COMPLEXITIES OF MELT GENERATION AND STORAGE IN AN OFF-RIFT SETTING ON THE SNAEFELLSNES PENINSULA OF ICELAND REVEALED THROUGH DETAILED STUDY OF THE BERSERKJAHRAUN VOLCANIC FIELD

Post-glacial magmatism (< 12 ka) on Iceland is dominated by eruption of tholeiitic magmas along the main rift zones that represent the site of active crustal accretion. Smaller volumes of alkali basalts have also been erupted in an off-rift setting, 50-150 km from the main rift zones, along the Snaefellsnes Peninsula. We are investigating how the differences in crustal structure and tectonic environment (limited extension, low geothermal gradient, low magmatic productivity) influence melt aggregation and crustal storage in the off-rift setting compared to the main rift zones. This study focuses on the ~4 ka Berserkjahraun volcanic field, which lies in the centre of the peninsula at the western end of the WNW-ESE-trending Ljósufjöll volcanic system, and comprises a series of four main cinder cones and their associated lava flows. Whole rock analyses of ~40 lava samples show the high-MgO nature of the lavas (9-17 wt%) and the diversity of melts being supplied from the mantle (La/Sm_N 2.2-3.3). These data are being integrated with petrographic observations and field data to constrain the temporal-compositional trends within the Berserkjahraun eruptions. The earliest inferred unit in the extreme east is both compositionally distinct (low La/Sm_N: 2.2 vs. 2.6-3.3) and also petrographically distinct in having two different plagioclase populations (sieve-textured vs. clear). Mineral and scoria glass analyses are in progress, using the new electron microprobe facility at the University of Iowa. These compositions will be used to determine the depths of crystallization, in order to reconstruct the details of magma storage and ascent beneath this off-rift zone and the origins of the crystal cargos.