Kwangsian crustal anatexis within the eastern South China Block: geochemical, zircon U-Pb geochronological and Hf isotopic fingerprints from the gneissoid granites of Wugong and Wuyi-Yunkai Domains

摘要

Gneissoid granites were traditionally thought to be the components of the Precambrian basement in the eastern South China Block, but twenty-four gneissoid granite samples from the Wugong, northern Wuyi, southern Wuyi and Yunkai Domains gave zircon U-Pb ages of 424-456Ma, 410-457Ma, 426-438Ma and 415-450Ma, respectively. The existence of the abundant Kwangsian gneissoid granites, which have incorrectly been assigned to the Precambrian basement, indicates that the 'so-called' Precambrian stratigraphic sequence of the Cathaysia Block should be termed 'Complex' rather than 'Group'. These gneissoid granites are peraluminous granites with relatively high Al 2O 3, MgO, TiO 2, FeOt and CaO/Na 2O but low CaO, Al 2O 3/(MgO+FeOt) and Rb/Sr ratios. They exhibit strongly negative Ba, Sr, Nb, P and Ti and positive Pb anomalies in primitive mantle-normalized diagrams, and have initial 87Sr/ 86Sr ratios ranging from 0.70924 to 0.72935 and negative εNd(t) values from -6.4 to -11.4, similar to those of paragneissic enclaves and Precambrian sedimentary rocks in the eastern South China Block. The zircons crystallized in the Kwangsian gneissoid granites gave εHf(t) values ranging from +2.4 to -19.4 with the peak at -4 (almost all grains clustering at -1 to -12) and Hf model ages of 1.1-2.1Ga. The synthesis of these geochemical and in-situ zircon Hf isotopic data indicates that the Kwangsian gneissoid granites dominantly originated from Proterozoic metapelitic and meta-igneous components with insignificant input of juvenile mantle-derived magmas. A model of two-stage crustal anatexis is proposed for the Kwangsian granitic magma in the eastern South China Block, with (1) the formation of 460-430Ma granite through the breakdown of hydrous minerals under the condition of the doubly thickened crust, and (2) the generation of the 430-400Ma granite accompanying promoted melting along a path of isothermal decompression due to the increasing thermal weakening for the collapse of the thickened crust. The crustal anatexis has probably occurred in an intracontinental tectonic regime that was geodynamically linked to the far-field response to the assembly of the Australian-Indian plate with the Cathaysia Block during middle Paleozoic (Kwangsian) time. © 2011 Elsevier B.V.