17:15 〜 19:15
[SMP26-P05] Tectonic and microstructural characteristics of an Archean shear zone; insights from the Dharwar craton, southern India
キーワード：太古代、変形、石英微細構造、フラクタル次元、電子後方散乱回折
The Dharwar Craton, a Meso-Neoarchean terrane (3500-2500 Ma) in the southern India, comprises two main geological units: TTG (Tonalite-Trondhjemite-Granodiorite) complexes and greenstone belts, with multiple generations of granitic intrusions. The craton is divided into three regions: the Western Dharwar Craton (WDC), Central Dharwar Craton (CDC), and Eastern Dharwar Craton (EDC). The Gadag-Mandya Shear Zone (GMSZ), which trends N-S to NW-SE, serves as the boundary between WDC and CDC.
Detailed fieldwork conducted along a 174 km stretch of the Gadag-Mandya Shear Zone (GMSZ) has revealed the presence of granitic mylonite and occasionally amphibolite mylonite. Observations indicate that the width of the GMSZ ranges from 1-20 m in the north to about 500 m in the south, with a strike-slip sinistral sense of movement. Limited studies have detailed the structural evolution, microstructural characteristics, and regional-scale variation deformation mechanisms of this key shear zone from the Archean Eon. To address these gaps, we will present the regional-scale variation in the microstructural properties of recrystallized quartz grains in the granitic mylonite and deformed granitic rocks of the GMSZ. Our study involves microstructural analysis, SEM-EBSD analysis, fractal analysis, and grain size analysis of dynamically recrystallized quartz grains to understand the deformation conditions within the GMSZ.
Microstructural analysis shows that the dominant dynamic recrystallization structures in the studied samples are bulging recrystallization (BLG) and sub-grain rotation recrystallization (SGR), which indicate low to medium grade deformation conditions. In some samples high temperature deformation characteristics such as mylmekite and chessboard pattern are preserved possibly associated with syn-magmatic deformation. Major metamorphic minerals aligned along the foliation in the GMSZ are muscovite and chlorite in the northern and central part. In the southern part biotite and amphibole aligned along the foliation plane indicating possibly an elevated deformation grade.
We conducted fractal dimension analysis of recrystallized quartz grains in mylonitic samples to understand their shape characteristics. The fractal dimension (D), which reflects the grain shape, is calculated by equating the area (S) of the recrystallized quartz grain to that of a circle with diameter (d). We selected more than 60 recrystallized quartz grains with clear boundaries from the central part of thin section samples and manually traced them (Takahashi et al., 1998). Using HAYERA and ImageJ software, we determined the perimeter (P) and area (S) of each traced grain and calculated the diameter (d). The fractal dimension (D) is then expressed as the slope of the line obtained by plotting the perimeter (P) against the diameter (d) on a logarithmic graph.
Our preliminary results show that, in the northern part of GMSZ, the fractal dimension (D) is 1.13, which plot granulite facies and synkinematic granite fields (based on Kruhl and Nega, 1996) . In the central part, fractal dimension (D) is 1.13-1.18, which plot upper greenschist facies to lower amphibolite facies fields. In the southern part, fractal dimension (D) is 1.05, which plot close to granulite facies and synkinematic granite fields. The correlation coefficients are more than 0.95 in all the samples.
We conducted SEM-EBSD analysis at the Geological Survey of Japan, (AIST) to evaluate crystal orientation and grain size analysis of dynamically recrystallized quartz grains in mylonite samples. Quartz c-axis fabric shows that medium to high temperature mylonite with Y-maximum (dominant prism slip) in the central part of the GMSZ, and low temperature mylonite with Z-maximum (dominant basal slip) in the southern part of the GMSZ. The average size of quartz grains in the northern part of the GMSZ is 34 micro meters. In the central and southern parts, the averages are 46 micro meters and 21 micro meters respectively. Additionally, grain sizes tend to have peaks in the 10-30 micro meters and 70-90 micro meters ranges.
Based on our initial microstructural studies, we have identified a low-to-medium grade deformation condition in the GMSZ. In future research, we aim to understand the deformation conditions within the GMSZ based on additional fieldwork, microstructures, detailed temperature and differential stress estimates.
Detailed fieldwork conducted along a 174 km stretch of the Gadag-Mandya Shear Zone (GMSZ) has revealed the presence of granitic mylonite and occasionally amphibolite mylonite. Observations indicate that the width of the GMSZ ranges from 1-20 m in the north to about 500 m in the south, with a strike-slip sinistral sense of movement. Limited studies have detailed the structural evolution, microstructural characteristics, and regional-scale variation deformation mechanisms of this key shear zone from the Archean Eon. To address these gaps, we will present the regional-scale variation in the microstructural properties of recrystallized quartz grains in the granitic mylonite and deformed granitic rocks of the GMSZ. Our study involves microstructural analysis, SEM-EBSD analysis, fractal analysis, and grain size analysis of dynamically recrystallized quartz grains to understand the deformation conditions within the GMSZ.
Microstructural analysis shows that the dominant dynamic recrystallization structures in the studied samples are bulging recrystallization (BLG) and sub-grain rotation recrystallization (SGR), which indicate low to medium grade deformation conditions. In some samples high temperature deformation characteristics such as mylmekite and chessboard pattern are preserved possibly associated with syn-magmatic deformation. Major metamorphic minerals aligned along the foliation in the GMSZ are muscovite and chlorite in the northern and central part. In the southern part biotite and amphibole aligned along the foliation plane indicating possibly an elevated deformation grade.
We conducted fractal dimension analysis of recrystallized quartz grains in mylonitic samples to understand their shape characteristics. The fractal dimension (D), which reflects the grain shape, is calculated by equating the area (S) of the recrystallized quartz grain to that of a circle with diameter (d). We selected more than 60 recrystallized quartz grains with clear boundaries from the central part of thin section samples and manually traced them (Takahashi et al., 1998). Using HAYERA and ImageJ software, we determined the perimeter (P) and area (S) of each traced grain and calculated the diameter (d). The fractal dimension (D) is then expressed as the slope of the line obtained by plotting the perimeter (P) against the diameter (d) on a logarithmic graph.
Our preliminary results show that, in the northern part of GMSZ, the fractal dimension (D) is 1.13, which plot granulite facies and synkinematic granite fields (based on Kruhl and Nega, 1996) . In the central part, fractal dimension (D) is 1.13-1.18, which plot upper greenschist facies to lower amphibolite facies fields. In the southern part, fractal dimension (D) is 1.05, which plot close to granulite facies and synkinematic granite fields. The correlation coefficients are more than 0.95 in all the samples.
We conducted SEM-EBSD analysis at the Geological Survey of Japan, (AIST) to evaluate crystal orientation and grain size analysis of dynamically recrystallized quartz grains in mylonite samples. Quartz c-axis fabric shows that medium to high temperature mylonite with Y-maximum (dominant prism slip) in the central part of the GMSZ, and low temperature mylonite with Z-maximum (dominant basal slip) in the southern part of the GMSZ. The average size of quartz grains in the northern part of the GMSZ is 34 micro meters. In the central and southern parts, the averages are 46 micro meters and 21 micro meters respectively. Additionally, grain sizes tend to have peaks in the 10-30 micro meters and 70-90 micro meters ranges.
Based on our initial microstructural studies, we have identified a low-to-medium grade deformation condition in the GMSZ. In future research, we aim to understand the deformation conditions within the GMSZ based on additional fieldwork, microstructures, detailed temperature and differential stress estimates.