| 其他摘要 | Northern Hebei province and Chifeng in Inner Mongolia are located in the east part of Central Asia orogene belt, are the most special areas in China, and the study areas for the investigation of Yanshan activity. The North China Craton (NCC) is situated in the south of the Xilamulun suture. The Mafic dykes from Yangshugou and Xinfangzi in Hebei Peovince were found in these areas. In addition, there is a transitional plate between Xilamulun suture and Soker-Hegengshan suture; there the rhyolites in Chifeng were studied here. As the direct result of lithospheric extension, late Mesozoic mafic dykes are widespread in Northern NNCC. The zircon U-Pb age of the Yangshuigou mafic dykes is 134.9?9.2Ma, indicating that these mafic dykes are the result of earlier Cretaceous magmatism. The SiO2 contents of the rocks range from 52.02wt. % to 57.79 wt. %, and straddle in the field of alkaline and sub-alkaline series. In addition, the mafic dykes are characterized by higher Na2O (3.77-5.26wt.%) than K2O(2.17-3.32wt.%), depleted in MgO(1.88-3.46 wt.%), enrichment in light rare earth elements(LREEs) and large ion lithophile elements(LILEs , Rb, Ba, and Sr)and weak positive Eu anomalies(δEu =0.98-1.19, average in 1.08), as well as depletion in heavy rare earth elements(HREEs)and high field strength elements(HFSEs, Nb and Ta). Systemic geochemistry research indicates that these mafic dykes, in this study, were derived from partial melting of an enriched lithospheric mantle, however, there were remnant of olivine、Cr-spinel、 eclogite and amphibolite faces in the source. In summary, the origins of the studied dykes were related to post-collision extension of Xing-Meng Orogene. The zircon U-Pb age of Xinfangzi mafic dykes is 142.9±7.9Ma, the result of earlier Cretaceous magmatism. The SiO2 =50.01-51.78 wt. %, and located in the field of sub-alkaline series but also near alkaline series. In addition, the mafic dykes are characterized by higher Na2O (3.02-3.79 wt.%) than K2O (1.06-1.32 wt.%), enrichment in MgO (5.18-6.05 wt.%), LREE and lLILEs(Rb, K, and Sr)and weak positive Eu anomalies(δEu =1.06-1.11), depletion in HREEs and much lower Yb and Lu,and HFSEs(Nb、Ta、Zr and Hf)are normal in this mafic dykes. εSr(t)=1.14-2.70,εNd(t)=2.69-2.89. Systemic geochemistry investigation indicates that these mafic dykes were derived from partial melting of depleted mantle; however, there were remnant of eclogite face in the source. Based on the above interpretations, the origins of the dykes were related to post-collision extension of Xing-Meng Orogene blet. The zircon U-Pb age of Zhongxintun rhyolites is 136.3±2.39 Ma, the result of middle earlier Cretaceous magmatism. The SiO2 = 69.71-77.31wt. %, located in the dacite and rhyolite series, have a lower contents of Fe2O3, MgO and CaO, but K2O is much richer than Na2O. And have a wide range of Cr, Ni and Sr, the studied rhyolites are characterized by enrichment in LREE, LILEs(Rb, Zr, Hf, La and Ce), depletion HFSEs(Nb, Ta and Ti), Sr and P. Systemic geochemistry researches show that the magma experienced the fraction of plagioclase, olivine, clinopyroxene, spinel, titanite and rutile. In addition, these rhyolites were formed in an extentional setting, and lower crust partial melting may contribute to the magma formation. |
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