The mode of emplacement of Neogene flood basalts in Eastern Iceland:The plagioclase ultraphyric basalts in the Grænavatn group

• Published in: Journal of Volcanology and Geothermal Research
• Main Authors: V. Óskarsson, Birgir, B. Andersen, Christina, S. Riishuus, Morten, Sørensen, Erik Vest, Tegner, Christian
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: Compound flow; Flood basalt; Neogene; Non-Newtonian; Plagioclase ultraphyric basalt; Simple flow; Grænavatn; Iceland
• Online Access: https://pure.au.dk/portal/da/publications/the-mode-of-emplacement-of-neogene-flood-basalts-in-eastern-iceland(d0d3765d-6d0d-4605-814f-e844239c4446).html; https://doi.org/10.1016/j.jvolgeores.2017.01.006

Plagioclase ultraphyric basalt lava with high fraction of solids have a mode of emplacement that is poorly understood. In this study we conduct detailed mapping of a PUB group in eastern Iceland, namely the Grænavatn group, and assess the group architecture, flow morphology and internal structure with additional constraints from petrography, petrology and crystal size distribution, to derive information on emplacement dynamics of plagioclase ultraphyric basalts. We also derive information on the plumbing system of the group with reference to the source of the macrocysts. The group is exposed in steep glacially carved fjords and can be traced for more than 70km along strike. The flows have mixed architecture of simple and compound flows. Individual flow lobes have thicknesses in the range of 1-24m and many reach widths and lengths exceeding 1000m. The flows vary from rubbly to slabby pahoehoe, but are predominantly of pahoehoe type. The aspect ratio of the group and the nature of the flows indicate fissure-fed eruptions. The plagioclase macrocrysts (5-30mm) are An-rich, exhibit bimodal size distribution and the modal proportions within the group varies from 15-40%. Clinopyroxene macrocrysts are also present ranging from 1-6%. The lowermost flow is thickest and carries the greatest crystal cargo load. The morphology of the lava flows suggests low viscous behavior, at odds with the high crystal content. The very calcic plagioclase macrocrysts (An80-85) are in disequilibrium with the groundmass and plagioclase microlaths therein (An50-70), meaning that the crystal-laden magmas quickly ascended from deeper crustal levels to the surface. The flows with highest crystal content may have maintained high temperatures by heat exchange with the primitive macrocrysts in the flows and developed non-Newtonian behavior such as shear thinning. Such conditions would have enabled the flows to advance rapidly during episodes with high effusion rates forming the simple flows, and subsequently maintained by insulated lobe-by-lobe ...