胶北太古宙早期锆石U-Pb定年及Hf同位素研究:华北克拉通古老陆壳增生及再循环的证据

U-Pb dating and Hf isotope study of Early Archean zircons from the Jiaobei Terrane,North China Craton: Evidence for growth and recycling of ancient continental crust

古老陆壳物质的发现与鉴别是探索地球早期陆壳形成与演化历史的重要内容之一,锆石U-Pb年龄结合Hf同位素研究是该研究的重要手段。本文通过对胶北地体内一个长英质副片麻岩中的锆石开展系统的原位U-Pb定年和微量、稀土元素分析,获得了多个太古宙早期的锆石。根据这些锆石的阴极发光图像、Th/U比值及稀土元素球粒陨石标准化配分模式,它们具有典型岩浆锆石的特征,其中2个分析点给出了3413Ma和3400Ma（～3.4Ga）的锆石U-Pb年龄,7个分析点给出3547±19Ma（MSWD=1.16）的锆石U-Pb年龄,指示太古宙早期的陆壳岩浆事件;结合华北克拉通其它地区的类似研究结果,暗示华北克拉通可能曾经存在比现今出露面积更大的太古宙早期的古老陆壳。这些古老锆石的Hf同位素分析显示,它们的εHf（t）值在-6.19～0.95之间,平均为-2.54,两阶段Hf模式年龄在3737～4353Ma之间,平均值为～4.1Ga,远大于锆石的U-Pb年龄,指示华北克拉通存在～4.1Ga的地壳增生作用及古老陆壳（〉3.55Ga）的再循环。

Identification and discovery of ancient continental crust is an important part of exploring the crustal evolutionary history of the early Earth,and zircon U-Pb age combined with zircon Hf isotope analysis is the major method of the research. In this study,we have conducted in situ U-Pb dating,rare earth element concentrations and Hf isotope analyses on detrital zircons from one felsic paragneiss sample collected from the Jiaobei Terrane,and many Early Archean detrital zircons have been identified. They are igneous origin on the base of analyses of their CL images,Th /U ratios and chondrite normalized REE patterns. The result of the U-Pb analyses indicates that two zircons yield ～ 3. 40Ga（ 3413 Ma and 3400Ma）,and other seven zircons yield a weighted mean ^207 Pb /^206 Pb age of3547 ± 19Ma（ MSWD = 1. 16）,suggesting that two magmatic events took place at ～ 3. 40 Ga and 3. 55 Ga,respectively,combined with similar reports of other places in the North China Craton,implying that the Early Archean continental crust is much larger than present exposure of ancient continental crust. Most of these ancient detrital zircons have generally negative εHf（ t） values from-6.19～ 0. 95（ average =- 2. 54）,two-stage Hf model ages of 3737 ～ 4353Ma（ average = ～ 4. 1Ga） which are much older than their U-Pb ages,implying the recycling of ancient continental crust（ 〉3. 55Ga） and crustal growth prior to 4. 1Ga in the North China Craton.