AN OVERVIEW OF THE ALKALINE OFF-AXIS MAGMATISM OF THE SNAEFELLSNES PENINSULA, ICELAND

Post-glacial (< 12 kyr) magmatism on Iceland is largely confined to well-defined rift zones, with tholeiitic magmas generated by decompression melting in response to plate separation above a hotspot. Small volumes of alkaline magmas are also erupted along the Snaefellsnes peninsula (western Iceland), although the reasons for melt generation at such large distances (50-150 km) from the main rift zones remain unclear. We have completed a systematic sampling of primitive (MgO >8 wt%) lavas from throughout the Snaefellsnes peninsula. Most flows are porphyritic with olivine ± clinopyroxene ± plagioclase macrocrysts, and often contain compositionally and texturally distinct macrocryst populations that indicate complex origins for the crystal cargo. Glass and clinopyroxene geobarometry results are broadly consistent with melt storage predominantly in the lower crust. In places, there are large differences in incompatible element ratios over short distances (e.g. western Ljosufjoll: La/Sm 2.5 to 5.2 over <15 km). A significant step in MREE/HREE enrichment occurs at 22.3°W, where flows to the east have lower Dy/Yb (1.9 ± 0.1) compared to flows to the west (Dy/Yb 2.2 ± 0.1). This boundary coincides with a transition in morphology and productivity of magmatism, from the voluminous ENE-aligned central volcanoes (Snaefellsjökull, Lysuskard, Ljosufjoll) that follow the trace of the extinct Snaefellsnes-Hunafloi rift zone west of 22.3°W, to the ESE-alighned minor isolated basalt eruptions east of 22.3°W. We speculate that melting is driven by upwelling of plume outflow mantle caused by topography on the base of the lithosphere related to this aborted rift. Several lines of evidence show that the lithology of the enriched mantle melting beneath Snaefellsnes is fertile peridotite rather than pyroxenite: olivines have similar Ni-Mn-Ca compositions to olivines in the rift zone tholeiites, and whole rock Fe/Mn and Zn/Fe values are low and similar to the rift zone tholeiites.