Petrogenesis and geodynamic implications of the Baimashan granitic complex in central Hunan, South China

Abstract

The Baimashan granitic complex in central Hunan is a giant multiphase complex pluton within the South China Plate, which contains three phases (the Shuiche, Longtan‐Xiaoshajiang, and Longcangwan superunits). It is located in the Jiangnan Orogenic Belt and is of great significance for the study of the regional tectonic magmatic evolution. Herein, we utilized zircon U–Pb dating, major and trace element geochemistry, and zircon Hf isotope geochemistry for three of the Baimashan granitic complexes; data for the Longcangwan superunit were obtained from previous studies. The new zircon U–Pb ages of 418 ± 3.2 Ma and 223 ± 2.6 Ma for the Shuiche and Longtan‐Xiaoshajiang superunits, respectively, suggest the existence of Late Silurian and Late Triassic magmatism. Both superunits exhibited weak peraluminosity and high‐K calc‐alkaline and weak negative Eu anomalies, which are similar to those of I‐type granites. The Longcangwan superunit showed strong peraluminosity and high‐K calc‐alkaline and stronger negative Eu anomalies; Hf isotope and mineralogical evidence suggest that it is a fractionated I‐type granite. The zircon Hf(t) values and two‐stage depleted mantle Hf model ages revealed that the magma of the Baimashan granitic complex originates from the partial melting of metamorphic igneous rocks; the crust–mantle differentiation age in the source area was 1.5–1.7 Ga for Longtan‐Xiaoshajiang and Longcangwan and 1.7–1.9 Ga for Shuiche. The superunits formed in a post‐collisional extensional tectonic setting. In the case of the Shuiche supersuit this is related to the remote effect of the interaction of the South China Plate with the Australian‐Indian margin of Eastern Gondwana. The formation of the Longtan‐Xiaoshajiang and Longcangwan superunits is linked to the remote effect of the Indosinian and South China Plate collision and the superimposition of the subduction of the Palaeo‐Pacific Plate. The weakly fractionated I‐type granite that formed during the Caledonian in South China is closely related to scheelite mineralisation.