Zircon U-Pb ages and Hf isotopes and their geological significance of Yiyang TTG gneisses from Henan province,China
Zircon U-Pb ages and Hf isotopes and their geological significance of Yiyang TTG gneisses from Henan province,China
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作者：
Diwu Chun
在北中国克拉通南缘产出大量灰色片麻岩的英云闪长花岗闪长岩。本文报道了益阳地区英云闪长岩的地球化学、锆石年代学和Hf同位素组成，并讨论了其地质意义。片麻岩中SiO_2含量较高(59~73%)，Al_2O_3含量较高(15%)。CaO、MgO、Al_2O_3、、_2含量随着SiO_2含量的增加而降低，呈负相关，而P_2O_5、Na_2O/K_2O比值呈正相关指示镁铁质矿物、磷灰石和斜长岩。片麻岩的稀土配分模式以强分馏稀土配分模式((La/Yb)N=9~23)为特征，无Eu异常，重稀土元素相对轻稀土含量较平坦。Rb、Ba、Sr等不相容的微量元素含量较高，而高场强元素(如Nb、Ta)含量较低。岩石具有典型的低Y(13.25~25.27μg/g)和Yb(1.27~1.98μg/g)的特征。La-Icp-MS锆石U-Pb定年结果为2.3Ga，代表片麻岩的形成年龄。片麻岩中的大多数锆石具有负的e_(Hf)(T)值，表明原岩主要来自于中国大陆西南边缘的古地壳岩石。少数锆石则显示出正的E_(μ)(T)值，可能来自亏损的地幔，这表明片麻岩中的锆石可能来自亏损的地幔。TTG片麻岩最古老的两阶段Hf模式年龄约为2.8Ga，表明益阳片麻岩的源岩年龄至少为2.8Ga。尽管2500 Ma是NCC地壳生长的重要时期，但我们结合前人的研究表明，古元古代也有新的地壳形成。
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 SiO_2(59～73%) and Al_2O_3 (15%).The concentrations of CaO、MgO、Al_2O_3、TiO_2 decrease with the increasing of SiO_2,showing negative correlation,whereas the P_2O_5,Na_2O/K_2O 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=9～23)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(13.25～25.27μg/g)and Yb(1.27～1.98μg/g)contents,and exhibit high Sr(507.14～606.85μ/g)contents,resembling the geochemical characteristics of high-Al TYG 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 negativee_(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 positivee_(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.