Presentation Information
[SMP31-P08]Strain analysis from platy minerals in severely deformed conglomerate
*Renya Suzuki1, Miki Tasaka1, Toshiaki Masuda1, Kenji Kusunoki1 (1.Shizuoka University)
Keywords:
Strain analysis,Hitotsu-nashi conglomerate schist,Muscovite,Chlorite,Crystallographic preferred orientation
Microstructural analysis in metamorphic rocks has developed along two independent approaches: finite strain analysis based on strain markers, and analyses based on crystallographic preferred orientation (CPO) of constituent minerals. However, quantitative comparisons of these two approaches within a single sample remain limited. In this study, we examine the Hitotsu-nashi conglomerate schist, which contains well-preserved strain markers, and investigate the geometrical relationship between finite strain derived from clast shapes and CPO of platy minerals.
The studied sample is an Hitotsu-nashi conglomerate schist distributed in the Naradani area, Gifu Prefecture, Japan. It is a greenschist containing strongly elongated granitic clasts with a maximum aspect ratio of approximately 30. Based on the deformation texture, thin sections were prepared parallel to the XZ plane (perpendicular to foliation and containing the lineation) and the XY plane (parallel to foliation). The axial ratios and long-axis orientations of clasts were measured on both sections to reconstruct the three-dimensional finite strain ellipsoid. The strain parameter in the Flinn diagram yields k_F = 0.35, indicating that the finite strain ellipsoid plots in the flattening field.
Crystallographic preferred orientations of muscovite and chlorite (grain size ~5 μm) were measured using EBSD. The (001) poles of both minerals are concentrated parallel to foliation, and their c-axes are perpendicular to the shortening axis of the finite strain ellipsoid, showing a consistent geometrical relationship. Furthermore, following Masuda and Omori (2021), the pole figure patterns of mica were used to evaluate the orientation concentration, yielding concentration parameters of K_CPO = −2.5 and ψ = 0.1. These values indicate a strong planar fabric and are consistent with the geometrical characteristics of the finite strain ellipsoid.
These results demonstrate that the CPO of sheet silicate minerals reflects the principal geometric axes of the finite strain ellipsoid. This study suggests that, even in metamorphic rocks lacking strain markers, the geometrical characteristics of finite strain can be inferred from the CPO of sheet silicates.
The studied sample is an Hitotsu-nashi conglomerate schist distributed in the Naradani area, Gifu Prefecture, Japan. It is a greenschist containing strongly elongated granitic clasts with a maximum aspect ratio of approximately 30. Based on the deformation texture, thin sections were prepared parallel to the XZ plane (perpendicular to foliation and containing the lineation) and the XY plane (parallel to foliation). The axial ratios and long-axis orientations of clasts were measured on both sections to reconstruct the three-dimensional finite strain ellipsoid. The strain parameter in the Flinn diagram yields k_F = 0.35, indicating that the finite strain ellipsoid plots in the flattening field.
Crystallographic preferred orientations of muscovite and chlorite (grain size ~5 μm) were measured using EBSD. The (001) poles of both minerals are concentrated parallel to foliation, and their c-axes are perpendicular to the shortening axis of the finite strain ellipsoid, showing a consistent geometrical relationship. Furthermore, following Masuda and Omori (2021), the pole figure patterns of mica were used to evaluate the orientation concentration, yielding concentration parameters of K_CPO = −2.5 and ψ = 0.1. These values indicate a strong planar fabric and are consistent with the geometrical characteristics of the finite strain ellipsoid.
These results demonstrate that the CPO of sheet silicate minerals reflects the principal geometric axes of the finite strain ellipsoid. This study suggests that, even in metamorphic rocks lacking strain markers, the geometrical characteristics of finite strain can be inferred from the CPO of sheet silicates.