摘要
本文报道了内蒙古大青山地区早前寒武纪变质岩石的锆石Hf同位素和稀土组成。两个古元古代晚期(1.9～2.1Ga)变质碎屑沉积岩样品中碎屑锆石的n(176Hf)/n(177Hf)c、tDM1(Hf)和tDM2(Hf)分别为0.281079～0.281502、2548～3000Ma、2612～3153Ma和0.280916～0.281451、2533～2717Ma、2600～3404Ma;一个古元古代早期(2.37Ga)变质辉长岩样品中岩浆锆石的εHf(t)和tDM1(Hf)分别为1.50～6.68和2449～2647Ma,表明大青山及邻区在新太古代晚期—古元古代早期存在强烈的构造岩浆热事件,既有地幔添加又有壳内再循环作用。三个样品的边部变质锆石εHf(t)、tDM1(Hf)和tDM2(Hf)分别为-9.49～3.91、2201～2686Ma、2285～2887Ma;-7.29～-2.42、2350～2540Ma、2499～2740Ma和-5.46～-0.53、2319～2507Ma、2443～2687Ma,Th/U比值普遍小于0.1。与核部锆石相比,边部变质锆石tDM2(Hf)变小,Th/U比值和稀土含量降低,但稀土模式十分类似。研究表明,变质锆石增生边的形成及其Hf同位素、稀土和U—Th组成受核部锆石和变质作用的双重制约。变质增生边的形成至少部分与核部锆石溶解以后的再结晶有关,变质流体起了重要作用。
This paper reports Hf isotope and REE composition of zircons from metamorphic rocks in the Daqing Mountains, Inner Mongolia. Detrital zircons from two meta- sedimentary rock samples have [n（176 Hf）/n（177 Hf） ]c, tDM1 （Hf） and tDM2 （Hf） ranging from 0. 281079 - 0. 281502, 2548 3000 Ma and 2612-3153 Ma and from 0. 280916-0. 281451, 2533-2717 Ma and 2600-3404 Ma, respectively. The εHf （t） and tDM1 （Hf） of magmatic zircon from one meta--gabbro sample range from-5.45 -6.68 and 2319-2647 Ma, respectively. These indicate that late Neoarhcaean--early Palaeoproterozoic tectono--magmatic events occurred in the Daqing Mountains and adjacent areas, with both mantle addition and crustal recycling. Metamorphic zircon rims commonly show young tDM2 （Hf） when comparing with detrital and magmatic zircon cores, suggesting that Lu--Hf compositions of the rocks and minerals were changed during metamorphic processes. Metamorphic zircon rims are similar in REE pattern to adiacent detrital and magmatic cores, but commonly being lower in total REE content. Most of the metamorphic rims are smaller than 0. 1 in Th/U ratio, being quite different from the cores which Th/U ratios are generally higher than 0.5. It is speculated that metamorphic fluid was high in n（^176 Hf）/n（^177 Hf） ratio and U and low in Th and REE and played an important role in metamorphic zircon formation.
出处
《地质论评》
CAS
CSCD
北大核心
2009年第4期509-520,共12页
Geological Review
基金
中国地质科学院地质研究所基本科研业务费资助项目(编号J0901)
国土资源部重点项目(编号1212010711815)的成果
关键词
锆石
HF同位素
稀土模式
早前寒武纪
大青山
Zircon
Hf isotope
REE pattern
Early Precambrian
Daqing Mountains Inner Mongolia
