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.
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