the Iceland mantle, plume during a major rift relocation.