° Daczko, N.R., Clarke, G.L. and Klepeis, K.A. 2001. Transformation of two-pyroxene hornblende granulite to garnet granulite involving  simultaneous melting and fracturing of the lower crust, Fiordland, New Zealand. Journal of Metamorphic Geology, 19, 547-560.


Granulite facies gabbroic and dioritic gneiss 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 anorthositic leucosome are surrounded by fine-grained garnet granulite that patchily recrystallized the host two-pyroxene-hornblende granulite at conditions of T>750°C and PÅ14 kbar. Major and trace element whole rock geochemical data indicate that the recrystallization was essentially isochemical. The garnet reaction zones cut contacts between the gabbroic gneiss and dioritic gneiss, but change in morphology at the contacts to zones with a septum of coarse-grained garnet surrounded by anorthositic 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. 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.

Garnet Reaction Zones

Dr Nathan R. Daczko
GEMOC ARC National Key Centre
Department of Earth and Planetary Sciences
Macquarie University NSW 2109

Phone: +61 2 9850 8371
Fax: +61 2 9850 6904