Experimental crystallization of the Beauvoir granite as a model for the evolution of Variscan rare metal magmas
Résumé
Abstract New experiments on the strongly peraluminous, Li-, P- and F-rich Beauvoir granite (Massif Central, France) provide a basis for understanding crystallization and chemical fractionation in Variscan rare metal magmas. Crystallization experiments were performed on two natural granite compositions under H2O-saturated conditions at 100, 200 and 300 MPa, from 540°C to 700°C and between ~NNO+3 and NNO-1.4. Experimental charges were examined by SEM and their products (glasses and crystals) analyzed for major elements by EMPA. Trace element concentrations in selected glasses were also determined by LA ICP-MS. Despite experimental durations commonly exceeding 1000 h and some up to 4000 h, kinetic problems were encountered in particular in the 100 MPa charges whereas, at 200 and 300 MPa, results consistent with previous melting experiments were obtained. Beauvoir melts crystallize quartz, plagioclase, K-feldspar and mica as major phases. At NNO-1.4, mica is a biotite whereas it is a Li-mica between ~NNO+3 and NNO-1. Apatite, Fe-Ti oxides, either hematite or magnetite, topaz, amblygonite, cassiterite and columbite-tantalite appear as accessory phases between ~NNO+3 and NNO+1. Experimental plagioclases are albitic (An < 4.5 mol%) and more Ca and K-rich than natural albites in the granite whereas experimental K-feldspars are more sodic (Ab < 45 mol%) than the natural crystals. The less evolved starting melt crystallized Li phengites whereas the most evolved yielded Li-, F-rich micas near the polylithionite-zinnwaldite series, similar to natural micas in the granite. Equilibrium crystallization increases A/CNK, F and P and concentrates Li, Be, B, Rb, Cs, W, U in the melt. Nb and Ta are also enriched, their behavior being controlled by the solubility of columbite-tantalite in the melt. Other elements are either unchanged (Mn, Zn, Ti) or depleted (Sr, Pb) during magmatic fractionation. Sn is concentrated in Li-mica and hematite and it behaves compatibly at high fO2. Beauvoir melts crystallize at very low temperatures, below 670°C for the two compositions studied and solidus temperatures, determined from previous melting experiments and confirmed by the new crystallization experiments, are near 550°C. The experiments demonstrate that most of the mineralogical and geochemical characteristics that make the Beauvoir granite distinctive result from magmatic rather than hydrothermal post-magmatic processes. Albitic plagioclase, Li-mica, topaz and amblygonite are of magmatic origin. Glass major element compositions suggest that the two granite samples represent crystallized liquids. Trace element fractionations for most elements at Beauvoir can be accounted for by magmatic crystallization-differentiation processes. Implications for the mineralogy, fO2, volatile concentrations, crystallization and conditions of emplacement, fractionation mechanisms and origin of the Beauvoir granite are discussed.
Origine : Fichiers produits par l'(les) auteur(s)