[S-CG62] Rheology, fracture and friction in Earth and planetary sciences

[SCG62-P17] On microboudin paleopiezometer: Implication to glaucophane in the Sanbagawa Metamorphic Belt, Wakayama, Japan

*Kazuya Toyama¹, Katsuyoshi Michibayashi^1,2 (1.Department of Earth and Planetary Sciences, GSES, Nagoya University, 2.Volcanoes and Earth's Interior Research Center, IMG, JAMSTEC)

Keywords:Microboudin, Paleopiezometer, Glaucophane, Sanbagawa Belt

A microboudin paleopiezometer was proposed to estimate differential stresses during deformation using minerals such as piedmontite in metacherts^[1] and tourmaline^[2] in fault rocks. However, these paleopiezometers enable the calculation of only the relative values of differential stresses. This study focuses on glaucophane in metacherts from the Sanbagawa Belt in Wakayama, Japan, and proposes a method to determine the differential stress and its evolution by reevaluating the microboudin paleopiezometer. Thin sections were prepared perpendicular to the foliation and parallel to the lineation (XZ plane). The major minerals are quartz, albite, hematite, muscovite, and glaucophane. Focusing on the microboudin structures of glaucophane, we measured shape parameters such as the boudin aspect ratio (major axis/minor axis) and interboudin gap. To efficiently measure several hundred glaucophane crystals in thin sections, an image analysis program was newly developed using Python v. 3.12.4 (Spyder v. 5.5.1). In addition, the crystal fabrics and grain sizes of quartz were measured using the SEM-EBSD equipment at Nagoya University. Among the 242 glaucophane grains in the thin sections, 195 grains exhibited microboudin structures, whereas 47 grains did not. The average aspect ratios were 7.20 and 3.46, respectively. Of the boudinaged 195 grains, 104 were boudinaged into two, 63 into three, 22 into four, and 6 into five. Additionally, the quartz c-axis fabric exhibited a Type I cross-girdle with an average grain size of approximately 30 µm. We applied the measured parameters to the microboudin paleopiezometer proposed by Li & Ji (2020) (hereafter, the LJ method). However, because the LJ method did not adequately explain the relationship between the interboudin gap and boudin aspect ratio, a new method(the LJ-T method) was derived by modifying some parts of the LJ method. Applying the average grain size to the dynamic recrystallization grain size paleopiezomter^[3], the differential stress was estimated at 59.4 MPa and shear stress at 42.0 MPa. This shear stress corresponds to a glaucophane aspect ratio of 5.11. Subsequently, using the relationship^[2] between the shear stress, tensile strength, and aspect ratio, the glaucophane tensile strength was calculated as 859 MPa. Furthermore, assuming the interboudin gap as the amount of strain, it was confirmed that the younger the boudin, the greater the differential stress. In conclusion, the microboudin paleopiezometer for glaucophane is a powerful tool for elucidating the differential stress history of metamorphic rocks.

References: [1] Masuda et al. 2004 J. Metamor. Geol. 22, [2] Li & Ji 2020 J. Struct. Geol. 130, [3] Twiss 1977 Pure Appl. Geophys. 115.