Emplacement mechanisms of pegmatites in the Chotonagpur Granite Gneiss Complex, Eastern India: insights from laboratory experiments

The Chotonagpur Granite Gneiss Complex (CGGC) is crisscrossed by numerous syn-tectonic pegmatitic bodies in the entire terrain. We chose a set of locations in the CGGC to minutely study their structural characteristics, with an objective to explore their emplacement mechanisms. The field evidences show periodic wavy interfaces of pegmatites with the walls, indicating their emplacement in an overall ductile regime. We conducted laboratory experiments to replicate the pegmatitic intrusion processes in analogue models. In these experiments, analogue materials of complex rheology (visco-elastic and visco-elastoplastic) were chosen as hosts, and viscous fluids (water and commercial low-viscosity oil) were injected into the host at varying volumetric flow rates (VFR), 0.100 ml/sec to 1.670 ml/sec. The experimental results show a systematic transition from rupturing to wall-instability-driven fluid intrusion mechanisms with increasing VFR. By combining field and laboratory observations, this study suggests that pegmatites can eventually attain varying geometrical patterns depending on the dominance of these two competing intrusion mechanisms. We also consider the injecting fluid to host viscosity ratio as an additional factor, and performed experiments with varying viscosity ratios: (i) low (oil and UST gel), (ii) moderate (coloured water and UST gel) and (iii) high (coloured water and gel wax). This rheological factor significantly modulates the rupturing versus instability mechanisms in determining the three-dimensional intrusion geometry. We complement this investigation with a fractal analysis of the intrusion trajectories, showing specific fractal dimensions (D) for the two intrusion mechanisms. Finally, a model is proposed to establish a linkage between the intrusion shape and the modes of failure.