豫北岭头杂岩体地球化学特征、锆石U-Pb年龄及地质意义

Geochemical characteristics and zircon U-Pb age of the Lingtou complex in North Henan and its geological significance

豫北岭头杂岩体位于太行山东麓中性岩带内,本次工作对其进行了元素地球化学分析及锆石U-Pb测年研究。该杂岩体包括两期侵入体,岩性分别为二长闪长岩、石英二长岩,前者为主体岩性、w(SiO_(2))为50.49%~57.10%,后者w(SiO_(2))为65.47%~66.74%、偏酸性。岭头杂岩体的总碱含量(Alk值)为6.73%~9.35%,里特曼指数(σ)为3.68~6.05,为中—高钾钙碱性系列;w(Al_(2)O_(3))介于14.33%~17.44%之间,山德指数(A/CNK)为1.21~1.48,为过铝质岩石;镁铁指数(MF)为70.6~81.4,反映岩浆的分离结晶作用程度较高。稀土元素总量为(114.34~161.09)×10^(-6),轻、重稀土比值为2.96~5.92,δEu值为0.94~1.10;稀土配分曲线呈右倾型,为轻稀土富集型,Eu异常不明显。富集Ba、K、La、Sm、Y等大离子亲石元素,亏损U、Nb、Ti等高场强元素。岭头杂岩体的w(Th)/w(U)比值的平均值为4.97,含镁指数较高(Mg^(#)为31.07~44.22),指示该杂岩体的岩浆源区既有地幔物质、又有地壳物质。用LA-ICP-MS锆石U-Pb测年方法,测得二长闪长岩的年龄为(134.20±0.64)Ma、石英二长岩的年龄(128.8±1.4)Ma,后者反映了岭头杂岩体的最终形成年龄,属早白垩世。测年结果与南太行其他地区获得的中生代侵入岩U-Pb年龄较一致,说明南太行地区在早白垩世发生了较广泛的岩浆活动,反映出华北克拉通的岩石圈减薄事件可能波及了豫北岭头一带。

Lingtou complex in North Henan Province is located at the intermediate rock belt in the east foot of Taihang Mountains.Element geochemical analysis and zircon U-Pb dating research are carried out for the intrusive body in this work.The complex includes two phases of intrusive bodies,monzonitic diorite and quartz monzonite.The former is the main rock,with SiO_(2) of 50.49%-57.10%.The latter is slightly acidic,with SiO_(2) of 65.47%-66.74%.Lingtou complex belongs to medium-high potassium calc-alkaline series,with alkaline contents of 6.73-9.35,Rittman Index(σ)of 3.68-6.05,Al_(2)O_(3) of 14.33%-17.44%,Sandner Index(A/CNK)of 1.21-1.48,belonging to peraluminous rock.Magnesium-iron Index(MF)is 70.6-81.4,showing the high degree of separation and crystallization of magma.The REE total amount is(114.34-161.09)×10^(-6),LREE/HREE ratio is 2.96-5.92,andδEu value is 0.94-1.10.The REE distribution curve of Lingtou complex is right-inclining,which is characterized by LREE enrichment,and Eu anomaly is not obvious.Lingtou complex is enriched with large ion lithophile elements(Ba,K,La,Sm,Y,etc)and depleted with high field strength elements(U,Nb,Ti,etc).The average value of w(Th)/w(U)for Lingtou complex is 4.97,the magnesium index is high(Mg^(#)=31.07-44.22),indicating that the magma source of the intrusive body are both mantle and crustal materials.Using LA-ICP-MS zircon U-Pb dating method,the age of monzonitic diorite is(134.20±0.64)Ma,and the age of quartz monzonite is(128.8±1.4)Ma.The latter age reflects the final formation age of Lingtou complex is in the Early Cretaceous.The dating results are consistent with the U-Pb ages of Mesozoic intrusive rocks obtained in other areas of the South Taihang,indicating that extensive magmatic activity occurred in the South Taihang area during the Early Cretaceous.It is reflected that the lithospheric thinning event of the North China Craton might have affected the Lingtou area in North Henan.