Ultrahigh‐temperature metamorphism in the Tuguiwula area, Khondalite Belt, North China Craton

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AbstractIn this study, sapphirine‐bearing granulites and sapphirine‐absent garnet–sillimanite gneisses from the Tuguiwula area in the eastern segment of the Khondalite Belt, North China Craton (NCC) are interpreted to show a P–T evolution involving cooling at pressures of 8–9 kbar from >960°C to the solidus (~820°C) and late subsolidus decompression. This interpretation is based on the sequence of mineral appearance and thermodynamic modelling of phase equilibria. Sapphirine is observed to coexist with spinel within the peak assemblages. This observation conflicts with the traditional view that spinel generally appears prior to sapphirine and thus indicates pre‐Tmax compression. For ultrahigh‐temperature (UHT) metapelites at Tuguiwula, a clockwise P–T path may be more likely, which would be consistent with the clockwise P–T evolution of the extensive “normal” granulites (Tmax <900°C) and UHT granulites at other localities in the eastern segment of the Khondalite Belt. At Tuguiwula, for UHT metapelites with low bulk‐rock Mg/(Mg+FeT), the oxidation state/Fe3+ content is interpreted to be a significant factor in controlling the mineral assemblages. We find that these compositions tend to contain sapphirine under oxidized conditions but spinel (without sapphirine) under reduced conditions. This difference may account for the simultaneous presence of both sapphirine‐bearing UHT granulites and sapphirine‐absent garnet–sillimanite UHT gneisses at Tuguiwula. LA‐ICP‐MS U–Pb dating of metamorphic zircon in the UHT metapelites yields mean 207Pb/206Pb ages of c. 1.92 Ga (two samples), which are interpreted to record the timing of cooling of the UHT rocks to the solidus. The UHT metamorphism is interpreted to have been generated by mantle upwelling and emplacement of mafic magmas within a post‐orogenic setting.