Experimental studies on partial melting of massive samples of granite

Abstract

As a basis of modern petrology, the equilibrium relations describing the melting of granite were established mainly on melting experiments of Powder samples. Such experiments, however, have serious limitations in providing information about the variations in compositional and fabric features of the minerals and in the composition and distribution of the melt. Our experiments using massive samples indicate that melt occurs mainly at the quartz-plagioclase and quartz-potash feldspar boundaries and the composition of the melt is dependent on local characteristics in the melting system, showing no correlation with the bulk composition of the rock samples. At lower temperatures (740 ∼ 760°C, 0.2 GPa), the melt plots at or near the eutectic point in Q-Ab-Or-An-H₂O diagram, indicating equilibrium melting. At higher temperatures (790 ∼ 800°C, 0.2 GPa) the melt becomes lower in SiO₂ and higher in Na₂O, deviating makedly from the eutectic line but without disappearance of any mineral phase, suggesting a non-equilibrium process. It is obvious that the phase-equilibrium relations in natural massive granites may be greatly different from those deduced from powder experiments.