Phase equilibria modeling of anatexis during ultra-high temperature metamorphism of the crust

Ultra-high temperature (UHT) granulite-facies metamorphism is commonly identified in Mg–Al-rich rocks. Many such lithologies are thought to be metasedimentary rocks based on the presence of detrital zircon. However, whether a metasedimentary protolith is required to form diagnostic mineral assemblages and how much melt is lost during burial and heating to reach UHT conditions varies significantly according to the prograde pressure–temperature path. In this study, integrated thermodynamic modeling, accessory mineral modeling, and trace element modeling has been conducted on an average metapelite composition and average mid-ocean ridge basalt (MORB) composition under open-system conditions. The results show at least three and two melt loss events occur before reaching UHT conditions in initially water-saturated pelitic and basaltic rocks, respectively. Around 22–27 vol% S-type granite and 12–17 vol% I-type granite would be produced during these evolutions, respectively. Melt extraction leads to an increase of Al and Mg in metapelite residua. Mineral assemblage sapphirine + quartz is then allowed to form at higher grades; however, metabasalt does not form these UHT diagnostic assemblages, regardless of the amount of melt lost. During equilibrium melting, crustal differentiation induced by UHT metamorphism would significantly reduce the amount of heat-producing elements (HPEs) in the melt-depleted residuum, which progressively decreases a rock's heat production capacity. In reality, however, new monazite grains likely form during peak metamorphism and some monazite grains would be shielded by porphyroblasts, resulting in an increase of Th in the residua during heating. Even so, we suggest the efficiency of heat production would still decrease, given the heat production rate of Th is much lower than U. Considering some granitic rocks with relatively high heat production are emplaced into the granulite terrane, we suggest that radioactive heat production may be a contributing driving force for UHT metamorphism; however, it is not sufficient. Most heat required to generate UHT granulites must come from hybrid sources, such as advected heat from the mantle, conducted heat from nearby magmatic intrusions, radioactive heat production, and ductile shear deformation