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

Abstract

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) granulitefacies metamorphism in the Yunkai orogen of eastern South China to shed light on this controversy. Recently,highP 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 (LAICP-MS) zircon U-Pb dating have revealed a four-stage metamorphic evolution. The peak P stage (M1) is characterized by highP conditions of 10-11.8 kbar/760-830 degrees 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 degrees 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 degrees C. The final retrogression (M4) occurred under lower-grade conditions of 3.7-4.4 kbar/600-640 degrees C. Consequently,highP pelitic granulites in this region have undergone a clockwise P-T path, which indicates a continental collision setting. Zircon U-Pb dating from thehighP 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 longlived 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