The Implications of Petit-Spot Volcanism for the Origin of Alkaline Intraplate Magmas

Abstract:

The compositions of alkaline lavas are mostly similar even though they are observed in various tectonic contexts. This similarity has been used to suggest that these rocks are all produced by deep processes. Nevertheless, the formation of petit-spot seamounts, which are interpreted as low-degree melts extracted from the base of the lithosphere in response to plate flexure, demonstrates that alkaline lavas could also be produced by shallow tectonic processes. In this presentation, petit-spot lavas will be compared to intraplate basalts to reveal the processes that control the petrogenesis of intraplate lavas.

Petit-spot lavas are characterized by an alkaline basaltic composition rich in potassium (K₂O/Na₂O>0.7). This distinguishes them from oceanic island basalts, which are characterized by a lower alkali ratio. The K-rich nature of petit-spot melts is explained either by the melting of an asthenospheric mantle domain enriched in K₂O, TiO₂ and trace elements, or by the interaction of low-degree melts extracted from the low velocity zone (LVZ) with phlogopite-rich metasomatic lithologies present in the lower part of the lithospheric mantle; metasomatic cumulates formed during an early stage of LVZ melt migration. The latter model is supported by the recent discovery of metasomatized peridotite xenoliths in petit-spot lavas which demonstrates that low degree melts, similar in composition to the melts responsible for the formation of phlogopite-rich cumulates in continental lithospheric mantle, percolate through the oceanic lithospheric mantle producing a metasomatic enrichment. The involvement of metasomatic processes in the formation of petit-spot lavas provides a link to the metasomatic lithospheric model for the origin of alkaline magmas, a model that suggests that these rocks are not produced directly from the asthenosphere, but by the melting of hydrous veins produced by the percolation and differentiation of low degree asthonospheric melts across the lithospheric mantle.