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

The duration of assembly of igneous bodies controls the thermal evolution of the system, which in turn controls the spatial distribution of melt throughout the incremental construction. A preliminary method to reconstruct temporal evolution of magma storage within a reservoir is to perform thermal simulations from which the magmatic activity duration, melt fraction distribution and magma cooling rates can be quantified. In addition, rock microstructures (i.e., mineral morphologies, crystallisation sequence and dihedral angle at three-grain junctions) also carry important information on kinetics of magma solidification. In particular, the geometry of these dihedral angles can be used to decode magma cooling rates variations through an igneous body and, in combination with thermal simulations, provide valuable information on both emplacement and magma solidification kinetics.

We studied the 900 m thick incrementally-emplaced Beauvoir rare-metal granite (Central Massif, France) in which the size and sequence of intrusion of the 18 individual sills have recently been recognised from compositional variations of Li-mica (i.e., lepidolite). The construction of the composite Beauvoir intrusion was numerically simulated, with each successive sill emplaced once the entire reservoir cooled below a critical temperature.  Resulting values in emplacement rates are therefore linked to the chosen value of critical temperature. These simulations indicate that ~10 kyr likely elapsed between the emplacement of the first sill and the solidification of the last droplet of melt. The solidification time for each sill ranges from tens to thousands of years; this duration progressively increases during pluton construction. In such configurations, the magma cooling rate, and in particular that of the marginal regions of each sill, is high (e.g., >0.1 °C.yr^-1), resulting in a disequilibrium geometry of three-grain junctions involving two grains of plagioclase and one of lepidolite (measured at the edges of the platy lepidolite grains). These dihedral angles (Θ_lpp) have median and standard deviation values slightly lower than would be expected from an impingement texture. This evidence of early and rapid crystal growth under diffusion-limited conditions following a sill injection is supported by the presence of skeletal cores in lepidolite as well as plagioclase hopper-like morphologies.

This study demonstrates the power of a combined approach, using both thermal simulations, and rock microstructures to reconstruct the Beauvoir pluton assembly and to extract information about solidification kinetics through time. As the application of dihedral angles has so far been limited to mafic magmas, this study unpicks the use of dihedral angles in felsic magmas, opening up perspectives for their use to better understand magma storage and solidification kinetics in felsic bodies.