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（87.76×10-6–249.71×10-6）, 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（147.35×10-6– 282.17×10-6）, 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 ISr varying from 0.708 2 to 0.709 7, εNd（t） from-7.8 to-6.9 and εHf（t） from-7.4 to-3.2, in comparison with those of the HLT which are ISr=0.703 05–0.704 77, εNd（t）=-5–-3.4 and εHf（t）=-0.7–1.8）. The two-stage model ages of the MTS granites（T2DM（Nd）=1.51–1.59 Ga and T2DM（Hf）=1.26–1.48 Ga） are also higher than those of the HLT granites（T2DM（Nd）=1.21–1.34 Ga and T2DM（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 HLT 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.

Petrogenesis and Geotectonic Significance of Early-Neoproterzoic Olivine-Gabbro within the Yangtze Craton: Constrains from the Mineral Composition, U-Pb Age and Hf Isotopes of Zircons

The olivine-gabbroic rocks located at the Huangiing anticline within the Yangtze Craton are dated at circa 857-854 Ma by LA-ICP-MS method. The rocks belong to the sub-alkaline series and consist of pyroxene （35%-40%）, plagioclase （40% -45%）, olivine （8%-10%） and spinel （3%-5%）. Olivine has Fo values of 73-83 that is classified as chrysolite. Pyroxene has relative low contents of FeO （6.60 wt.%-8.23 wt.%） but high CaO （20.23 wt.%-21.25 wt.%） contents, however, plagioclase has high A1203 （31.78 wt.%o-32.37 wt.%）, CaO （16.08 wt.%-16.25 wt.%） and An （79-80） values, but low Na20 contents （1.95 wt.%-2.11 wt.%）. Spinel are magnesioferrite with characteristics of high contents of MgO （13.65 wt.%- 13.68 wt.%）, FeO （23.27 wt.%-23.40 wt.%） and A1203 （62.43 wt.%-62.74 wt.%）. Chemical compositions of these minerals are similar to those of gabbro rocks that were formed in the post-orogeny environment. The olivine-gabbro samples have negative zircon eHf values （-16.57±0.47） that resemble the mafic rocks in the same region, indicating that they are derived from the extremely enriched mantle source. On the compilation of documented Neoproterozoic mafic rocks in the Yangtze Craton, it is proposed that the mantle in the northern Yangtze Craton has experienced different degrees enrichment during the Neoproterozoic.