Origin of Exceptionally Abundant Phonolites on Ua Pou Island (Marquesas, French Polynesia): Partial Melting of Basanites Followed by Crustal Contamination

Abstract

On the basis of the first systematic mapping of Ua Pou, long known for its exceptionally abundant phonolites, we estimate that these rocks cover 65% of the surface of the island whereas mafic lavas cover 27% and intermediate ones 8%. The silica-undersaturated suite was erupted in a restricted time span (2·9–2·35 Myr), following the emplacement of tholeiites derived from a young HIMU-type source at c. 4 Ma. Primitive basanites, derived from a heterogeneous mantle source with a dominant EM II + HIMU signature, represent likely parental magmas. The series is characterized by a Daly gap defined by a lack of phonotephrites. We consider that the most likely model for the origin of evolved lavas is partial melting at depth of primitive basanites, leaving an amphibole-rich residuum and producing tephriphonolitic magmas. These tephriphonolitic magmas may have evolved by closed-system fractional crystallization towards Group A phonolites. Three other groups of phonolites could have been derived from tephriphonolitic magmas by open-system fractional crystallization processes, characterized respectively by seawater contamination (Group B), assimilation of nepheline syenite-type materials (Group C) and extreme fractionation coupled with assimilation of the underlying oceanic crust (Group D). The prominence of evolved lavas is a consequence of their origin from partial melting of mafic precursors followed by crustal contamination.