Formation of A-type charnockite and constraints on deep crustal anatexis in early Paleozoic orogen, South China

Charnockite is generally regarded as a product of high-temperature melting; however, its specific origin, generation, and preservation mechanisms, as well as its relationship to high-grade metamorphism or deep crustal reworking, remain poorly constrained. During the early Paleozoic, the South China Block underwent intense orogeny that resulted in significant crustal shortening and thickening, subsequently inducing widespread anatexis and extensive S-type granites. This study identifies ~431 Ma charnockites containing granulitic enclaves that were exposed in the Yunkai massif, providing key insights into the early Paleozoic crustal reworking and deep crustal melting behaviors in South China. The body displays A-type characteristics with crustal reworking zircon isotopic features (δ¹⁸O = 8.0–9.8 ‰; ε_Hf(t) = - 11.5 to - 3.4). The charnockite and its enclaves show identical mineral assemblages and comparable orthopyroxene chemical compositions. The two anhydrous minerals of orthopyroxene and garnet are identified as of peritectic and magmatic origins given their textural features and geochemical compositions. Moreover, petrographic observations and bulk geochemical data argue that the peritectic minerals were derived from the entrainment of their granulitic sources. Crystallization phase modeling indicates orthopyroxene would have been completely hydrated and formed biotite when water contents exceed ∼0.3 wt.% near the solidus. Water-in-zircon analysis and thermodynamic modeling indicate low magma water contents (∼0.15 wt.%; 135 ppm, zircon water medians) for the Gaozhou charnockite from early crystallization to final solidification. CO₂‐rich fluids flushed the charnockite reservoir further contributing to the stabilization of the orthopyroxene. Accordingly, the Yunkai charnockite reveals deep crustal melting processes involving anhydrous minerals entrained in a low-water environment. This low-water environment correlates with high-temperature melting of granulite-facies rocks in the lower crust and the presence of CO₂-rich fluids within the system. Regional magmatic-metamorphic-tectonic data indicate that the formation of the Yunkai A-type charnockite occurred within a post-orogenic extension regime, representing the peak of intracrustal reworking in South China.