Small pluton construction by the stacking and differentiation of magma sheets: Insights from the Beauvoir granite (Massif Central, France)

Abstract

Constraining the links between the assembly of granitic intrusions and their magmatic differentiation processes remains a major challenge in igneous petrology. Although it has been suggested that most granitic intrusions are emplaced incrementally, the assembly processes that ultimately form highly differentiated granites that commonly host mineralisation remain to be identified. To tackle this issue, we focus on petrographic and geochemical analyses of drill core samples from the Beauvoir intrusion (Massif Central, France), a small, ~800 m thick, Li-Be-Nb-Ta mineralised pluton that is transected by a 900 m long borehole, to provide new insights on pluton assembly and differentiation processes. The Beauvoir granite contains early-crystallised quartz and topaz associated with albite, lepidolite (Li-mica), K-feldspar and late amblygonite (Li-phosphate). Based on new petrographic data (mineral morphologies and textural relationships) and on the chemical variations of lepidolite throughout the granite, we demonstrate that the pluton formed by the stacking of at least eighteen decametric, crystal-poor, sill-like magma sheets (referred as sub-units), without significant magma mixing between each of these injections. From the use of geochemical models to explain the different compositions of these sub-units, we constrain the relative chronology of this highly-differentiated, stacked intrusion with an overall over-accretion mechanism. The recognition of internal magmatic contacts within sub-units, as well as the ubiquitous presence of reversely and oscillatory zoned minerals throughout the granite, strongly suggest that each sub-unit grew from multiple, compositionally similar, small magma injections. Variations in the time-averaged magma emplacement rate dictated whether contacts formed, or whether the new batch mixed with the host magma. Once intruded, the early fractionation of quartz and topaz produced albite-, lepidolite-, amblygonite-saturated residual melts that were enriched in incompatible elements such as Li, Be, F and P. As recorded by the presence of channel-like albite-rich segregates representing fossilised liquid pathways, these low-viscosity residual liquids were partially extracted from their quartz-rich mush counterparts, eventually forming small pockets of eruptible magmas. Ultimately, these pockets may have been destabilised during host-rock fracturing events, providing a viable mechanism for the ascent and emplacement of rhyolitic and/or pegmatitic dikes within the surrounding host-rocks.