Plutonic rocks of intermediate composition on Gran Canaria: the missing link of the bimodal volcanic rock suite

Analysis of abundant plutonic fragments in the voluminous Miocene rhyolitic -basaltic composite ignimbrite P1, the initial felsic cooling unit overlyin g the basaltic shield on Gran Canaria, helps to solve the problem of bimoda l volcanism. Syenogabbros dominate the plutonic suite and provide evidence that magmas of intermediate composition formed abundantly at depth but did not erupt. The entire suite of plutonic fragments comprises (1) pyroxenitic to gabbroic cumulates that directly influenced the liquid line of descent of PI magmas, (2) gabbro, syenogabbro, syenodiorite and quartz-syenite frag ments showing moderate degrees of hydrothermal alteration (chloritization, partial melting), and (3) diverse xenoliths differing significantly in bulk -rock composition and texture from the erupted P1 magmas. The compositions of plutonic series (2) overlap with both bulk-rock and mineral compositions of the erupted magmas, whereas series (1) rocks show lower alkali and sili ca contents. Compositional variations among the plutonic rocks are compatib le with fractional crystallization as the major petrogenetic process, local ly modified by magma mixing and selective element contamination. Variations in trace element concentrations of the plutonic rocks, however, are inferr ed to be the result of evolved interstitial melts penetrating into, or drai ning out of, crystalline mushes during slow intratelluric solidification. T he presence of F-rich amphibole suggests that crystalline mushes were invad ed by F-bearing fluids. Overall compositional similarity, and the fact that selective contamination effects are similar, support the interpretation th at the series (2) plutonic fragments represent solidified portions of the m agmatic system that ultimately produced the erupted P1 magmas. This system extended from the zone of underplating at the mantle-crust boundary through the lower and into the upper crust of Gran Canaria and was roughly vertica lly zoned in composition as shown by geobarometric calculations.