Plutonic formation through sills stacking and amalgamation: The case of Beauvoir rare-metal granite

To better constrain the assembly, evolution and magmatic differentiation associated kinematics of granitic intrusions, we took advantage of a 1 km long drilled core of the Echassières-Beauvoir rare metal granite, allowing a high resolution sampling of a fully recovered plutonic body. Structural and textural data, coupled with high-resolution major and trace element composition of the mineral phases (in-situ measurements and chemical map), provide constraints on the differentiation processes and its dynamics. As Li-mica (lepidolite) is a liquidus phase present all along melt evolution, their compositions allow tracking melt batches and their differentiation state. We show that the granite formed through the stacking of deca- to hectometric crystal-poor sills, defining different sub-units within the granite. As each sub-units are compositionally different, the detailed study of mineral composition provides a dynamic record of the pluton assembly: although globally constructed from bottom to top, sill emplacement can also occur through off-sequence intrusion within or beneath partly crystallized sub-units. Those sub-units seem to be the result of the amalgamation of smaller sills; the latter displaying similar mineral composition from one to the other.

Once intruded these sills crystallize an assemblage of quartz-topaz-mica and alkali-feldspar, recording differentiation trends from core to rim. This differentiation leads to the formation of a quartz-rich mush and associated albite-saturated interstitial residual melts enriched in incompatible elements (e.g. Li, F, P). A part of these residual melts has been extracted from the quartz-rich mush under the form of differentiated magma channels. Now fully crystallized, those channels correspond to albite-rich segregates, forming lobate contacts with the surrounding granite. Locally, these albite-rich segregates are accumulated beneath the overlying subsequently intruded sill, indicating a protracted plutonic construction faster than the solidification of a single sill. Ultimately, the protracted differentiation of the last and upper sub-unit (≈130 m thick) has led to the accumulation of weakly-viscous evolved melt in the upper-part of magmatic reservoir. This newly formed liquid-rich lens could then be mobilized as erupted silica-rich magma, potentially corresponding to rhyolitic intruding the surrounding host-rocks.