扬子地块西缘峨山新元古代A_(2)型花岗闪长岩的成因及构造意义

Petrogenesis and tectonic implications of the Neoproterozoic Eshan A-type granodiorite,western Yangtze Block

在扬子地块西缘出露有大量的新元古代岩浆岩,这些岩石对于重建罗迪尼亚超大陆有着重要的意义。本文对云南峨山岩体的花岗闪长岩和似斑状黑云母二长花岗岩开展了详细的岩石学、岩石地球化学和年代学研究,结果表明,似斑状黑云母二长花岗岩侵位于826.6±2.5 Ma,而花岗闪长岩有着较年轻的结晶年龄818.3±2.8 Ma,花岗闪长岩比似斑状黑云母二长花岗岩有着更低的SiO_(2)含量,但是更高的Al_(2)O_(3)、MgO、Fe_(2) O_(3)、TiO_(2)和P_(2)O_(5)含量。在稀土元素配分曲线和微量元素蛛网图上,两种岩性呈现出相似的特征,都是具有右倾的稀土元素配分样式,呈现出Eu负异常,相对于大离子亲石元素(LILEs)更亏损高场强元素(HFSEs)。似斑状黑云母二长花岗岩富集Nd同位素组分[εNd(t)=-7.92~-5.08)],而花岗闪长岩与之有着相似的Nd同位素值[εNd(t)=-8.30~-4.71]。地球化学数据显示可能的岩石学成因是变质火成岩源区在826 Ma时发生部分熔融形成了峨山似斑状黑云母二长花岗岩并且残留下来了一个麻粒岩化的源区;麻粒岩源区在818 Ma时再次发生部分熔融形成了具有A型属性的峨山花岗闪长岩。结合前人的数据和本文的研究,认为扬子西缘在新元古代时期是一个活动大陆边缘,而华南地块当时在罗迪尼亚的位置更可能是在边缘而不是中心。

Abundant Neoproterozoic magmatic rocks were emplaced in western Yangtze Block,which are important for reconstructing the Rodinia supercontinent.In this paper,new petrologic,lithogeochemical and geochronological studies of the Eshan granodiorites and porphyritic biotite monzogranites were conducted to explore their origin and geodynamic implications.The porphyritic biotite monzogranites were emplaced at 826.6±2.5 Ma,while the granodiorites have younger crystallization age of 818.3±2.8 Ma.Relative to the porphyritic biotite monzogranites,the granodiorites exhibit lower SiO_(2) but higher Al2 O3,MgO,Fe_(2)O_(3),TiO_(2) and P_(2)O_(5) values.In chondrite-normalized REE patterns and trace element spider diagram,both lithologies are enriched in LREEs;compared with HREEs,they have negative Eu anomalies and are depleted in HFSEs relative to LILEs.The porphyritic biotite monzogranites have enriched Nd isotopic compositions[εNd(t)=-1.92~-5.08],consistent with those of the granodiorites[εNd(t)=-8.30~-4.71].The geochemical data indicate that the porphyritic biotite monzogranites are likely to have been derived by partial melting of meta-igneous rocks,leaving a granulite source at ca.826 Ma and subsequent partial melting of the granulite at ca.818 Ma,thus producing the Eshan granodiorites with A-type affinities.Combined with data available,the results obtained by the authors reveal that the western Yangtze Block was an active continental margin and the South China Block occupied an external location rather than an internal location within the Neoproterozoic Rodinia.