Hydrous shear zones in the lower arc crust are sites of melt transfer
Recent studies of the lower arc crust exposed in Fiordland, New Zealand, concluded that shear zones are sites of melt migration and mass transfer through the deep crust. The 4-10 km-wide George Sound Shear Zone (GSSZ) in central Fiordland comprises two main rock types, relatively dry two-pyroxene gneisses, and hornblende-bearing gneisses.
Petrographic analysis of samples collected in a transect across the shear zone shows a range of hydration reaction textures from rims of hornblende + quartz around pyroxene grains to complete replacement of pyroxene grains. Plagioclase is recrystallized and partially replaced by clinozoisite. Additionally, biotite mode increases from outside the shear zone towards higher strain rocks. BSE images and polarised light microscopy show microstructures indicative of former melt-present deformation: (a) interconnected mineral films of K-feldspar along grain boundaries, (b) grains that terminate with low dihedral angles, (c) interstitial grains, (d) undulose extinction in plagioclase and (e) serrated grain boundaries.
From the above observations, it is inferred that a felsic to intermediate hydrous melt migrated along the GSSZ through two-pyroxene gneiss host rocks. The melt migration along grain boundaries was deformation‐assisted, which hydrated pyroxene to hornblende + quartz, plagioclase to clinozoisite, and introduced high proportions of biotite into the assemblage.