Nicolas Esteves, Lydéric France, Michel Cuney, Pierre Bouilhol

The kinematics, modes of assembly, and the processes governing the evolution of magmas shape plutonic intrusions. Granite bodies have been suggested to emplace incrementally, with successive magmatic batches locally solidified as dikes or sills. Yet, the complexity and longevity of large-scale plutons hinders a unified model for their emplacement and concomitant differentiation. This is especially true for highly differentiated granites which usually lack continuous outcrops limiting our understandings of the detailed assembly of these igneous complexes and of the related magmatic processes. To tackle this issue, we focus on the Beauvoir intrusion (Massif Central, France), a small pluton (~800 m thick) of high economic interest (Li-Be-Nb-Ta) whose fully recovered 900 m borehole crosscutting the entire granite bring new insights on plutonic processes. The Beauvoir granite contains early-crystallised euhedral quartz and topaz associated with albite, lepidolite (Li-mica), K-feldspar and late amblygonite (Li-phosphate). Here based on numerous high resolution petrographic data (modal composition, intrusive and layering features, mineral morphologies and textural relations, etc.), and on the variation of lepidolite composition throughout the granite, we demonstrate that the whole intrusion formed through the stacking of at least eighteen decametric crystal-poor sills. Those intrusive bodies form the Beauvoir sub-units that emplaced successively without significant magma mixing with previous injections. Based on structural and geochemical features, we constrain the first relative chronology of a highly-differentiated stacked intrusion with an overall over-accretion mechanism. Once intruded, sill differentiation occurred via fractionation of quartz and topaz producing albite-, lepidolite-, amblygonite-saturated residual liquids, notably enriched in incompatible elements such as Li, Be, F and P. Channel like forming albite-rich segregates, representing escaped residual liquids from the solidifying quartz-rich mush often pounds beneath the overlying subsequently intruded sill, indicating a protracted plutonic construction faster than the solidification of a single sill. Alternatively, such evolved residual melts locally accumulate to form weakly-viscous potentially eruptible melt lenses, which possibly fed the rhyolitic dikes intruding the surrounding host-rocks.