| Summary: | A number of lava flows in Iceland contain large plagioclase phenocrysts and some contain abundant olivines (picrites). The origin of these macrocrysts and their relationship to their host magmas are of fundamental petrologic importance. Two separate pillow/hyaloclastite formations, Maelifell and Midfell, in the Hengill Central Volcano in SW Iceland contain macrocrysts (large phenocrysts) of olivine, plagioclase and clinopyroxene and gabbro nodules with the same mineralogy. The composition of the macrophenocrysts and the gabbro minerals are similar and within a relatively narrow range. These are embedded in a primitive basalt matrix, best represented in pillow-rim glasses. Closely adjacent is the highly plagioclase porphyritic formation of Sandfell also included in the present study. Strontium isotope ratios, 87Sr/86Sr, are easily measurable in the glasses and there is also enough Sr in the plagioclase macrophenocrysts so that 87Sr/86Sr can be measured with the same precision in individual crystals. Sr isotopic ratios were measured in two sample sets, macrocrysts and plagioclase from the gabbros. The results were compared with 87Sr/86Sr ratios from the pillow rim glasses, previously published by Magna et al. (2011). For Sandfell only plagioclase was included as it has no nodules. The long half-life of 87Rb, parental to 87Sr, and the low abundance of Rb in Icelandic basalts means that any differences in 87Sr/86Sr detected should be a long term one and not related to or caused by any recent petrologic processes but reflecting different mantle differences. The resulting data show that the glasses are significantly different from both the macrocrysts and gabbro plagioclases. The plagioclase macrocysts and the gabbro show no significant difference. The main conclusion is therefore that the macrocrysts and host magma are genetically unrelated. The 87Sr/86Sr ratios of the xenoliths and megacrysts resemble the average crustal ratios of the Reykjanes Peninsula (87Sr/86Sr = 0.703099-0.703164). Their origins are most likely ...
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