High ductility of K-feldspar and development of granitic banded ultramylonite in the Ryoke metamorphic belt, SW Japan

Abstract

Granitic mylonites from an upper greenschist facies mylonite zone in the Ryoke metamorphic belt, SW Japan show three types of microstructure with increasing mylonitization: (1) mylonite, (2) banded mylonite and (3) banded ultramylonite. The banded mylonites and ultramylonites include bands of quartz, biotite, K-feldspar, plagioclase > K-feldspar (>quartz + biotite) and K-feldspar > plagioclase > quartz. The inequality sign indicates abundance in modal proportion within a band. Microstructural and textural investigations by cathodoluminescence (CL), scanning electron microscopy (SEM) and electron back-scatter diffraction (EBSD) show that the microstructural changes from mylonites to banded ultramylonites have occurred through fracturing of plagioclase porphyroclasts, K-feldspar replacement by myrmekite, K-feldspar precipitation in fractures and tails and dislocation creep of quartz and K-feldspar. Development of the banded structure was promoted by high mobility via solution transfer of K-feldspar. Clear spatial distribution of constituent minerals in polyphase feldspar-rich bands, instead of homogeneous mixing of them, indicates that the grain boundary sliding was not the dominant deformation process.