Geothermobarometry on anatectic melts – a high-pressure Variscan migmatite from northeast Sardinia

A high-pressure amphibole-bearing migmatite occuring at the northeastern coast of Sardinia, a few kilometres northeast of the town of Olbia, was studied to improve our knowledge about its evolution using P-T pseudosections. For this purpose, thermodynamic calculations with PERPLE_X were undertaken in the system Na2O-K2O-CaO-FeO-MnO-MgO-Al2O3-TiO2-SiO2-H2O using a haplogranitic melt model. The calculations were achieved for average compositions of the mesosome and the tonalitic leucosome with different contents of H2O to construct various pseudosections in the P-T range 2-20 kbar and 550-900 °C. We can demonstrate that the molar ratios Na/K and Si/Al of the anatectic melt are of high geothermobarometric value. Particularly the contouring of the melt field in the P-T pseudosection for the mesosome by isopleths for constant Si/Al shows a significant decrease of this ratio with rising pressure. This ratio is hardly dependent on variable H2O contents of the amphibole-bearing migmatite as demonstrated by the contouring of T-H2O pseudosections. The application of the Na/K and Si/Al ratios in melt yielded P-T conditions close to 13 kbar and 700°C which were assigned to the migmatization event. The evaluation of contoured P-T pseudosections for the leucosome composition resulted in P-T conditions of about 10.5 kbar and 700°C for the crystallization of amphibole in the leucosome melt and 9 kbar and 680°C for the final crystallization of the melt pointing to a pressure release of the rock at slightly decreasing temperatures.