PETROGENESIS OF THE BASALT-ANDESITE-DACITE ASSOCIATION OF GRENADA, LESSER-ANTILLES ISLAND-ARC, REVISITED

The basalt-andesite-dacite association of Grenada is produced by fract ional crystallization of primary magmas that contain about 15 weight p ercent MgO and 1-2 weight percent H2O. The compositions of primary mag mas indicate that they were last in equilibrium with the mantle at dep ths 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 deriv ed from the subducted slab. The degree to which source regions are enr iched is likely related to mantle wedge dynamics. Water pressure suppr esses plagioclase crystallization in derivative magmas, leading to dev elopment 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 mag mas will have a greater tendency to stagnate at mid-crustal levels. Th e observation that basalts predominate in young island arcs while ande sites 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.