Fluid-absent melting of high-grade semi-pelites: P-T constraints on orthopyroxene formation and implications for granulite genesis

The fluid-absent melting behavior of two, natural high-grade semi-pelitic rocks were studied using a piston-cylinder apparatus in the P-T range 7--15 kbar and 800--1050°C. The aim was to constrain P-T conditions required for the formation of orthopyroxene, the metamorphic index mineral marking the transition from the amphibolite to the granulite facies. The starting materials (garnet-biotite-gneiss) were obtained from a transitional amphibolite-granulite terrane in order to simulate closely the transition process in nature. The results indicate that temperatures >875°--900°C are required to stabilize orthopyroxene in rocks of semi-pelitic bulk compositions under fluid-absent conditions at mid-to-lower crustal depths. Such high temperatures conflict with the temperature estimates obtained in transitional amphibolite-granulite terranes using Fe-Mg exchange thermometers, implying either that these thermometers record temperatures much below the peak metamorphic temperature or that regional granulite formation occurs through a process other than fluid-absent melting.