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

Abstract

We studied a high-pressure amphibole-bearing migmatite cropping out along the northeastern coast of Sardinia, a few kilometres northeast of Olbia, in order to improve our knowledge about its evolution using pressure–temperature (P–T) pseudosections. 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. Calculations were conducted for average compositions of the protolith 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 demonstrate that the molar ratios Na/K and Si/Al of the anatectic melt are of high geothermobarometric value. Particularly the melt field in the P–T pseudosection for the protolith, contoured by Si/Al isolines, shows a significant decrease in this ratio with rising pressure. This ratio is only weakly dependent on variable H2O contents of the amphibole-bearing migmatite as demonstrated by contoured 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, assigned as the conditions of partial melting. Evaluation of contoured P–T pseudosections for the leucosome composition resulted in 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 complete crystallization of this melt.