Geochronology And Geochemistry Of The Tiejiashan Granites And Supercrustal Rocks In The Anshan Area, Liaoning Province

Anshan is one of four≥3.8 Ga rock distribution areas in the world. The Tiejiashan granite studied in this paper is the largest and oldest K-rich granite in the North-China Craton. Two granite samples A9825 and A9837 give magmatic zircon ages of 2.91 Ga and 2.99 Ga, respectively, being different from the magmatic zircon ages ( 2.96 Ga) determined by Zhong (1984) and Song et al., (1996) and therefore probably meaning that the Tiejiashan intrusion should be further subdivided. The existence of inherited zircons, with the oldest zircon grain being 3.76 Ga in age, in the two granite samples indicates that the Tiejiashan granite was derived from ancient continental crust. It is consistent with the geochemical characteristics of the Tiejiashan granite, which is high in K2O and low in Na2O and shows obvious negative Eu and Ba anomalies, with tDM(Nd) andεNd(t) being 3.38 ~ 3.88 Ga and -7.21 ~ -2.26, respectively. Granite sample A0432 does not give a believable magmatic zircon age because of strong lead loss. However, it gives a metamorphic zircon age of 2.91 Ga obtained for the first time in the Tiejiashan granite and even in the Anshan area. This metamorphic age is same as the age of magmatic zircon from granite sample A9825, undoubtedly indicating that an important tectonothermal event happened around 2.9 Ga ago. The magmatic zircons giveεHf(t) and tDM(Hf) of -9.36 ~ -2.29 and 3.32 ~ 3.57 Ma, with exception of analysis A9837-16. The inherited zircons from samples A9825 and A9837 haveεHf(t) and tDM(Hf) values ranging from -9.10 to -5.73 and from 3.42 to 3.55 Ga, respectively. The metamorphic zircons from sample A0432 show similarεHf(t) and tDM(Hf) values to those of the magmatic zircons, being -9.10 ~ -5.73 and 3.42 ~ 3.55 Ga, respectively. Based on the Hf isotope compositions of the zircons, combining with element and Nd isotope compositions of the whole rocks, it is suggested that the formation of the Tiejiashan granite was related to recycling of ancient continental crust and that the REE distribution patterns of some portions of the granite were changed during its formation and/or latter evolution processes.There are some supracrustal rocks of high maturity in the Tiejiashan granite. The most important supercrustal rocks, including mica quartz schist, biotite fine-grained gneiss and fine-grained leuco-gneiss, are high in K2O and low in Na2O, with strongly negative Eu and Ba anomalies. tDM(Nd) andεNd(t) are 3.18~3.51 Ga and -3.89~0.64, respectively. These rocks were considered to be enclaves in the Tiejiashan granite and to be source material from which the Tiejiashan granite was derived according to their similarity in geochemical composition. However, new SHRIMP U-Pb data show that the detrital zircons from three supercrustal rock samples (A9819, A0435 and A0436-1) are commonly younger than 3.0 Ga in age, indicating that they formed later than the Tiejiashan granite and so excluding the possibility that they are the enclaves in the Tiejiashan granite. These supracrustal rocks may be a part of the Anshan Group of 2.5 Ga. The other type of supercrustal rock is mica quartz schist with high SiO2. It shows anomalous REE and Nd isotope compositions, with LREE being lower than HREE in content and an anomalous tDM(Nd) value being 6.53 Ga.