A PETROGRAPHIC AND COMPOSITIONAL STUDY OF PLAGIOCLASE IN THE ALKALINE OFF-RIFT BÚÐAHRAUN LAVA FLOW, SNÆFELLSNESS, ICELAND

The young off-axis magmatism on the Snaefellsnes Peninsula in western Iceland erupts in a region of limited extension, > 100 km from the main rift zones. It is not clear how this distinct tectonic setting affects how melts are stored and transported to the surface. Mineral chemistry and textural features allow details of the “magmatic plumbing” to be inferred, which can be compared to those of the rift zone. Budahraun is a post-glacial (< 12 kyr) cinder cone eruption on Snaefellsnes that formed a primitive alkaline basalt lava with a macrocryst assemblage of olivine, clinopyroxene, and plagioclase. Crystal size distributions for olivine and clinopyroxene show kinked trends that indicate open system processes and macrocryst accumulation. Compositional data show two distinct macrocryst populations for both olivine (Fo% 78 & Fo% 86) and clinopyroxene (green high Mg# & black low Mg#). This highlights the complex history of macrocrysts present in the flow that likely resulted from successive accumulation of distinct crystal mushes by the ascending magma. In contrast, there appears to be just a single compositional population of plagioclase macrocrysts, which have cores of An% 82. The plagioclase macrocrysts are normally zoned with more evolved rims similar in composition to groundmass crystals (An% 74). The relationships between the various macrocryst compositional types is unclear at present. Glomerocrysts of plagioclase ± olivine ± clinopyroxene are present throughout the flow. Work is in progress to better characterize the compositional assemblages present in a representative number of glomerocrysts, as analysis of co-existing phases should help to define crystal associations and their ultimate origins. These data, coupled with depth constraints from clinopyroxene geobarometry, should lead to more robust models for the origins of the diverse crystal macrocryst cargo present in the Budahraun flow.