GeochemicalCharacteristics and Geological Significance of Siliceous Rocks in ModingFormation of Early Devonian in Eastern Margin of Youjiang Basin
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摘要: 右江盆地东缘南宁地区泥盆系发育大量碎屑岩至碳酸盐岩夹层状硅质岩、硅质团块和硅质结核的海相沉积。层状硅质岩最早出现于早泥盆世莫丁组（D1m），主量、微量和稀土元素研究表明：莫丁组硅质岩具较高的SiO2含量（91.86%-95.57%，均值为94.36%），较低的Al/(Al+Fe+Mn)值（0.05-0.30，均值为0.12），较高的Y/Ho值（24.20-48.79，均值为37.14）和U/Th值（0.90-11.84，均值为5.72），具明显的Ce负异常（δCe=0.32-0.83，均值为0.62）和Eu正异常（δEu=1.15-2.92，均值为1.76）。岩石地球化学综合判别表明，莫丁组硅质岩为热液成因。结合古生物、沉积特征及前人在邻区的研究成果，认为莫丁组硅质岩形成于开阔海盆的沉积环境。区域对比显示，右江盆地早泥盆世以来的海侵由南东向北西发展，海水自南东向北西逐渐变浅，至埃姆斯期晚期海侵范围持续扩大，盆地东缘已发展成为开阔海盆。莫丁组硅质岩的热液成因进一步证实了右江盆地基底在埃姆斯期晚期发生裂解，而热液活动自东向西，自边缘至内部逐渐减弱的规律表明盆地基底的裂解可能受控于盆地边界的深大断裂，在盆地边缘基底断裂围限的区域热液活动更强、裂解程度更高。Abstract: A series of Devonian clastic rocks to carbonate rocks with intercalated siliceous rocks, siliceous masses and siliceous nodules were developed in Nanning area in eastern margin of Youjiang Basin. The study of the major, trace and rare earth elements shows that the layered siliceous rocks, firstly found in Moding Formation (D1m) in early Devonion, carried higher SiO2 values (91.86 to 95.57%, average 94.36%), lower Al/(Al+Fe+Mn) values (0.05 to 0.30, average 0.12), higher Y/Ho values (24.20 to 48.79, average 37.14), and higher U/Th values (0.90 to 11.84, average 5.72)with obvious negative Ce anomalies (0.32 to 0.83, average 0.62) and positive Eu anomalies (1.15 to 2.92, average 1.76). The rockswereof hydrothermal genesis under overall assessment, considered to form in the depositionalenvironment of open sea basin according to paleontologicaland sedimentary characteristics and former research. The regional comparison tells that since early Devonianthe transgression of Youjiang Basin developed from southeast to northwest as sea water gradually became shallower, continuing to expand in late Stage of Emsian while the eastern margin of Youjiang Basin developed into an open sea basin. Hydrothermal genesis of siliceous rocksin Moding Formation further confirmed that the basement of Youjiang Basin had been rifted in late stage of Emsian. Gradually decrease of hydrothermal activity from east to west and from margin to interior suggests that the rifting may be affected by the deep and big fault from the basin edge, while hydrothermal activity was stronger and rifting wasgreater on the edge of the basin basement with faults surrounded.
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Key words:
- eastern margin of Youjiang Basin /
- Moding Formation /
- siliceous rocks /
- hydrothermal genesis /
- rifting /
- Early Devonion
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