Enigmatic provenance of carbonate clasts in Cryogenian glacial diamictite of the Nantuo Formation in South China
Enigmatic provenance of carbonate clasts in Cryogenian glacial diamictite of the Nantuo Formation in South China
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DOI：
10.1016/j.precamres.2022.106734
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发表时间：
2022-08
影响因子：
3.8
通讯作者：
M. Nolan;S. Xiao;B. Gill;Rachel Reid;Maxwel F. Schwid
中科院分区：
文献类型：
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作者：
M. Nolan;S. Xiao;B. Gill;Rachel Reid;Maxwel F. Schwid
扬子区南沱组的冰川杂岩记录了马里诺冰期的主要环境变迁。虽然南沱组中硅质碎屑物质的来源已被碎屑锆石分析所限制，但该组中丰富的碳酸盐碎屑的源区尚未确定。鉴于中元古代神农架群是华南地区唯一的主要前低温期沉积碳酸盐岩序列，它被认为是扬子北方南沱白云岩碎屑的来源，并可能是整个扬子区低温期碳酸盐碎屑的主要供应者，但这种推测性的物源尚未得到验证或正式调查。为了验证这一假设，对湖北宜昌附近采集的18个碎屑的δ 13 C和δ 18 O值进行了沿着岩相学分析，以确定岩性、成岩蚀变及其与神农架群碳酸盐岩的关系。这些碎屑的δ18O和δ13C值与神农架群的δ18O和δ13C值不同，不可能来源于神农架群。因此，神农架群不可能是南沱碳酸盐碎屑的唯一物源。相反，我们认为研究区的南沱碳酸盐碎屑来源于（1）华南地区由于侵蚀而丧失的前Marinoan碳酸盐序列或（2）另一个克拉通（例如，在低温纪与扬子江相邻的印度江。鉴于古地理和碎屑锆石的物源分析表明，在扬子板块和印度板块之间的分离在低温期，我们赞成前者的起源，神农架群是不是唯一的来源，碳酸盐碎屑从南沱组。
Glacial diamictites of the Cryogenian Nantuo Formation of the Yangtze Craton of South China record major environmental transitions during the Marinoan glaciation. Although the provenance of the siliciclastic materials in the Nantuo Formation have been constrained via detrital zircon analysis, the source areas for the abundant carbonate clasts within the formation have yet to be determined. Given that the Mesoproterozoic Shennongjia Group is the only major extant pre-Cryogenian sedimentary carbonate succession in South China, it has been proposed as a source of the Nantuo dolostone clasts in the northern Yangtze Craton and may have been a major supplier of Cryogenian carbonate clasts across the entire Yangtze Craton, but this speculative provenance has not been tested or formally investigated. To test this hypothesis, δ13C and δ18O values of 18 clasts collected near Yichang, Hubei Province were analyzed along with petrographic examination to determine lithology, diagenetic alteration, and relationship to the Shennongjia Group carbonates. Our data show that the δ18O and δ13C values of these clasts are distinct from and thus unlikely sourced from the Shennongjia Group. Therefore, the Shennongjia Group cannot be the sole source of Nantuo carbonate clasts. Instead, we propose the Nantuo carbonate clasts in the study area were derived from either (1) pre-Marinoan carbonate successions in South China that were lost due to erosion or (2) another craton (e.g., the Indian Craton) adjacent to the Yangtze Craton during the Cryogenian. Given that paleogeographic and detrital zircon provenance analysis suggest separation between the Yangtze Craton and the Indian Craton in the Cryogenian Period, we favor the former origin, and that the Shennongjia Group is not the exclusive source of carbonate clasts from the Nantuo Formation.