10:00 AM - 10:15 AM
[SMP25-05] A shape change model of isolated K-feldspar inclusions within a micro-shear zone developed in Teshima granite, the Ryoke metamorphic belt
Keywords:Micro-shear zone, Numerical modeling, K-feldspar inclusion, Deformation time
The isolated mineral inclusions possess many kinds of important information for geological history of natural rocks. Especially, shape change patterns (i.e. size versus aspect ratio pattern of the inclusion) of the inclusions were used as a tool to estimate deformation time and deformation conditions in some of previous studies. Toriumi [1981, 1987] firstly proposed mathematical equation representing shape change patterns of the isolated mineral inclusions. More recently, Okamoto and Michibayashi [2005] evaluated deformation time and deformation conditions of natural rock deformed in uniaxial deformation by modeling shape change patterns of garnet inclusions based on the mathematical equation. In this study, we present a numerical model explaining shape change of isolated K-feldspar inclusions within quartz host grain in micro-shear zone developed in Teshima granite, the Ryoke metamorphic belt based on the mathematical equation proposed by Toriumi [1987]. Moreover, we estimated deformation time and shear strain of the micro-shear zone on the basis of modeling and observing data of the K-feldspar inclusions.
To calculate shape change pattern according to various deformation conditions from the model, we estimated deformation temperature (~500-600 °C) and paleo-stress (156 MPa) based on deformed quartz microstructures within the micro-shear zone. Shape change patterns were obtained by calculating the model at the estimated temperature ranges from 500 to 600 °C with 25 °C of interval and at the estimated paleo-stress. Subsequently, the shape change patterns were corrected through combining shape change patterns in zero stress condition to compensate post-deformational annealing effects during exhumation stage of the micro-shear zone. Deformation time was measured by finding out the best-matched shape change pattern calculated by the model in comparison with a shape change pattern observed in the micro-shear zone and by cooling rate of the Ryoke belt proposed by Okudaira et al. [2001]. As a result, deformation time in the micro-shear zone is calculated as 2250-4000 yr under conditions of T = 540-550 °C, σ = 156 MPa. Shear strain in the micro-shear zone is 34-45.
To calculate shape change pattern according to various deformation conditions from the model, we estimated deformation temperature (~500-600 °C) and paleo-stress (156 MPa) based on deformed quartz microstructures within the micro-shear zone. Shape change patterns were obtained by calculating the model at the estimated temperature ranges from 500 to 600 °C with 25 °C of interval and at the estimated paleo-stress. Subsequently, the shape change patterns were corrected through combining shape change patterns in zero stress condition to compensate post-deformational annealing effects during exhumation stage of the micro-shear zone. Deformation time was measured by finding out the best-matched shape change pattern calculated by the model in comparison with a shape change pattern observed in the micro-shear zone and by cooling rate of the Ryoke belt proposed by Okudaira et al. [2001]. As a result, deformation time in the micro-shear zone is calculated as 2250-4000 yr under conditions of T = 540-550 °C, σ = 156 MPa. Shear strain in the micro-shear zone is 34-45.