Shearing during progressive retrogression in granitoids: Abrupt grain size reduction of quartz at the plastic-brittle transition for feldspar

Abstract

The Kashio shear zone, Chubu district, Japan, affects the southeast margins of the Ryoke granitic plutons as well as the Ryoke metasedimentary rocks, and is truncated by the Median Tectonic Line near the shear zone centre. Coarse quartz grains within the granitic rocks have been dynamically recrystallized to quartz aggregates towards the pluton margins. The grain size data show an abrupt reduction from various sizes (59–148 μm) to a steady-state size around 35 μm at approximately 400 m from the boundary of the granitic plutons without any tectonic boundary such as a fault being present. This abrupt grain size reduction accompanies brittle deformation in feldspar that overprints earlier plastic deformation and develops strong mylonitic fabrics. In contrast, feldspar away from the pluton margins contains only weak microstructural evidence for plastic deformation and still locally preserves magmatic polycrystal structures amongst the inhomogeneously recrystallized quartz grains. These features, combined with an isochronological study, suggest that the Kashio shear zone began at high temperature (≥450°C) as a ductile event that affected relatively wide regions in the Ryoke granitic plutons before rapid cooling by ca 70 Ma. At this latter stage, strong mylonitization at lower temperatures (≥300°C) in a semiductile regime occurred in a narrow zone towards the shear zone centre. Here, recrystallized quartz aggregates reached a steady-state grain size by ca 60 Ma. That is, cooling within the granitic plutons resulted in strain localization, with the abrupt grain size reduction of quartz aggregates correlating with the plastic-brittle transition of feldspar.