华北克拉通北缘东段海沟岩体的锆石U-Pb年龄、Sr-Nd-Hf同位素组成及其动力学背景

海沟岩体位于华北克拉通北缘东段,由二长岩和二长花岗岩组成。本文对该岩体主微量元素、Sr-Nd-Hf同位素和锆石的U-Pb年龄进行了系统研究。高精度锆石U-Pb年龄测试结果表明,二长岩和二长花岗岩形成的精确年龄分别为322.9±3.4Ma和320.3±3.5Ma,为海西期构造-岩浆活动产物。海沟岩体为准铝质-弱过铝质的I型花岗岩,其K₂O/Na₂O比值介于0.51~0.93之间,A/CNK介于0.87~0.94之间。微量元素方面表现为轻稀土富集、轻重稀土分馏明显、弱铕正异常、大离子亲石元素(如Rb、Ba、K)富集,以及高场强元素(如Nb、Ta、Ti、P)相对亏损等。锶同位素初始比值为(⁸⁷Sr/⁸⁶Sr)_i= 0.7054~0.7059, ε_Nd(t)=-10.4~-8.6,亏损地幔的钕模式年龄分别介于1.84~1.99Ga和1.84~1.93Ga之间。二长岩和二长花岗岩锆石的ε_Hf(t)变化范围分别为-7.6~-0.8和-11.5~-5.4,对应的两阶段Hf模式年龄分别为1.39~1.81Ga和1.67~2.06Ga。同位素特征表明两者可能来自中元古代和古元古代的下地壳物质的部分熔融,并可能有较多地幔物质的加入。根据其形成时代、岩石地球化学、同位素特征以及区域地质演化史,认为海沟岩体的形成可能与古生代古亚洲洋消减作用密切相关。

Zircon U-Pb ages and Sr-Nd-Hf isotopic composition of the Haigou granitoids at the northeastern margin of North China Craton: Implications for geodynamic setting

The Haigou granitoids,located in the northeastern margin of the North China Craton,consists of monzonite and monzogranite.This study presents zircon U-Pb age,geochemical compositions and Sr-Nd-Hf isotopes of the Haigou granitoids,to discuss the origin and tectonic setting of the magmas.The zircon U-Pb dating for the monzonite and monzogranite yields 322.9±3.4Ma and 320.3±3.5Ma respectively,indicating the magmas emplaced during the Hercynian Period.Petrochemical data indicate that they belong to calc-alkaline I-type granitoids with metaluminous affinity.They have similar geochemical features such as K2O/Na2O ratios being between 0.51 and 0.93,A/CNK ranging from 0.87 to 0.94,LREE enrichment with slightly positive Eu anomalies.In addition,trace element geochemistry also displays LILE(such as Rb,Ba,K) enrichment and HFSE(including Nb,Ta,Ti,P) depletion.The whole-rock Isr and εNd(t) of the Haigou granitoids are 0.7054～0.7059 and-10.4～-8.6 respectively.Two-stage Hf model ages(tDM2) of magmatic zircons from monzonite and monzogranite are 1.39～1.81Ga and 1.67～2.06Ga respectively,all manifesting relative low εHf(t) values(-7.6～-0.8 for monzonite and-11.5～-5.4 for monzogranite) .Above-stated isotopic systematics of the granitoids implies that the magmas were derived possibly from the partial melting of ancient lower crust of the Mesoproterozoic or Paloproterozoic ages with abundant mantle contribution.Considering their ages,geochemical and isotopic signatures and regional geological evolution,it is suggested that subduction of the Paleoasian oceanic crust might be responsible for formation and emplacement of the Haigou granitoids magmas.