Petrology,Geochemistry And Zircon SHRIMP U-Pb Geochronology Of Granite In Chicheng Coumty, North Hebei Province

The granitic plutons which emplaced in the Hongqiyingzi complex are exposed in Maying,Yunzhou reservoir,Dushikou and Zhenningpu, in the northern margin of Northern China Craton(NCC). The granitic bodies in the region of interest include Hongdunliang,Seshenya,Bingshanliang and Donggoulou et al. Some of the granitoids emplaced into the Hongqiyingzi complex which consisted of biotite plagioclase gneiss, some emplaced in Yangdun gneiss, and some other bodies were covered by upper Jurassic volcanic sedimentary rock. On the basis of the geochemical and isotopic characteristics, the granitic plutons could be classified two types. The first, calc-alkalic granitic rocks that are mainly exposed in Hongdunliang, Sheshenya, and Donggoulou area have different exposure area, but the same attitude and similar petrological feature. They mainly component of plagioclase,potassic feldspar,quartz and little blotite, with typical granitic texture and massive structure. The granites rich in SiO₂ content (73.93%～76.44%) and lower Al₂O₃ content (11.56%～12.95%),Na₂O(3.88%～4.38%) and lower K₂O content (4.37%～5.06%), and is characterized by lower total rare-earth element content (∑REE) (92.51～240.53 ppm), (La/Yb)N between 6.66 and 18.27, and distinct negative Eu anomaly of Eu (δEu=0.15～0.38), and a weak Ce anomaly. This type present the features: the fSm/Nd average range from -0.31 to -0.53 and theεNd(t) average range from -13.25 to 15.80, so, the monzonitic granites is calc-alkalic series granites, stemming from continental crust. The second, the granitic pluton, exposed in Bingshanliang, is granodiorite, which mainly component of plagioclase (40%～45%),potassic feldspar (30%～35%),quartz (10%～15%) and little blotite. The granodirite body is characterized by lower SiO₂ content (66.31%), higher Al₂O₃ (15.13%),Na₂O (4.40%) and K₂O(5.51%), higher∑REE(373.95 ppm), the ratio of (La/Yb)N is 30.77, a weak Eu anomaly and apparent Ce anomaly(δCe=0.70). Its fSm/Nd is -0.50 and theεNd(t) is -12.18(<0), illustrates the granodiorte stemmed from continental crust too.For the biotite monzogranite, we tested some samples from Hongdunliang and Donggoulou, and just analyses only one spot on the sample from Sheshenya for lacking of time. The zircon in the samples from Hongdunliang body (sample 072503) have apparent magmatic genetic clitellum, and its ratio of Th/U is between 0.50 and 1.45, the average SHRIMP U-Pb isotopic age ²⁰⁶Pb/²³⁸U from 14 zircons is 143.9±2.2Ma; and the zircon in the samples from Donggoulou (sample 072802) also have apparent magmatic genetic clitellum in cathodoluminescence images, and its ratio of Tu/U range from 0.61 to 0.96, the average of analytic datas of zircon U-Pb ²⁰⁶Pb/²³⁸U age is 138.0±2.2 Ma; with regard to the body exposure at Sheshenya, the zircon also have magmatic genetic clitellum in the cathodoluminescences, and based on the only one spot on the zircon, its ratio of Th/U is 2.19, the ²⁰⁶Pb/²³⁸U age is 136.0±3.3Ma. For the granodiorite, we used the body of Bingshanliang to zircon SHRIMP U-Pb dating. The zircons in samples from the Bingshanliang body (sample 072602) have apparent clitellum in the cathodouminescences, and higher ratio of Th/U (range from 1.18 to 2.73), the average of analytic datas of zircon U-Pb ²⁰⁶Pb/²³⁸U age is 228.0±3.1Ma.Based on our study of zircon SHRIMP U-Pb dating, the ²⁰⁶Pb/²³⁸U age from the zircon of the granodiorite exposed in Bingshanliang is 228Ma, formed at Indo-Chinese epoch. On the other hand, the average of ²⁰⁶Pb/²³⁸U age, obtained from the monzonitic granite, which exposure at Hongdunliang,Sheshenya and Donggoulou, ranges from 136Ma to 144Ma, the average value is 143Ma, so, the monzonitic granites in the studying region formed at Yanshan epoch. The age of granitic plutons in Chicheng is different from the concept of other scholars who regarded the granits is formed in Haixi epoch.The study of incorporation of granite forming and lithopheric thinning could get in touch with craton breaking The SHRIMP U-Pb age data has extraordinarily important geologic meaning. First of all, it strongly show that there are two stages of magmatic activities in Chicheng region, northern Chicheng—Chongli—Pingquan discordogenic fault, which offers exact foundation to the study of property of tectonic framework of the rock—building block in the studying area. Besides, integrating with the braking of Northern China Craton in Mesozoic era, it offers the petrologic proof for the braking of Northern China Craton, restricts that the Northern China Craton wrecked begin at Indo-Chinese epoch and attained the most intense phase at Yanshan epoch.Finally, the study of the granites in the northern margin of Northern China Craton denotes the thinning of lithopheric, which is an extraordinary apparent about the breaking of Northern China Craton in Mesozoic era. Then, the study of the granites extends some significant petrologic proofs and some geochronologic proofs for the study of braking of Northern China Craton in Mesozoic era, which have extraordinary important geotectonic significations.