秦岭勉略带中鞍子山蛇绿杂岩的地球化学——古洋壳碎片的证据及意义

Geochemistry of the Anzishan Ophiolitic Complex in the Mian-Lue Belt of Qinling Orogen——Evidence and Implication of the Palaeo-ocean Crust

鞍子山蛇绿杂岩位于秦岭勉略带东段,它由蛇纹石化变质橄榄岩和MORB型变质镁铁质岩组成。蛇纹石化变质橄榄岩主要是方辉橄榄岩和少量二辉橄榄岩,代表了古洋幔的上部岩石;变质镁铁质岩为斜长角闪岩,其原岩是基性火成岩类,它多以亏损LREE元素和具有很高的^(143)Nd/^(144)Nd值为特征,类似于现代洋壳中的洋中脊玄武岩(MORB),代表了古洋壳的残余部分。这表明鞍子山蛇绿杂岩是勉略混杂岩带中又一个典型的蛇绿岩块,其可能形成在一个孤后盆地或小洋盆的扩张洋脊环境。然而,鞍子山蛇绿杂岩的Pb、Nd同位紊体系明显不同于勉略带西段的三岔子蛇绿岩,它们来源于两个不同的同位素地幔储源库,鞍子山蛇绿杂岩的形成时代可能比西段古洋壳更老,这意味着勉略带应有更复杂的构造演化历史。

The Anzishan ophiolitic complex is located in the eastern section of the Mian-Lue ophiolitic and tectonic melange belt, Qinling orogen, center China. It consists of serpentinized peridotites and metamorphased MORB-type mafic igneous rocks. The serpentinized peridotites (harzbur-gites and a few dunites) represent the upmost part of the palaeo-ocean mantle. Whereas the metamorphosed mafic igneous rocks, which are characterized by depleted LREE and very high Nd isotopic ratios similar to MORB, are remainders of the palaeo-ocean crust. These facts suggest that this complex is another typical ophiolite within the Mian-Lue ophiolitic and tectonic melange belt. The geochemical characters of the meta-mafic rocks reveal that this ophiolitic complex was most probably formed in a small ocean basin or back-arc basin. However, the Pb, Nd isotopic systems of the Anzishan ophiolitic complex from the eastern section are distinct from those of the ophiolites from the western section. The palaeo-ocean crust in Anzishan is probably older than the ophiolites in the western section of the Mian-Lue belt. Thus, the meta-mafic rocks of the Anzishan ophiolitic complex were derived from an isotopic mantle reservoir different from the those in the western section. This infers that the Mian-Liie ophiolitic and tectonic melange belt has a more complex evolution history than hitherto considered.