Anatectic migmatites from the roof of an ocean ridge magma chamber

A well-preserved contact aureole at the base of the sheeted dyke complex in the Troodos ophiolite, Cyprus, records the partial melting of the roof of a mid-ocean ridge magma chamber. Hydrothermally altered dykes within the lowermost 10-30 m of the sheeted dyke complex were recrystallized to pyroxene and hornblende hornfels and, closest to the underlying gabbros (<10 m), partially melted (up to >30%) at water-undersaturated conditions and temperatures greater than or equal to875degreesC. Geological considerations indicate that the duration of melting events was probably on the time scale of eruptive cycles (i.e. years to centuries). This resulted in disequilibrium partial melting and highly imperfect melt segregation within the contact aureole. We hypothesize that partial melting is a common process within the roof zones of axial magma chambers at fast-spreading ridges and that a component of geophysically imaged axial magma chambers may be partially melted upper crust rather than partially crystallized magma. Field relations suggest that where the highest degrees of melting occurred, a mixture of anhydrous hornfels and hydrous leucocratic melt disaggregrated, and was assimilated into the magma chamber.