Low-pressure ultrahigh-temperature (LP-UHT) metamorphism of pelitic and mafic granulites during postcollisional extension in the Eastern Kunlun domain, West China

Identifying ultrahigh-temperature (UHT) metamorphism in Fe-rich metapelites is challenging and commonly overlooked due to the lack of diagnostic mineral assemblages, but it is significant for understanding the areal extent and thermal-mechanical evolution of UHT metamorphic terranes. This study addresses this gap through comparative investigations on petrography, geothermobarometry, phase equilibrium modeling, and zircon U-Pb dating between Fe-rich pelitic gneiss and mafic granulite samples from the Eastern Kunlun domain, West China. Three stages of metamorphic evolution are recognized from both rock types. (1) The peak-pressure (Pmax) upper amphibolite to high-pressure granulite-facies stage (0.8−1.1 GPa, 740−850 °C) is recorded by the core compositions of garnet-plagioclase and biotite and amphibole inclusions. (2) The post-Pmax decompressional heating to the peak-temperature (Tmax) LP-UHT granulite-facies stage is inferred from the core-to-rim decreasing grossular component (XGrs) in garnet and core-to-rim or mantle increasing anorthite component (XAn) in plagioclase. The Tmax conditions (∼930−980 °C, 0.4−0.6 GPa) are well constrained in the mafic granulites based on thermobarometry results, high titanium (XTi) in amphibole and XAn in plagioclase from pressure-temperature (P-T) pseudosections, or T-XMelt (melt fractions) pseudosections specifically calculated for the sample (18CB126) that contained residual melt. In contrast, the pelitic gneisses mainly document lower Tmax conditions of 850−870 °C except for the rare records (up to ∼930−950 °C) from high-Ti quartz and the stability field of the mineral assemblage of cordierite + spinel + perthite + plagioclase + quartz in local domains from the inhomogeneous sample (21DK71). (3) The late post-Tmax near-isobaric cooling evolution is represented by the solidus assemblages, giving P-T conditions of ∼0.4−0.6 GPa and 750−810 °C. Zircon U-Pb dating from the pelitic gneisses yields two main metamorphic ages of ca. 436 Ma and ca. 408−402 Ma, which are interpreted to represent the Pmax and post-Tmax cooling stages, respectively. Our findings confirm the regional-scale occurrence of low-pressure UHT metamorphism in the Eastern Kunlun domain, and the recovered metamorphic P-T-time paths reveal a tectonic transition from earlier continental collisional thickening at ca. 436 Ma to the postcollisional extension before 408−402 Ma, where the UHT metamorphism is developed during the crustal thinning along with coeval asthenospheric upwelling related to possible lithosphere delamination.