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[HCG24-P02] Geoscientific aspect of formation of fracture in granite –a case study at the Toki Granite, central Japan–
Keywords:Deep geological disposal of high-level radioactive waste, Hydrothermal alteration, Cooling process, Microvoid
Fractures in the crystalline rock such as granite can act as fluid flow and contaminant transport conduits. Therefore, understanding of the fracture characteristics (population, orientation, infilling minerals and so on) is an important subject for the disposal of high-level nuclear waste. Fractures in granite were formed during long geological history from emplacement of granitic magma through solidification and cooling process to present. This suggests that geological events and/or processes might influence fracture formation.
The Toki granite is one of the Late Cretaceous plutonic bodies in Japan and is a nearly circular stock, approximately 14 × 12 km2 in aerial extent, comprising a zoned pluton with three lithofacies grading from muscovite-biotite granite at the margin through hornblende-biotite granite to biotite granite in the interior. Nineteen deep boreholes, ranging from about 500 to 1300 m length, were drilled into the Toki granite. Data on fracture characteristics were obtained by the borehole investigations. We compared fracture data with geological, geochronological and petrographical data to elucidate important geological event and/or process for fracture formation in this study.
In the Toki granite, the fracture population is high in the central part, while it is low in western and northeastern margin of the Toki granite. It decreases with decreasing elevation, except for the western part (Yuguchi et al., 2012). The fracture population roughly corresponds to cooling rate of granitic magma from about 800 to 300 °C, i.e. the parts with slow cooling correspond to areas with high frequencies of fractures, and rapid cooling corresponds to the opposite (Yuguchi et al., 2019). The fracture population shows either abrupt change (increment and decrement) or gradual change around the lithofacies boundary of granite (Yuguchi et al., 2019).
Recently, the authors found that the fracture population is related to areal extent of chlorite alteration of biotite and the areal fraction of microvoids (microscopic fractures and micropores) in biotite (Yuguchi et al., 2021). Biotite chloritization was associated with hydrothermal fluid during the sub-solidus cooling of the pluton (Yuguchi et al., 2015).
Those results indicate that formation of fracture in granite was strongly controlled by characteristics of granitic magma (lithological control) and magma cooling process (cooling rate and hydrothermal biotite chloritization). Now we acquire same type of data in other granitic rock body to conform applicability of our idea.
This work is supported by JSPS KAKENHI Grant Number 20K05410.
Reference
Yuguchi et al. (2012) Eng. Geol., 149-150, 35–46.
Yuguchi et al. (2015) American Mineralogist, 100, 1134–1152.
Yuguchi et al. (2019) Jour. Asian Earth Sci., 169, 47–66.
Yuguchi et al. (2021) PLoS ONE, 16, e0251198
The Toki granite is one of the Late Cretaceous plutonic bodies in Japan and is a nearly circular stock, approximately 14 × 12 km2 in aerial extent, comprising a zoned pluton with three lithofacies grading from muscovite-biotite granite at the margin through hornblende-biotite granite to biotite granite in the interior. Nineteen deep boreholes, ranging from about 500 to 1300 m length, were drilled into the Toki granite. Data on fracture characteristics were obtained by the borehole investigations. We compared fracture data with geological, geochronological and petrographical data to elucidate important geological event and/or process for fracture formation in this study.
In the Toki granite, the fracture population is high in the central part, while it is low in western and northeastern margin of the Toki granite. It decreases with decreasing elevation, except for the western part (Yuguchi et al., 2012). The fracture population roughly corresponds to cooling rate of granitic magma from about 800 to 300 °C, i.e. the parts with slow cooling correspond to areas with high frequencies of fractures, and rapid cooling corresponds to the opposite (Yuguchi et al., 2019). The fracture population shows either abrupt change (increment and decrement) or gradual change around the lithofacies boundary of granite (Yuguchi et al., 2019).
Recently, the authors found that the fracture population is related to areal extent of chlorite alteration of biotite and the areal fraction of microvoids (microscopic fractures and micropores) in biotite (Yuguchi et al., 2021). Biotite chloritization was associated with hydrothermal fluid during the sub-solidus cooling of the pluton (Yuguchi et al., 2015).
Those results indicate that formation of fracture in granite was strongly controlled by characteristics of granitic magma (lithological control) and magma cooling process (cooling rate and hydrothermal biotite chloritization). Now we acquire same type of data in other granitic rock body to conform applicability of our idea.
This work is supported by JSPS KAKENHI Grant Number 20K05410.
Reference
Yuguchi et al. (2012) Eng. Geol., 149-150, 35–46.
Yuguchi et al. (2015) American Mineralogist, 100, 1134–1152.
Yuguchi et al. (2019) Jour. Asian Earth Sci., 169, 47–66.
Yuguchi et al. (2021) PLoS ONE, 16, e0251198