华北克拉通南缘张士英岩体大陆动力学背景:来自地球化学、锆石U-Pb年龄和Hf同位素的证据

The continental dynamics of Zhangshiying pluton at the southern margin of the North China Craton: Evidence from geochemical,zircon U-Pb geochronology and Hf isotopic compositions

张士英岩体位于华北克拉通南缘,岩体主要由石英正长岩组成。石英正长岩的LA-ICP-MS锆石U-Pb年龄为122.8±1.5Ma。其Si O2含量为66.04%~67.80%,Na2O+K2O=9.03%~10.97%,K2O=4.40%~6.37%,K2O/Na2O>1属于钾质长英质岩石。A/CNK=1.26~1.58,A/NK=1.63~1.79属于过铝质岩石系列。石英正长岩的Mg#变化范围在12.9~39.4。富集LREE亏损HREE,轻重稀土分异明显,(La/Yb)N=15.48~21.12,Eu呈弱的负异常(δEu=0.54~0.99)。富集Rb、K、Th、U等大离子亲石元素,亏损Nb、Ta、P、Ti等高场强元素。张士英石英正长岩岩浆锆石εHf(t)集中在-17.6^-13.9,Hf两阶段模式年龄tDM2集中在1.7~1.9Ga。石英正长岩的岩浆Zr饱和温度高(936~998℃)。地球化学及同位素显示张士英石英正长岩源区主要为古老的壳源物质,并有少量年轻组分加入,这种年轻组分是幔源物质。岩体形成于拉张性构造环境下,拉张性的环境导致了幔源物质的上涌,底侵下地壳使其发生部分熔融。形成时代正好位于华北克拉通破坏峰期,张士英石英正长岩正是这一地质事件的响应。

The Zhangshiying pluton is located in the southern margin of the North China Craton. Zircon LA-ICP-MS U-Pb dating for Zhangshiying quartz syenite pluton yield 122. 8 _＋ 1.5Ma. Those quartz syenite are alkaline rocks with SiO2 = 66. 04% N 67. 80% , Na20 ＋ K20 = 9. 03% - 10. 97% , K2 0 = 4.40% - 6. 37%, K20/Na2 O 〉 1, A/CNK = 1.26 - 1.58, and A/NK = 1.63 - 1.79. Mg ranges from 12. 9 to 39.4. They are enriched in LREE and depleted in HREE. There is obvious partition of LREE from HREE with （La/Yb） N = 15.48 -21.12, and with slightly negative Eu-abnormal （δEu =0. 54 ~0. 99）, and it is enriched in LILE e.g. Rb, K, Th, U, and depleted in HFSE, e.g. Nb, Ta, P and Ti. The εHI（t） for quartz syenite ranges from - 17. 6 to - 13.9, with tDMz of 1. 7 1.9Ga. Its magma zirconium saturation temperature ranges between 940 ~ 1000℃. Geochemical and isotopic composition indicate that the source of the Zhangshiying pluton is mainly old lower crust and minor juvenile composition. The juvenile composition is mainly from mantle. The pluton was generated under an extensional setting. The extensional setting induced mantle magma upwelling, providing heat and causing the partial melting of the bottom crust. The age of pluton is at the same time of the lithospheric thinning and destruction of the North China craton. Zhang Shiying pluton is the response of this geological event.