Plutonic construction and residual liquid segregation: Insights from the Beauvoir granite mineral record

Constraining the assembly and evolution of granitic intrusions and the kinematics associated with their magmatic differentiation remain a major objective in igneous petrology. To progress on those issues, we took advantage of a 900m long drilled core of the Beauvoir granite, a rare metal granite intrusion from the French Massif Central, allowing a high-resolution sampling of a fully recovered plutonic body. Based on structural data, and high-resolution major and trace element composition of the mineral phases (in-situ measurements and chemical map) associated with cathodoluminescence imaging, we provide constraints on the differentiation processes and its dynamics during the construction of the intrusion. Mostly based on mineral composition and morphologies, we show that the granite formed via the stacking of deca- to hectometric crystal-poor sills, corresponding to the different sub-units of the Beauvoir granite. Furthermore, the detailed study of sill boundaries 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 partly crystallized sub-units.

Once intruded, early crystallised quartz and topaz will be accumulated at the base of their sills while residual melts progressively differentiate, that is recorded by the progressive metal (e.g., Li, Be) and fluxes (e.g., F and P) enrichment from bottom to the top of each sub-units. Textural and mineralogical evidences suggest an efficient extraction of these residual melts from the quartz-rich mush as these residual liquids are under the form of segregates dominated by albite, lepidolite (Li-mica) and amblygonite (Li-phosphate). These segregates can accumulate below an overlying sill or percolate through the upper solidification front of their sill, developing typical dendritic morphologies. In addition, these residual melts have also been observed as fragmented and dismembered mm to cm albite-rich ovoids, representing mixing features between two magmas. Such processes of residual melts collection and segregation contributed to the global enrichment in rare metals observed towards the top of the granite. This study eventually provides fundamental constraints on the processes leading to the construction and subsequent magma differentiation and melt extraction in rare metal granite bodies.