Genesis of Highly Fractionated I-Type Granites from Fengshun Complex: Implications to Tectonic Evolutions of South China

The South China Block is characterized by the large-scale emplacement of felsic magmas and giant ore deposits during the Yanshanian. We present zircon Hf isotopic compositions, whole-rock major and trace element compositions of the Fengshun complex, located in eastern Guangdong Province, South China. The Fengshun complex is a multi-stage magmatic intrusion. It is composed of two main units, i.e., the Mantoushan (MTS) syeno-monzogranites, alkali feldspar granites and the Hulutian (HLT) alkali feldspar granites. LA-ICPMS zircon dating shows that the complex emplaced in 166-161 and 139+/-2 Ma, respectively. Geochemically, the MTS granites show relatively various geochemical compositions with low REE contents (87.76x10(-6)-249.71x10(-6)), Rb/Sr ratios (1.19-58.93), pronounced Eu negative anomaly (0.01-0.37) and low Nb/Ta ratios (2.40-6.82). In contrast, the HLT granites exhibit relatively stable geochemical characteristics with high REE contents (147.35x10(-6)-282.17x10(-6)), Rb/Sr ratios (2.05-10.30) and relatively high Nb/Ta ratios (4.45-13.00). The isotopic data of the MTS granites display relatively enriched values, with I-Sr. varying from 0.708 2 to 0.709 7, epsilon(Nd)(t) from -7.8 to -6.9 and epsilon(Hf)(t) from -7.4 to -3.2, in comparison with those of the HLT which are I-Sr=0.703 05-0.704 77, epsilon(Nd)(t)=-5-3.4 and epsilon(Hf)(t)=-0.7-1.8). The two-stage model ages of the MTS granites (T-2Dm(Nd)=1.51-1.59 Ga and T-2Dm(Hf)=1.26-1.48 Ga) are also higher than those of the HLT granites (T-2Dm(Nd)=1.21-1.34 Ga and T-2Dm(Hf)=0.96-1.10 Ga). Thus the MTS and HLT granites might originate from different sources. The former is more likely derived from partial melting of Meso-Proterozoic basement triggered by upwelling of asthenosphere and/or underplate of the basaltic magma and then extensive fractional crystallisation, similar to the genesis of Early Yanshanian granitoids of the EW-trending tectono-magmatism belt in the Nanling range. In comparison, the latter might have involved with asthenosphere component, similar to the Early Cretaceous granitoids of NE-NNE-trending granitoid-volcanic belt in coastal region, southeastern China. We propose that the MTS granites were mainly formed in Paleo-Tethyan post-orogenic extensional tectonic setting whereas the HIT granites were formed in the back-arc extensional tectonic setting. The period at 139 Ma represents the initial time of roll-back of the paleo-Pacific Plate in SE-trending.