Origin and evolution of volatiles in the Central Europe late Variscan granitoids, using the example of the Strzegom-Sobótka Massif, SW Poland

Abstract

The results of the new Electron Microprobe Analysis of apatite, hornblende and biotite crystals of the hornblende-biotite variety of the Strzegom-Sobótka granite indicate that these rocks experienced several phases during their evolution. During the first phase, the melting of the continental crust was caused by heating from basaltic magma. Nevertheless, the system was closed, and there is no evidence of the influence of any mafic component. The volatile compositions of apatite crystals recorded the signature of this phase. Afterwards, small quantities of mafic material were delivered into the crystallizing melt by ascending basaltic magma. The volatile composition of hornblende, together with dramatic changes in the F/Cl ratios between apatite and hornblende, indicates that the injection of mafic material occurred between the crystallization of both minerals. During the crystallization of hornblende, the system was closed. During the last episode, the volatile composition recorded by biotite indicates that the system opened again. This is represented by the decrease of Cl in the melt. Moreover, the estimated F concentrations in the melt range from ~2000 to ~3000 ppm and do not change significantly within the crystallization interval, whereas the Cl concentrations decreased from 1470-900 ppm at 829-768 °C to as low as 100 ppm at 710-650 °C, most likely due to the continuous exsolution of aqueous fluid from silicate melt or the crystallization of Cl-bearing minerals.