Paleozoic peralkaline A-type magmatism of the Tongbai Orogen, Central China: Petrogenesis and tectonic implications

Abstract

The precise timing, petrogenesis, and geodynamic significance of the Paleozoic magmatic rocks in the southern Qinling–Tongbai Orogenic Belt (QTOB), Central China, are poorly constrained. We performed an integrated study of petrology, zircon UPb ages, geochemistry, and Hf–O–Nd isotopic compositions of the Huangyangshan pluton from the southern QTOB to constrain its genesis and tectonic implications. A granite sample from the pluton yields a weighted mean zircon 206Pb/238U age of 445 ± 6 Ma, representing the age of the pluton. The Huangyangshan pluton is dominated by alkaline granite and quartz syenite, has a peralkaline signature (A/NK = 0.82–0.96), and belongs to the shoshonite and high-K calc-alkaline series. Both the alkaline granite and quartz syenite have geochemical signatures similar to A-type granite, with high SiO2 and total alkali (Na2O + K2O) contents, high Fe-numbers and TiO2/MgO and Ga/Al ratios, and high field-strength elements (HFSEs), along with low MgO, CaO, and P2O5 contents. These samples are enriched in light rare earth elements (LREEs) and have flat heavy rare earth element (HREE) patterns and strongly negative Eu anomalies. They are enriched in most large-ion lithophile elements (LILEs) and HFSEs, with significant negative Eu, Ba, Sr, and Ti anomalies in primitive-mantle-normalized trace-element diagrams. The samples have relatively high Zr contents (175–776 ppm), yielding minimum magmatic temperatures of 768 °C–891 °C. The Huangyangshan pluton has initial zircon εHf(t) values of +6.6 to +10.0 and whole-rock εNd(t) values of +2.86 to +3.62. The analyzed zircons yield a mean δ18O value of 5.14‰ ± 0.26‰, indicating negligible incorporation of hydrothermally altered crustal materials. The geochemical signatures of the Huangyangshan pluton indicate that the magmas were derived from prompt partial melting of juvenile crust rocks. Combined with previous results, we suggest that the Paleozoic Huangyangshan peralkaline A-type granites resulted from the rifting of a passive continental margin in the northern South China Block, followed by the opening of the Paleotethyan Ocean.