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 calcalkaline 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 crustmantle 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.