Contrasting magmatic cannibalism forms evolved phonolitic magmas in the Canary Islands

Volcanic sequences on ocean islands record the temporal evolution of underlying magmatic systems and provide insights into how silicic crust is produced away from convergent margins. Assimilation has often been suspected to contribute, but the detection of such a process and its evolving maturity during migration across a mantle plume is less well documented. Here we present new major and trace element and Sr-Nd-Pb-U-Th-Ra-Pa isotope data that facilitate comparison of basanite to phonolite evolution on Tenerife (Canary Islands) with that shown by published data from La Palma. On both islands, (²³⁰Th/²³⁸U) ratios decrease with differentiation from parental magmas with ²³⁰Th excess toward different, silicic contaminants in secular equilibrium. On La Palma, this is inferred to reflect assimilation of small amounts of mafic wall rock. On Tenerife, both (²³⁰Th/²³⁸U) and (²³¹Pa/²³⁵U) ratios decrease toward 1 with increasing differentiation, and this is accompanied by a subtle increase in Pb isotope ratios. At the same time, (²²⁶Ra/²³⁰Th) ratios change from >1 to <1 (a hitherto unreported magnitude). The Tenerife assimilant is thus constrained to be a partial melt of syenite formed in equilibrium with residual feldspar. The differences reflect a primarily deeper, more mafic magma system beneath La Palma during its late shield-building stage, whereas recent magmatic evolution at Tenerife occurs primarily at lower temperatures in small, shallower magma systems formed during its post–basaltic shield stage. Differentiation takes millennia or less.