Assembly duration, cooling kinetics and associated microstructures of a small sized granite pluton

The duration of igneous body assembly controls the thermal evolution of magmatic systems and the spatial distribution of melt during incremental construction. Thermal simulations can be used to reconstruct magma storage histories by providing strong constraints on emplacement duration, melt fraction distribution, and cooling rates. In parallel, rock microstructures (mineral morphologies, crystallisation sequences, and dihedral angles at three-grain junctions) record information on magma solidification kinetics. In particular, dihedral angles can be used to constrain magma cooling-rate variations when combined with thermal modelling.

To better understand the assembly dynamics and magma solidification kinetics in small and highly differentiated granites, we investigated the 900 m thick, incrementally emplaced Beauvoir rare-metal granite (Central Massif, France), which is composed of 18 sills identified through Li-mica (lepidolite) compositional variations. Numerical simulations of pluton construction were performed by sequentially emplacing sills once the reservoir cooled below a critical temperature. The results suggest that ~10 kyr elapsed between emplacement of the first sill and complete solidification of the system, with an averaged construction rate as low as 10^-4 km³.yr^-1. The solidification times of individual sills ranged from tens to thousands of years. Rapid magma solidification resulted in disequilibrium three-grain junction geometries, while localised skeletal crystal habits bear witness to an early period of high magma undercooling related to sill emplacement. Our results highlight the value of integrating thermal modelling with microstructural observations to reconstruct magma storage histories, and extend the use of dihedral angles to felsic magmas, offering a new tool for probing solidification dynamics in granitic systems.