Metamorphic evidence of the Kuunga orogeny in South China: High-pressure pelitic granulites and gneisses from the Gaozhou Complex

The role of the South China Block in the Kuunga orogeny, a pivotal event marking the assembly of the Gondwana supercontinent, remains a subject of debate. This study investigates high-pressure (high-P) granulite-facies metamorphism in the Yunkai orogen of eastern South China to shed light on this controversy. Recently, high-P pelitic granulites with a mineral assemblage of garnet, K-feldspar, and sillimanite pseudomorph replacing earlier kyanite were identified in the Gaozhou Complex. Petrographic observations, phase equilibria modeling, geothermobarometry, and laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) zircon U-Pb dating have revealed a four-stage metamorphic evolution. The peak P stage (M1) is characterized by high-P conditions of 10−11.8 kbar/760−830 °C in the kyanite and rutile stability fields. This is followed by the peak temperature (T) metamorphism (M2) at slightly lower pressures and higher temperatures of 7.8−9.5 kbar/840−870 °C, which suggests a subsequent period of thermal relaxation. Subsequent decompression and cooling (M3) led to the formation of cordierite + spinel coronae, which reflects a change in P-T conditions to 4.5−5.3 kbar/730−790 °C. The final retrogression (M4) occurred under lower-grade conditions of 3.7−4.4 kbar/600−640 °C. Consequently, high-P pelitic granulites in this region have undergone a clockwise P-T path, which indicates a continental collision setting. Zircon U-Pb dating from the high-P granulites and gneisses yielded multistage metamorphic ages of ca. 520 Ma, ca. 440 Ma, and ca. 240 Ma, which correspond to the Pan-African, Caledonian, and Indosinian tectono-thermal events, respectively. These metamorphic ages, coupled with the clockwise P-T path, reveal a history of polymetamorphism associated with a long-lived subduction−continental collision event during the assembly of Gondwana and the subsequent Indosinian overprinting. These multiple orogenic processes provide significant insights into the tectonic evolution of the South China Block. Our findings contribute to the evidence of the Kuunga orogeny in South China during the assembly of Gondwana and offer a robust framework for interpreting the complex metamorphic histories of orogenic belts.