Experimental Crystallization of the Beauvoir Granite as a Model for the Evolution of Variscan Rare Metal Magmas

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