内蒙古大青山地区早前寒武纪变质岩的锆石Hf同位素组成和稀土模式

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（1.9～2.1 Ga）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、2548～3000 Ma、2612～3153 Ma和0.280916～0.281451、2533～2717 Ma、2600～3404 Ma; 一个古元古代早期（2.37 Ga）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2449～2647 Ma，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、2201～2686 Ma、2285～2887 Ma；-7.29～-2.42、2350～2540 Ma、2499～2740 Ma和-5.46～-0.53、2319～2507 Ma、2443～2687 Ma，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。

Hf Isotope Composition and REE Pattern of Zircons from Early Precambrian Metamorphic Rocks in the Daqing Mountains, Inner Mongolia

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(176Hf)/n(177Hf)]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 eHf (t) and tym (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 adjacent 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 w(176Hf)/w(177Hf) ratio and U and low in Th and REE and played an important role in metamorphic zircon formation.