桂北前寒武纪花岗岩类岩石的地球化学与成因

Geochemistry and petrogenesis of Precambrian granitoid rocks in northern Guangxi

桂北广泛发育前寒武纪花岗岩类岩石 ,按岩石组合特征可区分为二类 ,一类为花岗闪长质岩石 ,另一类为黑云母花岗质岩石。二类岩石的A/NKC值均在 1 .1 0以上 ,属铝过饱和岩石 ,但花岗闪长质岩石相对贫硅、钾 ,富铁、镁 ,贫铷 ,富锶、钡 ,并具有较高的稀土元素总量和轻重稀土元素比值 ;黑云母花岗质岩石则具更显著的铕负异常。二类岩石εNd(T)值的变化范围与四堡群基底相近 ,但花岗闪长质岩石更接近四堡群基底中的变质镁铁质 -超镁铁质杂岩 ,黑云母花岗质岩石则更靠近四堡群基底中的浅变质沉积岩。对岩石产出动力地质背景的全面分析表明 ,区内二类花岗质岩石是华夏地块与扬子地块碰撞俯冲作用的不同阶段由成熟度不同的四堡群变质基底在不同深度经部分熔融作用的产物。

Precambrian granitoid rocks are widely distributed in northern Guangxi. According to rock associations, these granitoid rocks can be grouped into two types, i. e., graniodioritic rocks and biotite granitic rocks. The A/NKC values of both types of rocks are higher than 1.10, and hence they can both be assigned to aluminum over_saturation rocks. However, geochemical differences between the two types of rocks are also quite obvious. Compared with the biotite granitic rocks, the granodioritic rocks are relatively poor in silica and potassium and enriched in iron and magnesium, and have lower rubidium abundance and higher strontium and barium content. As for REE content, the granodioritic rocks have higher ΣREE values and higher LREE/HREE ratios, whereas the biotite granitic rocks display quite clear negative europium anomalies. The variation ranges of ε Nd ( T ) values of the two types of granitoid rocks are similar to those of the basement rocks (Sibao Group), but the ε Nd ( T ) values of granodioritic rocks are more close to those of the metamorphic mafic -ultramafic rocks of the Sibao Group, and the biotite granitic rocks are close to the metasedimentary rocks of the Sibao Group in this aspect. Based on a comprehensive analysis of their tectonic setting, it is concluded that the two types of granitoid rocks were formed at different stages of the collision between the Cathaysia block and the Yangtze block, and were produced by partial melting of the Sibao Group with different maturities at different depths.