Zircon U-Pb Ages And Hf Isotopes And Their Geological Significance Of Yiyang Ttg Gneisses From Henan Province, China

Large volumes of gray gneisses of tonalite trondhjemite granodiorite (TTG) occur in the southern margin of the North China Craton (NCC). The data of geochemistry, zircon chronology and Hf isotopic composition for TTG genisses in the Yiyang area are reported in this paper, and then the geological significances are discussed. The gneisses are high in SiO2 (59 similar to 73%) and Al2O3 ( > 15%). The concentrations of CaO,MgO,Al2O3,TiO2 decrease with the increasing of SiO2, showing negative correlation, whereas the P2O5, Na2O/K2O ratios display positive correlations, indicating the department of mafic-mineral, apatite and plagioclase. The REE patterns of the gneisses are characterized by strongly fractionated REE patterns ((La/Yb)(N) =9 similar to 23) and without Eu anomalies, and the HREEs are flatter compared with the LREE contents. The concentrations of incompatible trace elements such as Rb, Ba, Sr are enriched, whereas the high field strength element (e. g. Nb and Ta) contents are low. The rocks have typical low Y (13.25 similar to 25.27 mu g/g)and Yb(1.27 similar to 1.98 mu g/g)contents, and exhibit high Sr (507.14 similar to 606.85Wg) contents, resembling the geochemical characteristics of high-Al TTG suite. In this case, hornblende is required to retain in the restite. The LA-ICP-MS zircon U-Pb dating yields 2.3Ga, representing the formation age of the TTG gneisses. Most zircons in the TTG gneisses have negative epsilon(Hf) (t) value, which suggests that the protoliths of the genisses were mainly derived from ancient crustal rocks at the southwestern margin of NCC. While a few zircons show positive epsilon(Hf)( t) value, indicting juvenile materials, probably from depleted mantle, were added to the protoliths of the gneisses. The oldest two-stage Hf model ages of the TTG gneisses are about2.8Ga, suggesting that the source rocks of the gneisses in the Yiyang are at least as old as 2.8Ga. Although the 2500Ma is the significant time of crustal growth in the NCC, there was also new crust formation during the paleoproterozoic as we show in this study, together with some former researches.