Generation of the charnockite by residua entrainment in water-deficient melts: insights from minerals composition and H2O-in-zircon

Charnockite is an orthopyroxene-bearing felsic rock and an important constituent of the deep crust, thus, it holds significant importance in uncovering crustal growth and differentiation mechanisms. However, its generation and preservation remain debated. By focusing on mineral records, we here aim to provide a more detailed understanding of these issues. The Gaozhou charnockite in the Yunkai terrane of South China was chosen for extensive mineralogical studies and thermodynamic modeling. The Gaozhou charnockite contains granulitic enclaves, and mineral assemblages within charnockite can be divided into three phases, with the peak phase mainly composed of orthopyroxene, and minor biotite. Meanwhile, the charnockite and enclaves show comparable compositions of biotite and orthopyroxene. Embayed textures and high TiO₂ content in biotite grains suggest high-temperature anatexis. On the other hand, the orthopyroxenes with inclusions and high Al₂O₃ content indicate peritectic origin. Moreover, reaction intergrowths of biotite with orthopyroxene grains have also been observed, suggesting that these grains were generated by the consumption of biotite. The Gaozhou charnockite has much lower zircon water content (135 ppm, median) as compared to that of the contemporary Opx-free granites (202-643 ppm, medians) in the Yunkai terrane, indicating dry primary parental melts. In addition, phase equilibria modeling constraints peak anatexis conditions at 860-870 ℃/6.2-7.0 kbar. Peritectic orthopyroxenes must have been generated by the incongruent melting of biotite under high-temperature granulitic facies in the lower crust. Subsequently, these grains were entrained and migrated by low water content melts to the upper crust. Therefore, overall observations favor a model of selective entrainment of source materials at a low magma water environment for the generation of the Gaozhou charnockite.