太古代の剪断帯における地質構造と微細構造の特徴；南インドダルワールクラトンからの知見

抄録

The Dharwar Craton, a Meso-Neoarchean terrane (3500-2500 Ma) in the southern India, 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 GMSZ has revealed the presence of granitic mylonite and occasionally amphibolite mylonite with a strike-slip sinistral sense of movement. Limited studies have detailed the structural evolution of this key shear zone in the Archean Eon. 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 to unravel its microstructural evolution.

Microstructural analysis show that the dominant dynamic recrystallization structures in the studied samples are bulging recrystallization and sub-grain rotation recrystallization, which indicate low to medium grade deformation conditions. In some samples high temperature deformation characteristics are preserved possibly associated with syn-magmatic deformation.

We conducted a fractal dimension (D) analysis of recrystallized quartz grains in mylonitic samples to understand their shape characteristics. Our preliminary results show that, in the northern part of GMSZ, D value is 1.13, which plot granulite facies and synkinematic granite fields (based on Kruhl and Nega, 1996). In the central part, D is 1.13–1.18, which plot upper greenschist to lower amphibolite facies fields. In the southern part, D is 1.05, which plot close to granulite facies and synkinematic granite fields. The average size of recrystallized quartz grains in the northern part of the GMSZ is approximately 150 µm. In the central and southern parts, the averages are 60 µm and 45 µm, respectively.

Based on our initial microstructural studies, we have identified a low-to-medium grade deformation condition in the GMSZ, with a potential increase in deformation temperature towards the south within the shear zone. In future research, we aim to validate these findings through EBSD analysis of recrystallized quartz grains to ensure the consistency and robustness of our results.