PHASE RELATIONS OF HIGH-GRADE AMPHIBOLITES REVISITED: EXPERIMENTAL CONSTRAINTS ON THE STABILITY OF GARNET-CLINOPYROXENE-PLAGIOCLASE AND ORTHOPYROXENE-CLINOPYROXENE-PLAGIOCLASE ASSEMBLAGES UNDER FLUID-ABSENT CONDITIONS

High-grade metabasites are characterized by Opx-Cpx-Pl ± Grt assemblages at low and intermediate pressure, and Opx-free Grt-Cpx-Pl assemblages at high pressure (e.g. Pattison, 2003). The former assemblage is type granulite facies whereas the latter has been categorized as either high-P granulite or amphibolite facies. Uncertainties in the fluid-absent phase relations of metabasites have hampered reconciling data from experimental studies with data from natural assemblages. One way to circumvent this problem is to utilize experimental starting materials from transitional amphibolite-granulite terranes where the targeted phase transitions are documented. We performed fluid-absent melting experiments in the P-T range 7-15 kbar and 775–1050°C on two Qtz-bearing amphibolite samples collected from the transitional amphibolite-granulite terranes of Three Valley Gap, British Columbia and Kapuskasing, Ontario. The two samples have similar mineralogy and bulk composition except for a higher Fe/Mg in the Kapuskasing sample. Our results indicate that temperatures of at least 775 and 830°C are required to stabilize Grt and Opx, respectively, by fluid-absent melting. Formation of Opx is favored at lower temperatures in more Mg-rich compositions whereas Fe-rich compositions favor Grt formation. There is only a limited P-T range where Grt and Opx coexist. Our results are consistent with previous experimental studies indicating that a minimum pressure of ~10 kbar is required for stabilization of Grt in high-grade metabasites. However, our study indicates a different topology for the super-solidus Grt-forming reaction Hbl+Pl+Qtz=Grt+Cpx+Melt. Rather than a poorly constrained near vertical dP/dT slope suggested by earlier experimental studies, our tightly bracketed phase reversal experiments require a distinctly negative slope for the reaction. This implies that the high-P Grt-Cpx-Pl assemblage is stable at significantly lower temperatures than the ‘classic’ Opx-Cpx-Pl granulite assemblage, a result that is more consistent with thermodynamic calculations and thermobarometric data from metamorphic terrains.