MAPPING THE PLUMBING SYSTEM OF AN OFF-RIFT VOLCANIC SETTING ON ICELAND USING GEOTHERMOBAROMETRY AND QUANTITATIVE TEXTURAL ANALYSES

Crustal accretion on Iceland is dominated by the emplacement of tholeiitic magmas along two NE-SW trending rift zones. A few peripheral regions erupt transitional to alkaline magmas not associated with rifting, through older (6-8 Ma) basement crust. The best example is Snaefellsnes Peninsula, located > 100 km from the nearest rift zone in western Iceland, which is the largest of these off-rift volcanic zones. The focus of this study is to investigate the differences between these off-rift plumbing systems & the main rift zones, given the differences in crustal extension and crustal structure, using geothermobarometry and quantitative textural analyses. A highly porphyritic (~14% phenocrysts) sub-glacial eruptive unit named Vatnafell (414 ± 11 ka) was used for all analyses. The large (1-13mm) phenocrysts of clinopyroxene, olivine, and plagioclase, as well as abundant glacially quenched glass, made this material ideal for crystal size distributions (CSDs) and pyroxene-glass equilibrium based barometric calculations. CSD plots for CPX and Ol, show kinked profiles indicating two distinct populations. Glomerocrysts composed of smaller rounded Ol chadacrysts enclosed in larger CPX oikocrysts, as well as a field observation of a horizon strongly enriched in large (> 5 mm) CPX & Ol crystals near the base of the unit, all suggest the incorporation of an olivine gabbroic/wehrlitic cumulate into the host magma. Reconnaissance electron microprobe analyses at the University of Iowa, show a bimodal composition for CPX (cores: mg# 83-88; rims/groundmass mg# 72-77), and calculations suggest core crystallization over a range of depths in the deep crust (based on variations in Al₂O₃: 4-7%) with little to no crystallization in the mid to shallow crust. Ongoing microprobe analyses will allow for a more complete picture of magma storage and transport in this extension-limited zone, and provide a comparison of plumbing systems between rift & off-rift volcanic zones on Iceland.