震旦纪地层黄铁矿硫同位素组成时－空变化特征及扬子地块与晚元古超大陆关系的论证

 扬子地区早震旦世时期沉积岩黄铁矿具有重的硫同位素组成,其δ¹⁴S值从早震旦世椿木组地层的24‰左右逐渐变得更正。在早震旦世扬子地区广泛沉积碳酸锰矿和黑色页岩的民乐组时期达到极正值+60‰,然后又降低至+16‰-+20‰。上述硫同位素组成的时-空变化特征支持了扬子地块属晚元古Rodinia超大陆的一个组成部分的假设。从晚震旦世早期陡山沱组底部开始,沉积岩中黄铁矿硫同位素的δ¹⁴SS出现负值,并继续降低至-27‰以下。早震旦世晚期-晚震旦世早期沉积岩中硫同位素组成特征反映了沉积盆地古海洋环境的剧烈变化,它与大约7亿年时Rodinia超大陆的解体以及扬子地块与其它大陆分离的地质推测相吻合。

TEMPDRAL AND SPACIAI, VARiAT10N IN δ34S VALUES OF PYRITE FROM S1N1AN STRATA DISCUSSION ON RELATIONSHIP BETWEEN YANGTZE BLDCK AND THE LATE PROTEROZOIC SUPERCONTINENT

The studied Sinian strata are distributed in Hunan, Guizhou and Sichuan Provnces, and include the Early Siruan Chumnu, Minle, and Nantuo, and the Late Siuan houshantuo and Uengying Formations. They are underlain unconformably by slightly metamorphosed nx:ks and orerlain confurmahly by the Early Cambrian siliceous rock,phosphorus nodule-bearing black shale and muddy limestone. Chunmu Formation is composed of pebbly sandstone, sandstone and silty mudstone. Minle Formation is composed of manganese carbonate ore, black shale, dolostone and dray shale. Nantuo Formation is the tillite horizon widely distributed in south China. Toushantuo Formation consists or dolostone, black shale, gray shale and siltstonc. Dcngying Formation is composed of limestone. The estimated age of the boundary between tire Sinian and Prc-Sinian strata is 74b Ma, and the Rb-Sr isochronic age of the manganese carbonate ore beds at the bottom of Minle Formation is (728±27) Ma.The Early Sioian strata have an anomalous heavy sulphur isotopic composition in pyrite The δ³⁴S values are around+24‰ at the bottom increase upwards, and can reach up to+60‰-64‰ in the manganese carbonate ore and the adjacent black shales. The very positmc δ³⁴S values of the Early Sinian strata are in accordance with the isotopic composition feature of the world-wide late Proterozoic black shales that, probably, reflects a global supercontinent tectonic setting. It seems that the Yangtze block had already joined the aurercontinent R odinia in the Early Sinian time.A marked variation in the sulphur isotopic composition of pyrite is observed in the Late Sinian strata. The negative δ³⁴S values occur at the bottom,decrease upwards, and maintain-22‰-30‰ which are typical of the normal marine sedimentary rocks. This variation in sulphur isotopic composition of the Sinian strata indicates great changes from close to open marine basin conditions. It is in accordancc with the suggestion that the breakup of the Rodinia around 0.7Ga scparatcd Yangtzc (plus Cathaysia) blocks from the other continents.