河北省北部杨树沟基性岩墙年代学和地球化学研究

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

性岩墙是研究大陆伸展裂解很好的证据，华北克拉通北缘在晚中生代发生了显著的拉伸作用，广泛分布着基性岩墙，河北省北部杨树沟基性岩墙就是其中的代表。锆石UHPb年龄显示，基性岩墙的形成时代约为134．9±9．2Ma，为早白垩世岩浆事件的产物。在岩石化学组成上，SiOz（52．02％～57．79％）位于碱性和亚碱性过渡区，另外，基性岩墙的Na_2O（3．77％～5．26％）大于K20（2．17％～3．32％），MgO（1．88％～3．46％）含量较低，富集轻稀土元素（LREE）和大离子亲石元素（LILEs，Rb，Ba和Sr）和轻微的Eu正异常（δEu-0．98～1．19，平均1.08），亏损高场强元素（HFSEs，Nb和Ta）及重稀土元素（HREE）。地球化学系统研究表明，研究区基性岩墙来源于富集岩石圈地幔的部分熔融作用，源区存在橄榄石、尖晶石、榴辉岩和角闪岩相的残余，同时，基性岩墙的形成可能与造山后伸展作用有密切联系。

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 SiO2 contents 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 Na20 （3.77%～5.26%） than K_2O （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.