Assessing the emplacement and differentiation processes in highly differentiated magmas through the study of Nb-Ta oxides (CT). Insights from the Beauvoir rare-metal granite (Massif Central, France)

Understanding the magma storage and differentiation processes that lead to the formation of small, highly differentiated granites is essential for constraining how evolved silicic magma reservoirs are assembled and differentiated, as well as how associated mineral ores form. Once saturated in rare-metals (e.g., Sn, Nb, Ta), these magmas start to crystallise metal-bearing minerals such as cassiterite (SnO₂) or columbo-tantalite (Mn,Fe)(Nb,Ta)₂O₆. Since the stoichiometry of columbo-tantalite (CT) allows two major substitutions, the CT composition will vary with melt composition. Therefore, the detailed study of these minerals can provide strong insights into the mechanisms related to emplacement, differentiation, and mineralisation processes in highly differentiated magma reservoirs.

To better understand how evolved, upper crustal, mineralised granites form, we investigated the internal texture and chemical composition of CT from the Beauvoir rare-metal granite (Massif Central, France). By combining chemical mapping and electron probe microanalysis of CT from various samples throughout the 900 m deep borehole that intersect the Beauvoir granite, we demonstrate that the CT composition can be used as a proxy of magma differentiation. More specifically, systematic variations in CT Mn* (Mn/Mn+Fe in atomic prop.) throughout the granite indicate that several compositionally distinct magma batches were involved during the construction of the Beauvoir granite. As these results are consistent with those obtained with lepidolite¹, we show for the first time that the CT composition can be used to identify several episodes of magma sheet stacking during the construction of a rare-metal granite body.

Once emplaced, the differentiation of these magma sheets is recorded by an increase in CT Ta* (Ta/Ta+Nb in atomic prop.), which mimics the progressive increase of Ta content in residual melts. The differentiation trends recorded by CT are also sensitive to the nature of the crystallising phases, and especially minerals competing for Fe. Notably, we show that when Fe-rich lepidolite crystallises, the CT’s Mn* remains stable, while it decreases with magma differentiation when CT is controlling the melt Fe budget. It reflects the lower solubility of the Mn-CT compared to Fe-CT in peraluminous magmas.

Overall, this study demonstrates that CT effectively records magma compositional evolution (differentiation and recharges) during rare-metal granite emplacement. These results highlight the potential of CT as tracers of magma plumbing processes and metal enrichment, with broader applicability to cogenetic granite–pegmatite systems.

• Esteves, N. (2025). From melt to pluton‎: Magmatic emplacement, differentiation & duration of the Beauvoir rare-metal granite(Doctoral dissertation, Université de Lorraine).