The Crustal Evolution From Indosinian To Yanshanian With Chicheng County, North Hebei Province, China

The granite in Chicheng area is divided into four rock, respectively, arehongdunliang granitoid, sheshenya granitoid, donggoulou granitoid and bingshanlianggranitoid. According to zircon LA-ICP-MS U-Pb dating, the granitoids in the areamainly belongs to the two times. The Bingshanliang granitoids belongs to theIndosinian epoch, and the Hongdunliang ranitoids、 Sheshenya ranitoids andDonggoulou ranitoids belongs to yanshanian period. Rock mass is divided accordingto the aluminum content, the granite in Indosinian belongs to I type granite, and thegranite in Yanshanian belongs to S type granite. Judge the structural environment ofgranite, the granite in Indosinian belongs to Post-collision granites, and the granite inYanshanian belongs to Syn-collision granite. Though the calculation results for εNd(t)and found that, the εNd(t) of Indosinian granite is between-12.18and-18.0, and theεNd(t) of Yanshanian granite is between-11.22and-16.46. It respresents the sourcerock of granite in this place is from continental crust. Due to the reasons for theformation of granite partial melting of the crust, characteristics of the chemicalcomposition of granite can infer the characteristics of the chemical composition of thecrust. Thus it can be seen, the chemical composition of crustal evolution can be shownby the chemical composition of granites evolution. Characteristics of Indosiniangranite: a high degree of magmatic differentiation; enrichment in LREE and loss inHREE; calc alkaline trend; high content of sodium and calcium; metaluminous toperaluminous characteristics. Characteristics of Yanshanian granite: a high degree ofmagmatic differentiation; enrichment in LREE and loss in HREE; calc alkaline trend;low content of calcium; peraluminous characteristics. According to compare thechemical of granite at two times, we can determine the process of chemicalcomposition of crustal evolution in this place from Indosinian to Yanshanian. Thecontent of Al was further enriched from Indosinian to Yanshanian, and it means thatcrustal composition from metaluminous to peraluminous, the content of calcium wasreduced, but there is not much change for the content of sodium and potassium. At the same time, the content of ferrum has not changed. Trac elements of crustal does nothave too much change, and it is characterized by a high degree of differentiation,enrichment in LREE and loss in HREE in Indosinian crust, this characteristic is moreobvious in Yanshanian crust. So, with the evolution of the crust, fractionation of REEare more obvious, and the fractionation in LREE and HREE become furtherstrengthen. According to the Nd model of Indosinian granite and Yanshanian granitewe can know that, the source rock of the Indosinian granite was formed at1700Maago and belongs to Mesoproterozoic, and the formation age of source rock ofYanshanian granite has greatly different between the different rock mass and their agewas from Mesoproterozoic to Neoproterozoic. The source rock of Yanshanian granitewas influenced by the material in the mantle in formation process, and it caused greatdifferences in Nd model age.