Petrogenetic mechanism of Neoarchean charnockitic gneiss in Tangtu, northern Liaoning Province

Charnockites (orthopyroxene-bearing granitoids) are a type of granitic rock with unique mineral compositions, usually genetically connected with Precambrian TTG gneisses and granulite high-grade terranes. Therefore, the study of charnockite is of great significance for understanding the growth, stability and evolution of early continental crust. This study focuses on charnockitic gneiss, whose protolith is charnockite. The origin and tectonic background of charnockite in Tangtu of northern Liaoning Province are discussed through micropetrography, zircon U-Pb dating, mineral chemistry, whole-rock geochemical analysis and other studies. Charnockitic gneisses is mainly composed of plagioclase, microcline, quartz, biotite, hornblende, orthopyroxene and other minerals. The content of SiO₂ in charnockitic gneiss samples ranges from 63.00% to 67.16%. Al₂O₃ content is 14.74% ~15.92%, with enrichment in Na and depletion in K (K₂O/Na₂O =0.45~1.10), δEu exhibits weak negative anomalies to positive anomalies(δEu=0.82~2.37), with relatively enriched large ion lithophile elements (Ba, Rb, etc.) and high field strength elements (Zr, Hf, etc.). The Sr/Y and (La/Yb)_N ratios vary significantly(Sr/Y=15.51~100.20. (La/Yb)_N =8.90~22.06), indicating the presence of two rock types of charnockitic gneisses in this area. Samples with Sr/Y values ranging from 15.51 to 18.42 and (La/Yb)_N values ranging from 8.90 to 12.78 correspond to tonalites, while those with Sr/Y values ranging from 64.54 to 100.20 and (La/Yb)_N values ranging from 20.37 to 22.06 correspond to granodiorites. The geochemical diversity may result from variances in melting depths. The relatively high contents of MgO, Cr and Ni in the rocks indicate potential crust-mantle interactions. The mineral chemistry study shows that all hypersthenes are metamorphic orthopyroxenes, plagioclases include oligoclase and andesine (X_An=0.24-0.32), microclines are orthoclases (X_Or=0.86-0.94, X_Ab=0.06-0.13). Biotite exists in two stages: brown biotite at the peak and green biotite during retrogressive metamorphism. Hornblende mainly surrounds orthopyroxene, which is the degeneration product of the orthopyroxene, and its composition belongs to the magnesiohornblende and tschermakitic hornblende within the calcium hornblende. Zircon U-Pb dating shows that the charnockite formed at 2589±11Ma, with the metamorphic age of 2532.9±9.1Ma. According to previous studies, the protolith of Tangtu charnockite in northern Liaoning Province was formed in subduction tectonic setting and was the product of partial melting of juvenile mafic lower crust.