Abstract
The dominant petrochemical evolution of St. Helena volcanics is from undersaturated alkali olivine basalt through nepheline normative trachybasalt and trachyandesite to trachyte and phonolite. A unique lava is described that shows extreme iron enrichment and internal fractionation towards a granitic mesostasis, and hence is quite unlike previously described volcanic rocks from this or any other Atlantic island. The unique chemistry is also reflected in the remarkable mineral assemblage andesine-titanomagnetite-ilmenite-ferrohypersthene-iron cordierite. Contamination is surmised to have played some role in the history of the lava, but the details of its evolution remain enigmatic.
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