陕西商县-丹凤地区丹凤群变质玄武岩的微量元素和同位素地球化学

Trace Element and Isotopic Geochemistry of Metabasalts from Dafeng Group (DFG) in Shangxian-Danfeng Area, Shaanxi Province

丹凤祥变质玄武岩可以分为两类：一类是ＬＲＥＥ富集型的钙碱性玄武岩，另一类是ＬＲＥＥ平坦型的拉斑玄武岩。ＬＲＥＥ富集型的变质玄武岩［（Ｃｅ／Ｙｂ）Ｎ为５～２０］贫Ｔｉ、Ｎｂ、Ｔａ，Ｔｈ＞Ｔａ，Ｎｂ／Ｌａ＜０．８，Ｈｆ／Ｔｈ＞８，Ｚｒ／Ｙ＜３，表明其形成于消减带之上的洋内岛弧环境。该玄武岩的εＮｄ（ｔ）＝＋４．９～＋７．３，εＳｒ（ｔ）＝３．０～＋３２，可能是亏损地幔端元（ＤＭＭ）与第二类富集地幔端元（ＥＭⅡ）混合形成的。大多数玄武岩的（２０６Ｐｂ／２０４Ｐｂ）ｉ比值（１６．５～１７．６）较低，（２０８Ｐｂ／２０４Ｐｂ）ｉ和（２０７Ｐｂ／２０４Ｐｂ）ｉ比值（分别为３６．２～３６．９和１５．３～１５．５）较高，ε１８Ｏ变化较大（５．５‰～９．２‰）。因此，根据微量元素和同位素地球化学资料，丹凤群变质玄武岩的源区可能包括下述组分：１）亏损的软流圈地幔（ＤＭ）；２）富集地幔（ＥＭ）；３）洋内岛弧下地壳（镁铁和超镁铁质岩）；４）消减带岩片带入的组分。而ＬＲＥＥ亏损的拉斑玄武岩具ＭＯＲＢ的特征，Ｔｈ／Ｔａ和Ｌａ／Ｔａ比值近似等于１，ＴｉＯ２＝１．６５％，Ｔｉ／Ｖ＝２２，指示来自亏损的软流圈地幔。

Danfeng Group(DFG) metabasalts can be divided into two types: one is LREE-enriched cafe-alkaline basalts, the other is LREE-depleted tholeiitic basalts. The LREE-enriched calc-alkaline basalt [Ce/Yb)N = 5～20] is poored in Ti, Nb and Ta contents, and Th>Ta, Nb/La<0. 8, Hf/Th>8, Zr/Y<3,which indicate that it was formed at intra-ocean island arc settings above subduction zone. For these basalts,their εNd (t)= + 4. 9～+ 7.3, εSr (t)=-3. 0～+32, which may be related to the mixing between depleted mantle member(DMM) and enriched mantle Ⅱ material(EMⅡ ). Most of basalts have lower (206Pb/204Pb)i ratios (16. 5～17. 6), and higher (208Pb/204Pb)i and (207Pb/204Pb)i ratios, and variative δ 18O (5. 5‰～9. 2‰ ).SO, the sources of DFG metabasalts could include following units on the basis of trace element and isotopic geochemical data: 1 ) Depleted asthenosphere mantle; 2) Enriched mantle ; 3) Low crust from intra-ocean island (mafie-ultramafic rocks ); 4 ) Component from subduction zone. Contrasting, the LREE-depleted tholeiite from DFG are predominantly MORB-like in character, such as Th/Ta and La/Ta ratios nearly 1,TiO2 = 1. 65%, Ti/V= 22, showing that it was derived from depleted asthenosphere mantle and formed at extension tectonic settings being not related to subduction, however, it is not certain yet whether it is a member of ophiolite or not.