Solution–precipitation of K-feldspar in deformed granitoids and its relationship to the distribution of water

We have investigated K-feldspar recrystallisation in granitoid mylonites within a ductile shear zone from the Ryoke metamorphic belt, SW Japan. Fine-grained K-feldspar (20 μm on average) occurs in the matrix and in pull-apart areas within fractured K-feldspar porphyroclasts. These fine grains are elongated and oriented parallel to the main foliation in the matrix, and their grain surfaces, observed with the scanning electron microscope, are not smooth, but rough due to the development of very fine (< 1 μm) round grains of K-feldspar on the surface of each grain. In pull-apart areas, the crystallographic orientation of fine-grained K-feldspar, as measured by electron backscatter diffraction (EBSD), is strongly controlled by that of the host porphyroclast, and shows rotations with shear components parallel to fractures. In the matrix, the crystallographic orientation of fine-grained K-feldspar is not consistent with intracrystalline plasticity, but rather with a growth rate that is slightly controlled by nearby porphyroclasts. All this, together with the growth features on grains, suggests that solution–precipitation of K-feldspar from K-rich aqueous fluid occurred during progressive deformation. Infrared (IR) mapping was performed to evaluate the distribution of water in pull-apart areas and the matrix. Water is heterogeneously distributed within K-feldspar porphyroclasts, which contain 150–2200 ppm H2O. In contrast, the water content is low (150–300 ppm H2O) and homogeneously distributed in fine-grained K-feldspar in the matrix and pull-apart areas, even though included in these analyses are grain boundaries that can generally contain abundant aqueous fluid. The results of EBSD analysis and IR mapping indicate that water is released during solution–precipitation of K-feldspar under mid-crustal conditions. The solution–precipitation process under a water-rich environment in the middle crust results in the formation of fine grains, possibly deforming dominantly by grain-size-sensitive creep, and with the release of aqueous fluid involved in the process.

► We focus on fine-grained K-feldspar in granitoid mylonites.
► Growth features of grains indicate a solution–precipitation process.
► Host-controlled crystallographic orientation may be related to grain growth.
► Water contents of fine-grained K-feldspar are low and homogeneous (250 ppm H2O).
► Aqueous fluid may be released during the solution–precipitation process.