Abstract:

Datan volcanic basin in Fengning is located in the zone of the northern North China Block, south of Xar Moron river fault
zone, west of the Yan-Liao polymetallic (Mo-U-Ag-Pb-Zn), and southwest of the Gu(yuan)-Hong(shanzi) uranium belts. It is adjacent to the Guyuan basin where there is a large Zhangmajing U-Mo deposit. The rhyolite-trachyte rock assemblage of Zhangjiakou was formed in the Datan volcanic basin and is the main wall rock of hydrothermal-type uranium deposit. The zircon SHRIMP U-Pb dating of trachyte yields an age of 140.3 ± 1.4 Ma (2σ, MSWD=1.09), which represents the emplacement age of the trachyte, and suggests that it was formed in the early Early Cretaceous. Trachyte contains high content of SiO2, K2O+Na2O ratio , SiO2=61.6%~63.1% , K2O+ Na2O=9.28%~10.2%, and falls in shoshonite series range of the K2O-SiO2 diagram and in alkaline series trachyte or dacite range of the TAS diagram. With the result of the CIPW and the content of Q =12.9%~18.4% (less than 20%), it belongs to the typical trachyte. Trachyte has high content of rare earth elements, enriched in light rare earth and the non-obvious negative Eu anomaly, ΣREE=(279~ 318)×10-6, δEu=0.77~0.88. The results show the right-leaning enrichment in light rare earth type with a high pressure type trachyte
characteristics of rare earth elements, enrichment in large ion lithophile element (K, Rb, Ba, Pb) and high field strength elements (Zr、 Y), and loss of these elements(Nb, Ta, Zr, Ti ). The Ti/Zr (8.40~14.72),Nb/Ta(14.07~15.26) ratio is low and Rb/Sr(0.95~1.60) ratio is high with high Sr-Ba dacite-rhyolite the characteristics of trace elements; High (87Sr/86Sr)i(0.7077~0.7096), lower εNd(t)(-12.09~-10.67), the larger TDM2(1796~1911Ma), lower (206Pb/204Pb)t(16.92~17.04), (207Pb/204Pb)t(15.39~15.42),（208Pb/204Pb)t(37.58~37.67). In the (87Sr/ 86Sr)i -t/Ga and (208Pb/204Pb)t-( 206Pb/204Pb)t diagram, it is located between the lower crust and the mantle, and is close to the EMⅠ enriched mantle. In the εNd(t)-(87Sr/86Sr)i diagram, it is located within EMⅠenriched mantle evolution line and Hannuoba two-pyroxenne grain xenoliths area. So the trachyte in the Datan basin can be explained by a two-stage model: In the paleoproterozoic, EMⅠenriched
mantle magma and a small part of the partial melting of ancient crust material were mixed and formed the young crust. In the early Early Cretaceous, the magma from partial melting of the young and thickened lower crust erupted and formed trachyte.

Key words: trachytes；petrogenesis；Sr-Nd-Pb isotopes；young lower crust；early Early Cretaceous；Datan basin