Identification of Neoarchean UHT (Ultrahigh-Temperature) Metamorphism From Intermediate Migmatitic Granulites in the Yinshan Block, North China Craton

Identifying UHT metamorphism in intermediate granulites is challenging due to the lack of diagnostic mineral assemblages. The Yinshan Block from the North China Craton (NCC) experienced granulite-facies metamorphism during the Neoarchean, but the P–T paths and tectonic regimes remain debated. In this paper, we conduct a comprehensive study of petrography, mineral chemistry, phase equilibrium modelling, and zircon dating for two intermediate migmatitic granulites from the Xiwulanbulang area, and for the first time, report UHT metamorphism from the Yinshan Block. The migmatitic granulites include mesosomes and leucosomes. The mesosomes contain two-pyroxene granulite-facies assemblages (Opx + Cpx + Pl + Amp + Qz) and can further be subdivided into amphibole-bearing residues and small former melt pockets. Multiple thermobarometers and phase equilibrium modelling suggest that the two samples record UHT peak conditions of 1000–1050 °C/8–9 kbar, with possible anticlockwise P–T paths dominated by the post-peak near-isobaric cooling. Phase equilibria modelling also suggests that the isopleths of the Al^T (Cpx) (tetrahedral Al in clinopyroxene), X_An (Pl) (anorthite content in plagioclase), and Ti (Amp) (Ti content in amphibole) in P–T pseudosections are potentially useful for defining the peak temperatures of two-pyroxene granulites. However, the Al^T (Cpx) isopleths are highly dependent on the bulk-rock O (Fe₂O₃) content in the two-pyroxene granulite-facies assemblages without amphibole, and care should be taken in using them to determine temperatures. Moreover, T–X_Melt pseudosections indicate that the amphibole-bearing residues and melt pockets preserve distinct aspects of the metamorphic evolution. The residues tend to record UHT solidi at 960–1000 °C, where amphibole can be stable above 1000 °C, making Ti (Amp) a reliable indicator for constraining the lower limit of the peak temperature; while the melt pockets record HT solidi likely at ~850 °C, with amphibole stable just above the solidi. Zircon dating yields a continuous metamorphic age spectrum of 2.55–2.47 Ga, approximately coeval with the regional magmatism. Combining other geologic data from the Yinshan Block, we prefer a sagduction model of vertical tectonics to account for the UHT metamorphism.