The Paleoproterozoic ultrahigh-temperature metamorphic evolution of Alxa Block, North China Craton: Insights from phase equilibria modeling, garnet trace element chemistry, and monazite geochronology

Abstract Ultrahigh-temperature (UHT) granulites offer valuable insights into the extreme thermal and tectonic evolution of orogenic belts. However, pervasive diffusive re-equilibration, accessory mineral dissolution, and post-peak recrystallization under sustained UHT conditions typically obliterate peak petrographic, mineralogical, and geochronological signatures in these granulites. To address this preservation challenge, we integrate petrography, phase equilibrium modeling, garnet trace elements, and in situ monazite U-Pb geochronology for a pelitic granulite with a characteristic mineral assemblage of garnet + spinel in the Alxa Block of the western Khondalite Belt, North China Craton. Our work constrains the peak pressure-temperature (P-T) conditions of ~990–1000 °C/8.4–9.4 kbar, with a near isobaric cooling retrograde P-T path for the pelitic granulite, which reveals two-stage garnet growth including a prograde peritectic and retrograde cooling phase. By establishing the growth sequence between monazite and garnet microdomains, we obtained the near-UHT metamorphic age of 1916 Ma from the inclusion-type monazites entrapped within garnet, shielding monazite’s chronometers from resetting and dissolution. The findings from this study, when integrated with regional metamorphic data, reveal a protracted suprasolidus heating duration of ~40 m.y., followed by retrograde cooling, which persisted for >100 m.y. These findings indicate prolonged extreme thermal regimes in the lower crust and ultrahot collisional orogenic processes in the Khondalite Belt during the Paleoproterozoic.