Global nature of the Paleoproterozoic Lomagundi carbon isotope excursion: A review of occurrences in Brazil, India, and Uruguay
Global nature of the Paleoproterozoic Lomagundi carbon isotope excursion: A review of occurrences in Brazil, India, and Uruguay
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DOI：
10.1016/j.precamres.2010.06.017
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发表时间：
2010-10
影响因子：
3.8
通讯作者：
A. Maheshwari;A. Sial;C. Gaucher;J. Bossi;A. Bekker;V. Ferreira;A. Romano
中科院分区：
文献类型：
--
作者：
A. Maheshwari;A. Sial;C. Gaucher;J. Bossi;A. Bekker;V. Ferreira;A. Romano
印度西北部阿拉瓦利超群、巴西米纳斯超群和乌拉圭Paso Severino组新剖面的古元古代碳酸盐岩中发现了正碳同位素偏移。位于巴西米纳斯吉拉斯的2.42 Ga Gandarela组含有红色碳酸盐相BIF，分级为白云岩和石灰岩，δ 13 C值范围为−1.6至+0.4‰ V-PDB。印度和巴西的海洋浅水碳酸盐岩（印度西北部的Jhamarkotra组和巴西米纳斯吉拉斯州的Cercadinho和Fecho do Funil组）中的正C同位素偏移（高达+ 11‰ V-PDB）与全球2.22-2.1Ga碳酸盐岩序列中观察到的Lomagundi偏移相当。在乌拉圭，彼德拉阿尔塔地体深水Paso Severino组中高达+11.6‰ V-PDB的δ 13 C值与约1000年的沉积相一致。2.15Ga，产于该单元顶部的英安岩U-Pb年龄为2146± 7 Ma。负δ 13 C值也存在于Paso Severino组的碳酸盐中，但不能排除与有机质矿化有关的起源。巴西巴伊亚州Rio Itapicuru绿岩带的薄碳酸盐层与深水黑色页岩有关，如Paso Severino组，碳同位素值高达+9‰ V-PDB。巴西巴伊亚的Medrado-Ipueira地区的Jacurici岩中的高变质级碳酸盐岩的碳同位素值高达+6.9‰ V-PDB，与根据与Medrado苏长岩的侵入接触和年龄推断的最小年龄2085± 5 Ma相一致。在印度、巴西和乌拉圭没有发现古元古代冰川事件的证据，这些冰川事件在全球2.45- 2.22 Ga的沉积序列中得到确认。巴西的Gandarela组和Cercadinho组与印度的带状片麻状杂岩和下Aravalli超群之间的不整合面可能解释了冰川记录的缺失。这里提出的成分和同位素数据研究古元古代碳酸盐岩继承允许他们融入全球记录的古元古代演化以及与其他继承类似的年龄。该研究强调了Lomagundi短途旅行的全球性质。此外，它表明，Lomagundi漂移记录在浅水（Aravalli和Minas超群）和深水碳酸盐（Paso Severino组和Rio Itapicuru绿岩带）否定了叠层石生产力和高盐度条件对浅水，开阔海洋和孤立盆地中沉积的碳酸盐的碳同位素值的重大影响。
Positive carbon isotope excursion is reported from Paleoproterozoic carbonates of the Aravalli Supergroup (northwestern India), the Minas Supergroup (Brazil), and new sections of the Paso Severino Formation (Uruguay). The 2.42Ga Gandarela Formation, Minas Gerais, Brazil, contains red carbonate-facies BIF grading into dolostones and limestones and yielding δ13C values ranging from −1.6 to +0.4‰ V-PDB. The positive C-isotope excursion (up to + 11‰ V-PDB) in marine shallow-water carbonates in India and Brazil (Jhamarkotra Formation in northwestern India, and Cercadinho and Fecho do Funil formations in Minas Gerais State, Brazil) is comparable to that observed in 2.22–2.1Ga carbonate successions worldwide that were deposited during the Lomagundi excursion. In Uruguay, δ13C values up to +11.6‰ V-PDB in the deep-water Paso Severino Formation of the Piedra Alta Terrane are compatible with deposition at ca. 2.15Ga, as indicated by the 2146±7Ma U–Pb age of dacites occurring at the top of the unit. Negative δ13C values are also present in carbonates of the Paso Severino Formation, but an origin related to organic-matter remineralization cannot be ruled out. Thin carbonate beds in the Rio Itapicuru greenstone belt, Bahia State, Brazil, are associated, as in the Paso Severino Formation, with deep-water black shales and have carbon isotope values up to +9‰ V-PDB. High metamorphic grade carbonates of the Jacurici terrane in the Medrado-Ipueira area, Bahia, Brazil, have carbon isotope values up to +6.9‰ V-PDB, consistent with their minimum age of 2085±5Ma inferred from the intrusive contact with and the age of the Medrado norite. No evidence was found in India, Brazil, or Uruguay for Paleoproterozoic glacial events recognized in the 2.45–2.22Ga sedimentary successions worldwide. Unconformities between the Gandarela and Cercadinho formations in Brazil and the banded gneissic Complex and the Lower Aravalli Supergroup in India might explain the absence of glacial record. Compositional and isotopic data presented here for studied Paleoproterozoic carbonate successions allow their integration into the global record of the Paleoproterozoic evolution as well as correlation with other successions of similar age. The study highlights the global nature of the Lomagundi excursion. Furthermore, it indicates that the Lomagundi excursion is recorded in both shallow-water (Aravalli and Minas supergroups) and deep-water carbonates (Paso Severino Formation and Rio Itapicuru greenstone belt) negating a significant impact of stromatolite productivity and hypersaline conditions on carbon isotope values of carbonates deposited in shallow-water, open-marine and isolated basins.