FORMATION OF EXTREMELY F-RICH HYDROUS MELT FRACTIONS AND HYDROTHERMAL FLUIDS DURING DIFFERENTIATION OF HIGHLY EVOLVED TIN-GRANITE MAGMAS: A MELT/FLUID-INCLUSION STUDY

Abstract

Quartz crystals from topaz–zinnwaldite–albite granites from Zinnwald (Erzgebirge, Germany) contain, in addition to primary and secondary fluid inclusions (FIs), abundant crystalline silicate-melt inclusions (MIs) with diameters up to 200m. These MIs represent various stages of evolution of a highly evolved melt system that finally gave rise to granite-related Sn–W mineralization. The combination of special experimental techniques with confocal laser Raman-microprobe spectroscopy and EMPA permits precise measurement of elevated contents of H2O, F, and B in re-homogenized MIs. The contents of H2O and F were observed to increase from 3 to 30 and 1.9 to 6.4wt%, respectively, during magma differentiation. However, there is a second MI group, very rich in H2O, with values up to 55wt% H2O and an F concentration of approximately 3wt%. Ongoing enrichment of volatiles H2O, F, B, and Cl and of Cs and Rb can be explained in terms of magma differentiation triggered by fractional crystallization and thus, is suggested to reflect elemental abundances in natural magmas, and not boundary-layer melts. Partitioning between melt and cogenetic fluids has further modified the magmatic concentrations of some elements, particularly Sn. The coexistence of two types of MIs with primary FIs indicates fluid saturation early in the history of magma crystallization, connected with a continuous sequestration of Sn, F, and B. The results of this study provide additional evidence for the extraordinary importance of the interplay of H2O, F, and B in the enrichment of Sn during magma differentiation by decreasing the viscosity of and increasing the diffusivity in the melts as well as by the formation of various stable fluoride complexes in the melt and coexisting fluid.