SIMULTANEOUS MELTING AND FRACTURING OF THE LOWER CRUST, FIORDLAND, NEW ZEALAND

Granulite facies gabbroic and dioritic gneisses in the Pembroke Valley, Milford Sound, New Zealand, are cut by vertical and planar garnet reaction zones in rectilinear patterns. In gabbroic gneiss, narrow dykes of trondhjemitic leucosome are surrounded by fine-grained garnet granulite that variably recrystallized the host two-pyroxene hornblende granulite at conditions of T>750°C and P=14 kbar. Major and trace element whole rock XRF geochemical data indicate that the recrystallization was essentially isochemical. The garnet reaction zones cut contacts between gabbroic gneiss and dioritic gneiss, but change in morphology at the contacts to zones with a septum of coarse-grained garnet surrounded by trondhjemitic leucosome. The dioritic gneiss additionally contains isolated garnet grains enclosed by leucosome, and short planar trains of garnet grains linked by spatially restricted leucosome. Partial melting of the dioritic gneiss, mostly controlled by hornblende breakdown at water-undersaturated conditions, is inferred to have generated the leucosomes. Garnet grains in the different textural settings analysed using a laser ablation ICPMS have similar major, but distinct trace element patterns. The form of the leucosomes is consistent with melt segregation and transport having been aided by fracture propagation; limited retrogression suggests that there was considerable melt escape. Dyking and melt escape is inferred to have propagated fractures into the gabbroic gneiss, where the dioritic gneiss-sourced melt scavenged water from surrounding rocks and induced the limited recrystallization to garnet granulite. The Pembroke example presents strong geological reasons for fracturing of the lower continental crust providing an efficient mechanism for the generation and initial ascent of felsic melt.