The ultrahigh-temperature porphyritic charnockites in the eastern Khondalite Belt, North China Craton: Implications for two-stage of anatexis events

Igneous charnockites are key components of many high-grade metamorphic terranes and provide crucial insights into crustal anatexis and crust-mantle interactions in the deep crust. However, the petrogenesis of ultrahigh-temperature (UHT) peraluminous charnockites—a distinctive subgroup of igneous charnockites—remains poorly understood because of their limited exposure. In this study, we investigate the UHT porphyritic charnockites from the Jining terrane in the eastern Khondalite Belt of the North China Craton, using petrography, mineralogy, geochemistry, geochronology, and EBSD fabric analyses to elucidate their petrogenesis and relationship with UHT metamorphism. The porphyritic charnockites are characterized by 40–50 vol% of partially interlocking, subhedral to anhedral phenocrysts of K-feldspar, plagioclase, and quartz (3–8 mm), commonly exhibiting embayed or subrounded margins. The remaining ∼50–60 vol% finer-grained interstitial matrix are composed of euhedral to subhedral plagioclase, quartz, K-feldspar, and orthopyroxene. Geochemically, these rocks are strongly peraluminous (ASI = 1.09–1.42) and enriched in high ferromagnesian components (TFeO + MgO = 8.28–10.87 wt%). Both macro- and microscopic structures indicate that the porphyritic charnockites underwent significant crystal accumulation and melt loss/extraction processes. Ternary feldspar thermometry reveals that matrix minerals crystallized at higher temperature condition (950–1000 °C) than the early-formed phenocrysts (850–900 °C), reflecting a complex thermal evolution. Zircon UPb dating, combined with previous data, reveals two-stage anatectic events at approximately 1.95 Ga and 1.92 Ga. These results suggest that the porphyritic charnockites formed initially through the accumulation of anatectic magma crystals, where were later remelted under UHT conditions. Consequently, the Jining terrane underwent multistage anatexis, with melt-bearing conditions sustained over tens of millions of years. These findings provide new insights into the processes of crustal anatexis and the evolution of the UHT orogenic lower crust.