MINERALOGICAL AND TEXTURAL INVESTIGATIONS OF AN OFF-AXIS, SUB-GLACIAL, ALKALINE VOLCANIC FORMATION ON SNAEFELLSNES PENINSULA IN ICELAND

Iceland is situated at the junction between the Greenland-Faeroe rise and the Mid-Atlantic spreading ridge, and is associated with plume driven mantle upwelling. Crustal accretion on Iceland is dominated by emplacement of tholeiitic magmas at two NE-SW trending rift zones, the onshore continuation of the mid-oceanic spreading ridge. The Snaefellsnes Peninsula in western Iceland is built on older Neogene flood basalts (6-8 Ma) belonging to a fossil rift zone, but now hosts an off-axis volcanic zone that erupts small volumes of alkaline magmas. We set out to explore how off-rift magmatic plumbing systems may differ structurally from systems in main rift zones, and the petrologic implications exerted by plumbing system structure on magma differentiation. Published major element data suggest differentiation at higher pressures, indicating that shallow-level chambers might be absent or less well-developed than in the main rifts, perhaps due to differences in upper crustal structure: thick piles of Tertiary (> 7 Ma) flood basalts (Snaefellsnes) vs. interbedded Plio-Pleistocene subaerial lavas and sub-glacial hyaloclastite deposits (main rifts).

This study focuses on Vatnafell, a sub-glacial eruptive unit (414 ± 11 ka), situated at the western end of the Ljósufjöll volcanic system in the Snaefellsnes volcanic zone. Samples are highly porphyritic (~14% phenocrysts), with large phenocrysts (1-10 mm) of clinopyroxene (7%), olivine (5%), and plagioclase (2%). Phenocrysts are subhedral to anhedral and often form glomerocrysts in which large clinopyroxene oikocrysts often enclose smaller rounded olivine chadacrysts, suggestive of incorporation of olivine gabbroic/wehrlitic cumulates by the host magma. Our initial focus is to quantitatively document crystal textures at different scales (glomerocryst mineralogy and texture; crystal size distributions), to be followed by detailed major and trace element chemistry. These data will be applied to calculations for reconstruction of magma storage and ascent in the Vatnafell plumbing system, and the origin of its crystal cargo.