鞍山3.8 Ga奥长花岗质岩石的地球化学和 Nd、Sr同位素组成及其意义

Geochemical and Nd and Sr Isotopic Compositions of 3. 8 Ga Trondhjemitic Rocks from the Anshan Area and their Significance

鞍山地区至少存在两种类型的3.8 Ga陆壳岩石。白家坟奥长花岗质岩石具有正片麻岩的性质。它们相对高SiO_2、Na_2O,低TFeO、MgO、CaO,稀土含量和轻重稀土分离程度都很低。岩石Nd、Sr同位素组成存在很大变化,其中t_(DM)和ε_(Nd)(3.8Ga)分别为3.07～3.87Ga和0.82～12.23。它们是由更早期但地壳滞留时间不长的TTG花岗质岩石部分熔融形成。Nd、Sr同位素组成变化与后期扰动有关。东山奥长花岗质条带状混合岩与白家坟奥长花岗岩存在明显区别,可能表明其成因的不同。根据实际资料,全球范围内≥3.8 Ga的TTG花岗质岩石普遍具有异常高的ε_(Nd)(t_0)正值以及稀土含量和轻重稀土分离程度低的组成特征,本文就出现这些现象的原因和意义进行了讨论。

In the Anshan area there are at least two types of 3. 8 Ga continental rocks. The trond-hjemitic rocks from the Baijiafen area have the character of orthogneisses. They are characterized by relatively high SiO2 and Na2O, low TFeO, MgO and CaO, rather low ∑REE contents and a low to moderate (La/Yb)n ratio. The Nd and Sr isotopic compositions vary greatly, with TDM and εNd(3. 8 Ga) being 3. 07-3. 8 Ga and 0. 82-12. 23 respectively. They were derived from older TTG rocks that only stayed in the crust for a short period of time. The banded trond-hjemitic rocks from the Dongshan scenic spot are quite different in many aspects from the Baijiafen trondhjemitic rocks, indicating that they might be of different origins. The high εNd(t0) positive values, low 2REE contents and low (La/Tb)n ratio are not only the features of the composition of the 3. 8 Ga TTG rocks from the Anshan area, but also, in general, those of Early Archaean TTG rocks all over the world. The causes for the occurrence of these features and their significance are also discussed.