A COMPARISON OF MICROSTRUCTURES AND DEFORMATION TEXTURES INSIDE AND OUTSIDE THE LOWER CRUSTAL DOUBTFUL SOUND SHEAR ZONE, FIORDLAND, NEW ZEALAND

Doubtful Sound, in SW New Zealand, exposes an exhumed section of lower crust that represents the root of an Early Cretaceous magmatic arc. Mafic-intermediate granulite gneisses occur below the extensional Doubtful Sound shear zone (DSSZ), a ~3 km thick zone that records the retrogression and transposition of granulite fabrics at the upper amphibolite facies. We compared mineral microstructures and textures within and below the DSSZ to infer processes influencing lower crustal rheology and strain partitioning during extension.

Below the DSSZ, dehydration zones around felsic veins and leucosome in migmatitic orthogneiss record granulite facies metamorphism. Aggregates of hornblende and orthopyroxene rimmed by garnet and clinopyroxene are set in a plagioclase matrix. Peritectic garnet, plagioclase-garnet symplectites, and elongate, inclusion-free plagioclase reflect the anatexis. Plagioclase exhibits a CPO and evidence of subgrain rotational recrystallization and grain boundary migration, indicating subsolidus deformation outlasted melting. Mafic aggregates are boudinaged and orthopyroxene developed subgrains. A comparison of mineral grain shapes indicates that plagioclase accommodated most of the strain.

Inside the DSSZ, deformation and heterogeneous retrogression of the granulites resulted in folia of hornblende, biotite, and fine plagioclase. Plagioclase aggregates are annealed but preserve grain boundary migration. Hornblende forms asymmetric fish. Evidence of high fluid activity and reaction softening includes biotite beards on plagioclase and hornblende and increased modal hornblende and biotite.

The observations suggest that magma, heat, and melting initially weakened the lower crust, when deformation was distributed broadly. Partially molten regions deformed by suprasolidus flow and solid portions deformed mostly by dislocation creep in plagioclase and the boudinage of strong minerals. Later, hydration weakened the lower crust and deformation occurred by mass transfer where hornblende and biotite accommodated most of the strain. The results illustrate the importance of cooling, hydration, and reaction softening in controlling strain partitioning and the rheological evolution of lower crustal shear zones.