169 / 2019-06-05 18:48:24

寒武纪早期混积浅水碳酸盐稀土组成特征—以汉南古陆周缘仙女洞组为例(Shallow seawater carbonate REE compositions in mixed siliciclastic-carbonate systems: A case study on Xiannüdong Formation of lower Cambrian neighboring Hannan Massif,

寒武系 华南 混积岩 稀土特征 成岩作用 (Lower Cambrian, South China, mixed siliciclastic-carbonate systems, REE composition, carbonate diagenesis)

鲕粒岩和微生物岩等浅水碳酸盐沉积物的稀土组成特征蕴含着指示海水成分，氧化还原性质，以及沉积物来源等方面的线索。寒武纪筇竹寺晚期，受气候和海平面变化等多重因素影响，汉南古陆风化剥蚀程度减弱，研究区内由浑水向清水碳酸盐沉积转变，浅水沉积物类型以鲕粒和微生物建造占主导，陆源碎屑物质含量显著降低。通过全岩，醋酸溶解组分和不溶组分，以及原位激光剥蚀分析结果显示，结构特征保存良好的鲕粒和微生物岩均受到早期成岩作用的影响，稀土组成与现代海洋环境下鲕粒和微生物岩经页岩标准化后的稀土特征存在明显差别。系统的岩石学和矿物学分析显示，该地区沧浪铺早期（对应仙女洞组）浅水碳酸盐在成岩过程中受到陆源碎屑物质的影响，一方面大量自生粘土矿物发育于鲕粒和微生物结构中的晶间孔隙，另一方面河流搬运而来的风化产物颗粒被鲕粒和微生物纹层粘结捕获。这两类非碳酸盐组分对碳酸盐成岩过程中微量和稀土元素的迁移产生影响。此外，这种特殊的稀土配型也可能与寒武纪早期（沧浪铺期）表层水体特殊的氧化还原性质存在关联。(The characteristics of rare earth elements (REE) in shallow water carbonates (e.g., oolite and microbialite) have recently been adopted to explorer the natures of seawater composition, redox condition, and possible terrigenous supply. During the early Age 3 of Cambrian (roughly corresponding to upper Qiongzhusi Formation), the extent of terrigenous supply from Hannan Massif began to decrease and accordingly the depositional circumstances changed from siliciclastic- to carbonate-dominant system that was characteristic of ooid and microbial deposits (Xiannüdong Formation) on inner ramp though episodic siliciclastic input was still exist. Both ooids and microbialites with well-preserved structures have suffered markedly diagenetic alteration evidenced by (1) orange-red luminescence, (2) features of enriched MREE and indistinct Ce anomaly when normalizing to shales, and (3) plentiful authigenic clay minerals observed within intergranular places, associated with a small number of (bonded) exogenetic siliciclastic particles. It is suggested that these non-carbonate components play an essential role in the migration of rare earth and trace elements during early diagenesis. Temporal oxygen-deficient water condition might also contribute to the modification on REE distributions of ooids and microbialites during early Cambrian.)