The generation of magmatism above subduction zones can be rather complicated - as illustrated in this simplified (honestly!) diagram. The down-going slab hydrates the overlying mantle. This can then melt, emitting basaltic (with added silica that was dissolved in the water), leaving a residue (called harzbergite) in the mantle. Alternatively the hydrated mantle might rise buyantly and melt at shallower depths. The complexity of magma is increased by fractional crystallisation as it rises or is ponded in magma chambers.
