5:15 PM - 6:45 PM
[SMP24-P18] Grain size reduction of albite porphyroblasts in the Sanbagawa metamorphic rocks, Kanto Mountains, Japan
Keywords:albite porphyroblast, grain size distribution, Sanbagawa metamorphism, albite spot
Albite porphyroblasts that are observed in the Sanbagawa metamorphic rocks are one of the old index of higher-grade Sanbagawa metamorphism. These porphyroblasts have formed presumably during or immediately after the peak of the metamorphism. The Sanbagawa metamorphic belt is known to have undergone intense deformation during exhumation. So far, the influence of the deformation of the host-rock have only been taken into account concerning the growth of the albite, not the breaking ups. In this report, we present some microstructures that indicate breaking ups of albite porphyroblasts that occurred probably as a result of strong shear deformation of the Sanbagawa metamorphic rocks. Pelitic schists sampled in the Nagatoro area of the Kanto Mountains reveal that fracturing of the albite porphyroblasts has occurred.
Samples with albite porphyroblasts visible to the naked eye contained albite grains elongated parallel to the lineation, ranging from 0.5 to 2.0 mm in length. Pressure shadows filled with quartz are found at both ends. The porphyroblasts often show cracks that are filled with quartz and/or calcite, or with K-feldspar. Finer-grained albites align parallel to the lineation and are thought to result from grain size reduction of the original porphyroblasts. Some porphyroblasts exhibit undulose extinction and subgrain boundaries, indicating breaking-ups. Texture of quartz largely suggests ductile behavior, which probably enabled finer albite grains to align due to shear deformation.
The pelitic schists lacking visible albite porphyroblasts contain albite grains in both micaceous and quartz-rich layers, with varying grain sizes. In micaceous layers, albite grains are generally small (<0.1 mm) and elliptical, while in quartz-rich layers, larger albite grains (~0.2 mm) coexist with smaller grains. The latter albite grains in the quartz-rich layers form lenticular aggregations, possibly originating from larger porphyroblasts. Quartz fabrics indicate ductile deformation, facilitating the dispersion of fine albite grains resulting from porphyroblast breakup into observed lenticular aggregations.
The deformation microstructures of albite porphyroblasts may constrain the conditions and history of deformation of the Sanbagawa metamorphic belt after the peak metamorphic condition.
Samples with albite porphyroblasts visible to the naked eye contained albite grains elongated parallel to the lineation, ranging from 0.5 to 2.0 mm in length. Pressure shadows filled with quartz are found at both ends. The porphyroblasts often show cracks that are filled with quartz and/or calcite, or with K-feldspar. Finer-grained albites align parallel to the lineation and are thought to result from grain size reduction of the original porphyroblasts. Some porphyroblasts exhibit undulose extinction and subgrain boundaries, indicating breaking-ups. Texture of quartz largely suggests ductile behavior, which probably enabled finer albite grains to align due to shear deformation.
The pelitic schists lacking visible albite porphyroblasts contain albite grains in both micaceous and quartz-rich layers, with varying grain sizes. In micaceous layers, albite grains are generally small (<0.1 mm) and elliptical, while in quartz-rich layers, larger albite grains (~0.2 mm) coexist with smaller grains. The latter albite grains in the quartz-rich layers form lenticular aggregations, possibly originating from larger porphyroblasts. Quartz fabrics indicate ductile deformation, facilitating the dispersion of fine albite grains resulting from porphyroblast breakup into observed lenticular aggregations.
The deformation microstructures of albite porphyroblasts may constrain the conditions and history of deformation of the Sanbagawa metamorphic belt after the peak metamorphic condition.