Chemistry of chromites from Arroio Grande Ophiolite (Dom Feliciano Belt, Brazil) and their possible connection with the Nama Group (Namibia)
Chemistry of chromites from Arroio Grande Ophiolite (Dom Feliciano Belt, Brazil) and their possible connection with the Nama Group (Namibia)
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DOI：
10.1016/j.jsames.2017.09.032
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发表时间：
2017-12
影响因子：
1.8
通讯作者：
R. C. Ramos;Edinei Koester;C. Porcher
中科院分区：
文献类型：
--
作者：
R. C. Ramos;Edinei Koester;C. Porcher
本文介绍了位于巴西/乌拉圭边界附近的Dom Feliciano带东南部的大阿罗约蛇绿岩蛇纹滑石片岩中发现的铬铁矿的矿物化学研究。利用电子显微镜扫描和电子探针技术，这项研究发现了一个超俯冲带签名的铬铁矿，以及交代作用的证据。它证实了以前的假设，即认为超俯冲带起源的大阿罗约变火成岩的原岩和交代成因的铬铁矿镁片岩。所研究的铬铁矿具有高的Cr#（0.65-0.77）和Fe 2 +#（0.88-0.95），低的MgO（0.85-2.47 wt%）和TiO 2（0.01-0.19 wt%）以及异常高的ZnO浓度（高达1.97 wt%）。的结果进行了比较，从碎屑铬铁矿的Schwarzrand和鱼河亚组的纳马组（纳米比亚）的化学数据，表明他们是在后者中发现的成分相似。而这些铬铁矿又被认为是来自西部的大洋性马尔莫拉地体（Gariep带）（今坐标）。考虑到后者和大阿罗约蛇绿岩中的大洋变镁铁质岩具有共同的整体岩石地球化学特征（在本文中解释为同一古洋底--马尔莫拉弧后盆地的碎片），有可能提出这样的假设，即来自所研究蛇绿岩的碎屑物质也可能在那马群中发现。在Schwarzrand亚群和Fish River亚群中发现了源自最东端Dom Feliciano带（即大阿罗约蛇绿岩所在地区）的沉积物（与Pelotas-Aiguá蛇绿岩花岗岩类有关），这一事实进一步证实了这一点。因此，我们认为，大阿罗约蛇绿岩碎屑沉积物也可能有助于在晚埃迪卡拉-下寒武世期间纳玛盆地的充填。
The present paper shows a mineral chemistry study in chromites found in serpentine-talc schists of the Arroio Grande Ophiolite, located in the southeastern Dom Feliciano Belt, near the Brazil/Uruguay border. Using electron microscope scanning and electron microprobe techniques, this study found a supra-subduction zone signature in the chromites, together with evidence of metasomatism. It corroborates previous hypothesis that suggested a supra-subduction zone origin for the protoliths of the Arroio Grande meta-igneous rocks and a metasomatic origin for the chromite-bearing magnesian schists. The studied chromites present high Cr# (0.65–0.77) and Fe2+# (0.88–0.95), low MgO (0.85–2.47 wt%) and TiO2(0.01–0.19 wt%) and anomalous high concentration of ZnO (up to 1.97 wt%). The results were compared with chemical data from detrital chromites from the Schwarzrand and Fish River Subgroups of the Nama Group (Namibia), demonstrating that they are compositionally similar with those found in the latter. These chromites, in turn, are believed to have been derived from the oceanic Marmora Terrane (Gariep Belt) in the west (present-day coordinates). Taking into consideration that oceanic metamafites from both the latter and the Arroio Grande Ophiolite share common bulk-rock geochemical features (in this paper interpreted as fragments of the same paleo-ocean floor – the Marmora back-arc basin), it is possible to raise the hypothesis that detrital material derived from the studied ophiolite might also be found in Nama Group. It is reinforced by the fact that sediments (related to the Pelotas-Aiguá Batholith granitoids) derived from the easternmost Dom Feliciano Belt, i.e. the region where Arroio Grande Ophiolite is located, is found in both Schwarzrand and Fish River Subgroups. Thus, we suggest that Arroio Grande Ophiolite detrital sediments might also have contributed to the Nama Basin infilling during Late Ediacaran-Lower Cambrian.