Petrogenesis of the basalt-andesite-dacite association of Grenada, Lesser Antilles island arc, revisited

The basalt-andesite-dacite association of Grenada is produced by fractional crystallization of primary magmas that contain about 15 weight percent MgO and 1-2 weight percent H2O. The compositions of primary magmas indicate that they were last in equilibrium with the mantle at depths greater than about 65 km, indicating that they must ascend rapidly after they segregate from residual mantle. It is inferred that mantle wedge diapirism typically leads to moderately large degrees of batch partial melting in the formation of primary are magmas (similar to 10-30%). The trace-element geochemical diversity of primary magmas mainly reflects variable degrees of source-region enrichment by fluids derived from the subducted slab. The degree to which source regions are enriched is likely related to mantle wedge dynamics. Water pressure suppresses plagioclase crystallization in derivative magmas, leading to development of the calc-alkaline, rather than tholeiitic, differentiation trend. Amphibole crystallization typically occurs only after melt MgO content decreases below about 4 weight percent. The island-are crust acts as a density filter. As are crust thickens, parental basaltic magmas will have a greater tendency to stagnate at mid-crustal levels. The observation that basalts predominate in young island arcs while andesites and dacites predominate in more mature arcs is explained by this hydrostatic control: the thicker the are crust, the more magmas must differentiate by crystal segregation to become buoyant enough to erupt at the surface.