小秦岭地区太华群锆石U-Pb年龄及华北克拉通南缘地壳演化

摘要

Geochemical,zircon U-Pb ages and Hf isotopic characteristics of two-feldspar gneiss from Taihua Group in Xiaoqinling area were analyzed.Geochemical results indicate that the protolith is high K calc-alkaline granitic rock with high REE content (229 × 10-6 ),obvious fractionation (w(La)N/w(Yb)N=45.39)of LREE and HREE and obvious negative Eu anomaly (0.43).The primitive mantle-normalized trace element spider diagram shows that it is enriched in LILE (e.g.Rb,Ba,Th and U)and relatively depleted in HFSE (e.g.Nb,Ta,Sr,P and Ti). The results of zircon U-Pb dating indicate that the forming age of protolith is early Paleoproterozoic (2.47 Ga).The results of zircon Hf isotope indicate that the range of initial N(176 Hf)/N(177 Hf)is small (0.281 284-0.281 328),εHf(t)values are positive (0.93-5.18);the two-stage Hf model ages are 2.70-2.89 Ga,and new continental crust (modern island arc volcano)Hf model ages are 2.54-2.73 Ga,which are close to the crystal age of zircon (2.47 Ga), so that new crustal growth is obvious in this period in the southern North China Craton.There are three important tectono-thermal events in early Neoarchean (2.7-2.9 Ga),late Neoarchean-early Paleoproterozoic (2.2-2.5 Ga)and middle Palaeoproterozoic (1.8-2.1 Ga)in the southern North China Craton.New crustal growth is obvious in early and late Neoarchean (εHf (t)>0), and new crustal growth is obvious for magmation in late Neoarchean-early Paleoproterozoic and middle Palaeoproterozoic,accompanying with recirculation and transformation of ancient crustal material (εHf (t)>0 orεHf(t)<0).%以小秦岭地区太华群二长片麻岩为研究对象，分析其地球化学、锆石U-Pb年龄和 Hf同位素特征。地球化学分析显示其原岩为高钾钙碱性花岗质岩石，稀土元素总含量较高（229×10-6），轻、重稀土元素分馏明显（w（La）N/w（Yb）N＝45．39），有明显的 Eu负异常（0．43）。原始地幔标准化微量元素蛛网图显示其富集Rb、Ba、Th、U等大离子亲石元素和亏损Nb、Ta、Sr、P、Ti等高场强元素。锆石U-Pb定年结果显示，其原岩的形成时代为古元古代早期（2.47 Ga）。锆石 Hf同位素分析结果显示：初始N（176 Hf）/N（177 Hf）值变化范围很小，为0．281284～0．281328，εHf（t）值均为正值（0．93～5．18）；二阶段 Hf模式年龄为2．70～2．89 Ga，新生地壳（现代岛弧火山）Hf模式年龄为2．54～2．73 Ga，接近于锆石结晶年龄（2.47 Ga），指示华北克拉通南缘在这一时期有明显的新生地壳生长。华北克拉通南缘在新太古代早期（2．7～2．9 Ga）、新太古代晚期-古元古代早期（2．2～2．5 Ga）和古元古代中期（1．8～2．1 Ga）有3期重要的构造热事件。新太古代早期及晚期有明显的新生地壳生长（εHf（t）＞0），新太古代晚期-古元古代早期和古元古代中期的岩浆活动有明显的新生地壳生长，同时还伴随有古老地壳物质的再循环和改造（εHf（t）＞0或εHf（t）＜0）。