Geochemical and isotopic composition of Pan-African metabasalts from southwestern Gondwana: Evidence of Cretaceous South Atlantic opening along a Neoproterozoic back-arc
Geochemical and isotopic composition of Pan-African metabasalts from southwestern Gondwana: Evidence of Cretaceous South Atlantic opening along a Neoproterozoic back-arc
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DOI：
10.1016/j.lithos.2014.05.034
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发表时间：
2014-08
期刊：
影响因子：
3.5
通讯作者：
T. Will;H. Frimmel;C. Gaucher;J. Bossi
中科院分区：
文献类型：
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作者：
T. Will;H. Frimmel;C. Gaucher;J. Bossi
对乌拉圭Dom Feliciano带南部Cuchilla Dionisio地体（La Tuna斜长角闪岩）和纳米比亚/南非Gariep带马尔莫拉地体的Chameis Subdae中的变基性岩进行的岩石地球化学和Sr-Nd-Pb同位素研究表明，这些岩石的成分非常相似，可能代表现代南大西洋两侧的同一单元。这两个岩浆岩的基性岩都是拉斑玄武岩和安山岩，具有亏损的稀土元素模式，通常低TiO 2（<1.5wt.%），非常低的Th/Nb比值和缺乏负Nb-Ta异常，所有这些特征都是“正常”洋中脊玄武岩（N-MORB）和/或弧后盆地玄武岩（BABB）的典型特征。此外，两套岩石的Nd同位素组成（ε Nd 630 Ma= 6.7-9.4）均为超温亏损，147 Sm/144 Nd比值均为超温，206 Pb/204 Pb和207 Pb/204 Pb初始比值均较低。La Tuna基性岩的87 Sr/86 Sr初始比值较低，而Chameis变辉长岩样品具有较高的可能与蚀变有关的比值。地球化学和同位素特征与同一成熟新元古代弧后盆地（马尔莫拉盆地）中两个岩石套的形成一致，支持了早期对这些地体变质沉积单元的物源研究得出的结论。根据Cuchilla Dionisio地体和马尔莫拉地体中的基性岩形成于同一弧后盆地的结论，得出结论：（1）拉普拉塔河与卡拉哈里河之间的主泛非缝合带位于南美洲Dom Feliciano带以西，（2）现代南大西洋的开启并不是沿着该缝合线，而是沿着新元古代马尔莫拉弧后盆地的轴向。
A lithogeochemical and Sr–Nd–Pb isotope study of former oceanic crustal rocks from the Cuchilla Dionisio Terrane in the southern Dom Feliciano Belt, Uruguay (La Tuna amphibolites) and metabasites in the Chameis Subterrane of the Marmora Terrane in the Gariep Belt, Namibia/South Africa shows that these rocks are compositionally very similar and probably represent the same unit on opposite sides of the modern South Atlantic. The mafic rocks from both terranes are tholeiitic metabasalts and -andesites and have depleted rare earth element patterns, generally low TiO2(< 1.5 wt.%), very low Th/Nb ratios and lack negative Nb–Ta anomalies, all features that are typical of ‘normal’ mid-ocean ridge basalts (N-MORB) and/or back-arc basin basalts (BABB). In addition, both rock suites have extremely depleted Nd isotope compositions (εNd630 Ma= 6.7–9.4), superchondritic147Sm/144Nd ratios, and low206Pb/204Pb and207Pb/204Pb initial ratios. The87Sr/86Sr initial ratios of the La Tuna mafic rocks are low, whereas the Chameis metagabbro samples have higher, possibly alteration-related ratios. The geochemical and isotopic signatures are consistent with the formation of both rock suites in the same mature Neoproterozoic back-arc basin (Marmora Basin), supporting conclusions drawn from earlier provenance studies of metasedimentary units from these terranes. Other mafic rocks from the Marmora Terrane are interpreted as ocean island basalts that formed in a within-plate setting.A corollary of the conclusion that the mafic rocks in the Cuchilla Dionisio and Marmora Terranes formed in the same back-arc basin is (1) that the main Pan-African suture between the Río de la Plata Craton and the Kalahari Craton lies to the west of the Dom Feliciano Belt in South America, and (2) that the opening of the modern South Atlantic did not occur along that suture but along the axis of the Neoproterozoic Marmora back-arc basin.