10:45 〜 12:15
[SMP26-P03] マグマ貫入による下部地殻の開口せん断破壊における応力状態、流体圧とその継続時間：東南極セール・ロンダーネ山地の岩石記録
キーワード：南極、下部地殻、応力状態、流体圧、流体活動時間、深部低周波地震
Fluid pressure and stress state are key factors to induce earthquakes[1]. Recent geophysical observations suggest magmatic and/or aqueous fluid activities can trigger deep crustal earthquakes under volcanic regions[e.g., 2]. For understanding the physio-chemical processes of deep crustal fracturing related to magmatic intrusions, we have investigated a series of granitic dikes and their reaction zones in high temperature metamorphic rocks that probably formed in such condition. We clarified the mode of fracturing, time-scales of fluid activities and stress state during the crustal fracturing.
Study area is located at high-temperature metamorphic terrain in the Sør Rondane Mountains, East
Antarctica. Felsic granulites are cut by fractures of granitic dikes and hornblende veins (Hbl veins) that branch from the granitic dikes (Fig. 1a, b). Both of them cut the gneissosity of the felsic granulite. Extensional shear displacement was observed for the granitic dike (Fig. 1c). Felsic granulite is largely composed of clinopyroxene (cpx), orthopyroxene (opx), plagioclase (pl) and hornblende (hbl). (m to ~100 m in length; Fig. 1a) Felsic granulites are hydrated along the fractures and form whitish reaction zones.
Reaction zones are composed of Bt zone (distance from dike [cm]=0–0.52), Hbl-Bt zone(0.52–1.61) Cpx-Hbl zone (1.61–2.72) and transition zone. Granitic dike is mainly composed of quartz (qtz), pl and K-feldspar, containing minor grains of biotite (bt) and chlorite. Apart from pl and qtz, each zone is mainly characterized by bt for Bt zone, hbl and bt for Hbl-Bt zone, cpx and hbl for Cpx-Hbl zone, and hbl, cpx and opx for transition zone. Hbl-rich veins are mainly composed of hbl, pl and qtz. XAn content of pl decreases from host rock (0.39–0.43) towards granitic dike (0.25–0.30). AlT1 content of hbl increases from host rock (1.32–1.64) toward granitic dike (1.65–1.99) and around Hbl vein (1.49–2.10). Application of hbl-pl thermometry[3] and Al in hbl barometer[4] to Bt zone tentatively suggest that magma intrusion had occurred at 672-746℃, 0.67-0.83 GPa (i.e., 25–31km).
Cl concentration in apatite increases toward the granitic dike from 0 to 1.2 wt%. Application of reactive transport modelling to the Cl profile[5] suggest timescales of fluid infiltration of 35-120 hours (Fig. 1d).
To estimate stress state, the orientations of granitic dike were measured with the 3D model and photographs of the outcrop of felsic granulite (120 m ×70 m ×80 m) and for analyzing the paleostress state by GArcmB software[6] .
The fractures largely orient WNW-ESE in strike, and the dip mostly vertically. Bayesian information criterion (BIC) indicates that stress state was constant (i.e., number of stress state n = 1) during the intrusion of magma (Fig. 1e). The stress orientations are σ1=dip direction/ dip : 102°/ 32°, σ2=292°/ 60°, σ3=(202°/ 60°) (Fig. 1e)
These results suggest following crustal fracturing processes by magmatic intrusion at 670–750℃, 0.7–0.8 GPa: Hydrous granitic magma intruded into gneissose felsic granulite, where σ1 and σ3 were sub horizontal and σ2 was subvertical. The granitic magma solidified, and released Cl-bearing aqueous fluid. The high-pressure Cl-bearing fluid activity lasted for 35–120 h, infiltrating into the felsic granulite to form reaction zones and forming Hbl veins vertical to σ3 through which Cl-bearing fluids were carried further. The observed extensional shear fracturing is an evidence of occurrence of earthquakes. The depth and time-scale, granitic dike intrusion observed in this study roughly match those of deep low frequency earthquakes[7,8] (Fig. 1f).
Yoshida (2021) Zisin Yukutake et al. (2019) GRL Holland & Blundy (1994) Contrib. Mineral. Petrol. Anderson & Smith (1995) Am. Mineral. Philpotts and Ague (2009) Cambridge University Press Yamaji, (2016) Isl. Arc. Hasegawa & Nakajima (2021) JGR Kurihara & Obara (2021) JGR
Study area is located at high-temperature metamorphic terrain in the Sør Rondane Mountains, East
Antarctica. Felsic granulites are cut by fractures of granitic dikes and hornblende veins (Hbl veins) that branch from the granitic dikes (Fig. 1a, b). Both of them cut the gneissosity of the felsic granulite. Extensional shear displacement was observed for the granitic dike (Fig. 1c). Felsic granulite is largely composed of clinopyroxene (cpx), orthopyroxene (opx), plagioclase (pl) and hornblende (hbl). (m to ~100 m in length; Fig. 1a) Felsic granulites are hydrated along the fractures and form whitish reaction zones.
Reaction zones are composed of Bt zone (distance from dike [cm]=0–0.52), Hbl-Bt zone(0.52–1.61) Cpx-Hbl zone (1.61–2.72) and transition zone. Granitic dike is mainly composed of quartz (qtz), pl and K-feldspar, containing minor grains of biotite (bt) and chlorite. Apart from pl and qtz, each zone is mainly characterized by bt for Bt zone, hbl and bt for Hbl-Bt zone, cpx and hbl for Cpx-Hbl zone, and hbl, cpx and opx for transition zone. Hbl-rich veins are mainly composed of hbl, pl and qtz. XAn content of pl decreases from host rock (0.39–0.43) towards granitic dike (0.25–0.30). AlT1 content of hbl increases from host rock (1.32–1.64) toward granitic dike (1.65–1.99) and around Hbl vein (1.49–2.10). Application of hbl-pl thermometry[3] and Al in hbl barometer[4] to Bt zone tentatively suggest that magma intrusion had occurred at 672-746℃, 0.67-0.83 GPa (i.e., 25–31km).
Cl concentration in apatite increases toward the granitic dike from 0 to 1.2 wt%. Application of reactive transport modelling to the Cl profile[5] suggest timescales of fluid infiltration of 35-120 hours (Fig. 1d).
To estimate stress state, the orientations of granitic dike were measured with the 3D model and photographs of the outcrop of felsic granulite (120 m ×70 m ×80 m) and for analyzing the paleostress state by GArcmB software[6] .
The fractures largely orient WNW-ESE in strike, and the dip mostly vertically. Bayesian information criterion (BIC) indicates that stress state was constant (i.e., number of stress state n = 1) during the intrusion of magma (Fig. 1e). The stress orientations are σ1=dip direction/ dip : 102°/ 32°, σ2=292°/ 60°, σ3=(202°/ 60°) (Fig. 1e)
These results suggest following crustal fracturing processes by magmatic intrusion at 670–750℃, 0.7–0.8 GPa: Hydrous granitic magma intruded into gneissose felsic granulite, where σ1 and σ3 were sub horizontal and σ2 was subvertical. The granitic magma solidified, and released Cl-bearing aqueous fluid. The high-pressure Cl-bearing fluid activity lasted for 35–120 h, infiltrating into the felsic granulite to form reaction zones and forming Hbl veins vertical to σ3 through which Cl-bearing fluids were carried further. The observed extensional shear fracturing is an evidence of occurrence of earthquakes. The depth and time-scale, granitic dike intrusion observed in this study roughly match those of deep low frequency earthquakes[7,8] (Fig. 1f).
Yoshida (2021) Zisin Yukutake et al. (2019) GRL Holland & Blundy (1994) Contrib. Mineral. Petrol. Anderson & Smith (1995) Am. Mineral. Philpotts and Ague (2009) Cambridge University Press Yamaji, (2016) Isl. Arc. Hasegawa & Nakajima (2021) JGR Kurihara & Obara (2021) JGR