Transformation of two-pyroxene hornblende granulite to garnet granulite involving simultaneous melting and fracturing of the lower crust, Fiordland, New Zealand

Granulite facies gabbroic and dioritic gneisses in the Pembroke Valley, Mil ford Sound, New Zealand, are cut by vertical and planar garnet reaction zon es in rectilinear patterns. In gabbroic gneiss, narrow dykes of anorthositi c leucosome are surrounded by fine-grained garnet granulite that replaced t he host two-pyroxene hornblende granulite at conditions of 750 degreesC and 14 kbar. Major and trace element whole-rock geochemical data indicate that recrystallization was mostly isochemical. The anorthositic veins cut conta cts between gabbroic gneiss and dioritic gneiss, but change in morphology a t the contacts, from the anorthositic vein surrounded by a garnet granulite reaction zone in the gabbroic gneiss, to zones with a septum of coarse-gra ined garnet surrounded by anorthositic leucosome in the dioritic gneiss. Th e dioritic gneiss also contains isolated garnet grains enclosed by leucosom e, and short planar trains of garnet grains linked by leucosome. Partial me lting of the dioritic gneiss, mostly controlled by hornblende breakdown at water-undersaturated conditions, is inferred to have generated the leucosom es. The form of the leucosomes is consistent with melt segregation and tran sport aided by fracture propagation, limited retrogression suggests conside rable melt escape. Dyking and melt escape from the dioritic gneiss are infe rred to have propagated fractures into the gabbroic gneiss. The migrating m elt scavenged water from the surrounding gabbroic gneiss and induced the li mited replacement by garnet granulite.