Paleoproterozoic granulites from Helingʹer: Implications for regional ultrahigh-temperature metamorphism in the North China Craton

The Inner Mongolia Suture Zone (IMSZ, incorporating the Khondalite Belt) forms part of an accretionary orogen developed during the Paleoproterozoic collision between the Yinshan Block and Ordos Blocks within the North China Craton (NCC). The granulites within the IMSZ are characterized by high-temperature (HT) to ultrahigh-temperature (UHT) metamorphism with diagnostic mineral assemblages for UHT conditions preserved in some localities such as Tuguiwula and Wuchuan. We report here a new locality in Helingʹer, located within the southern domain of the IMSZ where spinel–garnet–sillimanite-bearing granulites record UHT metamorphic conditions. The prograde assemblages occurring as inclusions in porphyroblastic minerals in these rocks include sillimanite + quartz + spinel, whereas coarse grained garnet + sillimanite + quartz + spinel + cordierite + plagioclase + K-feldspar + magnetite + ilmenite represent the peak assemblages. The retrograde stage is marked by orthopyroxene and cordierite rims developed on garnet and spinel. Biotite marks the final hydration stage of the high-grade metamorphism. Spinel and quartz occur in direct contact, with the spinel showing low Zn, Cr, and Fe3+. Application of Ti-in-zircon and ternary feldspar geothermometers yields temperatures up to ~ 940 °C for the peak stage. Mineral phase equilibria modeling based on the bulk chemistry and mineral assemblages shows that the peak assemblage of garnet + plagioclase + spinel + cordierite + sillimanite + quartz + ilmenite + magnetite was stabilized at 1030 °C at 6.5–7.5 kbar, thus we infer a 940–1030 °C temperature range for peak metamorphism in these rocks. The newly discovered UHT rocks in Helingʹer, dated as 1.92 Ga, together with the two previously reported UHT localities at Tuguiwula and Wuchuan, suggest that the Paleoproterozoic UHT metamorphism occurred on a regional scale within the IMSZ, probably in association with the subduction–accretion–collision tectonics and the incorporation of the NCC within the Columbia supercontinent.