Zircon U-Pb Geochronology,Geochemical Compositions of the Mafic Dykes From Yangshugou,Northern Hebei Province:Constraints on the Age and Petrogenesis

Mafic dykes are very important to research continental extension.The Northern boundary of North China Craton(NNCC)experienced extension during later Mesozoic and the Yangshugou mafic dykes is very representative in NNCC.The zircon U-Pb age of the mafic dykes provides an age of 134.9±9.2 Ma,indicating that the mafic dykes were the result of the earlier Cretaceous magmatism.The SiO2contents of the rocks range from 52.02%to 57.79%,straddling the fields of alkaline and sub-alkaline series.In addition,the mafic dykes are characterized by having higher Na2O(3.77%~5.26%)than K2O(2.17%~3.32%),depletion in MgO(1.88%~3.46%),enriching light rare earth elements(LREEs)and large ion lithophile elements(LILEs,Rb,Ba,and Sr)and having 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 the mafic dykes,in this study,were derived from partial melting of enriched lithospheric mantle,however,there were remnant of olivine,Cr-spinel,eclogite and amphibolite faces in the source.In summary,the origin of the studied dykes is related to the post-collision extension of the Xing-Meng Orogene.