水城-紫云-南丹裂陷盆地晚古生代硅质沉积物地球化学特征及其地质意义

Geochemical Features of Siliceous Sediments of the Shuicheng-Ziyun-Nandan Rift Basin in the Late Paleozoic and Their Tectonic Implication

水城-紫云-南丹裂陷盆地位于右江盆地北缘,晚古生代该盆地内广泛分布包括硅质岩在内的碳酸盐岩台盆相沉积,其沉积组合及地球化学特征不同于大陆架型以及增生杂岩中硅质岩。对盆地内硅质沉积物的地球化学特征研究表明,南丹上泥盆统榴江组底部和河池中下二叠统四大寨组硅质沉积物多为基性火山喷发相关的火山热液成因,其具有较低的Al/(Al+Fe+Mn)值和Al2O3/TiO2值,紫云中下二叠统四大寨组硅质沉积物的Al/(Al+Fe+Mn)值和Al2O3/TiO2值分别为0.62±0.16和11.70±4.30,为含有火山碎屑的非热液成因,其他硅质沉积物多为正常海相沉积。晚泥盆世—早石炭世盆地内硅质沉积物多与泥岩和灰岩共生,硅质沉积物常具有中等Ce负异常(多在0.60～0.85之间)以及较高的Y/Ho值(多在30～45之间),除去SiO2稀释作用的影响后,其具有较低的稀土元素含量(∑REE+Y含量相当于PAAS组成的3倍以下)。而伴随着盆地的裂陷和扩张,到早中二叠世,盆地内硅质沉积物多与深灰色厚层灰岩共生,硅质沉积物常具有明显的Ce负异常(0.06～0.61)以及与开阔洋盆海水相似的Y/Ho值(40～92),除去SiO2稀释作用的影响后,其具有较高的稀土元素含量(∑REE+Y含量相当于PAAS组成的2倍以上)。

The Shuicheng-Ziyun-Nandan rift basin （SZNRB） is located in the northern margin of the YoujiangBasin. During late Paleozoic, the basin was dominated by carbonate platform-basin faices deposition including siliceous rock. Sedimentary assemblage and geochemical features of the siliceous rock are different from that of siliceous rocks in the continental shelf and accretionary complexes. Geochemical analysis for siliceous sediments indicates that siliceous deposits from the bottom of the Upper Devonian Liujiang Formation in the Nandan area and the Lower-Middle Permian Sidazhai Formation in the Hechi area were genetically associated with basaltic volcanic eruption and hydrothermal activity, and showed lower A1/（AI＋Fe＋Mn） and A1203/TiO2 values. The siliceous sediments of the Lower-Middle Permian Sidazhai Formation in the Ziyun area have a A1/（AI-t-Fe＋Mn） value of 0. 62-t-0.16 and A1203/TiO2 value of 11.70~4.30, suggesting a pyroclast-bearing nonhydrothemal origin, while the other siliceous sediments were precipitated in normal marineenvironments. During the Late Devonian-Early Carboniferous, siliceous sediments were intercalated with mudstone and limestone, and show moderately negative Ce-anomalies （0. 6~0.85mostly） and higher Y/Ho values （30~45 mostly）, and low ~REE＋Y values three times lower than PAAS-Iike composition after deducing the effect of SiO2 dilution. With rifting and expanding of the basin and, siliceous sediments was interbedded with dark thick limestoneto the Early-Middle Permian. The siliceous sediments are characterized by large negative Ce-anomalies（0.06 n0. 61） and a similar Y/Ho values（40~92） as that in open-ocean seawater, and a high REE＋Y value twice higher than PAAS-like composition after deducing the effect of SiO2 dilution.