Two hundred chemical analyses made of igneous rocks from seven of the Cape Verde Islands and from Fernando de Noroñha show that these Atlantic islands are predominantly composed of plagioclase-free limburgite grading into plagioclase-bearing basanite. Minor amounts of nepheline latite, nepheline monzonite, and nepheline syenite occur as intrusive stocks together with flows and plugs of phonolite. The high average Sr, Ti, P, total Na + K, and normative Ne contents distinguish the series from the ankaramite-alkali basalt-trachyte suites found in most oceanic volcanic islands.

Nepheline latite and nepheline syenite rocks of the Cape Verde Islands and Fernando de Noroñha could have formed by the subtraction of a highly mafic olivine-bearing kaersutitic clinopyroxenite from a limburgitic magma. Comparison with published results from other oceanic islands and the sea floor shows that the Cape Verde Islands and Fernando de Noroñha are the most alkalic parts of the oceanic crust. A plot of Sr versus TiO2 for oceanic and sea-floor islands clearly delineates the extreme character of these island groups. This representation of magma type is proposed as being of greater value than the conventional alkali versus silica diagram for oceanic island comparisons.

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