Investigating magmatic plumbing systems in an off-rift setting on Iceland with geothermobarometry and quantitative textural data

English Abstract:

The emplacement of tholeiitic magmas along two NE-SW trending rift zones is the dominant mechanism of crustal accretion on Iceland. Small volumes of transitional to alkaline magmas are also erupted through older crust in several off-rift settings, including the Snæfellsnes Peninsula in western Iceland where the basement is formed by 6-8 Ma flood basalts. In this study, we are investigating how these off-rift magmatic plumbing systems compare to those in the main rift zones, given the significant differences in crustal structure and degree of crustal extension, through application of quantitative textural analysis and mineral geothermobarometry. Our focus is Vatnafell, a sub-glacial eruptive unit (414 ± 11 ka) at the western end of the off-rift Ljósufjöll volcanic system in the Snæfellsnes volcanic zone. Samples are highly porphyritic (~14% macrocrysts), with large pheno/antecrysts (1-12 mm) of clinopyroxene, olivine, and plagioclase. Crystal size distributions for olivine and clinopyroxene both show kinked profiles, indicating two distinct populations. Glomerocrysts in which large clinopyroxene oikocrysts enclose smaller rounded olivine chadacrysts are common, and a small horizon strongly enriched in large (> 5 mm) olivine and clinopyroxene crystals was found near the base of the unit. These observations suggest incorporation of olivine gabbroic/wehrlitic cumulates by the host magma. Preliminary analyses show a bimodal composition for clinopyroxene (cores: mg# 83-88; rims/groundmass mg# 72-77), and initial calculations suggest crystallization of cores over a range of depths in the deep crust (based on variations in Al₂O₃: 4-7 wt%) More extensive analyses of mineral compositions are in progress and will be used to reconstruct the vertical geometry of an extension-limited off-rift magmatic plumbing system and allow a more detailed comparison with plumbing systems hosted by the extension-dominated main rift zones on Iceland.