This article presents a detailed petrographic, mineralogical and geochemical study of clay, silt, sand and fine gravel generated in the Paraná–Etendeka large igneous province, once united and today exposed in contrasting climatic and orographic conditions on conjugate sides of the Atlantic Ocean. The comparison between low-relief wetter Uruguay and steeper hyper-dry Namibia allows to investigate the influence of weathering on basalt-derived sediments spanning in grain size from <2 μm to >2000 μm. In Namibia, as in Uruguay, X-ray diffraction analyses of mud samples from streams draining basaltic lavas show only smectitic clay minerals. Macroscopic and microscopic observations indicate that sand is composed in variable proportions of glassy to lathwork volcanic rock fragments, plagioclase and augitic clinopyroxene with minor pigeonite, and gravel dominantly consists of volcanic clasts with chalcedony or quartz geodes occurring locally. Geochemical data highlight the strong grain-size dependence of weathering intensity. Whereas fine gravel closely reflects source-rock composition in both Namibia and Uruguay, and mobile elements in sand are moderately depleted, α^AlE indices increase notably in silt and reach maximum in clay, with mobility sequence Na >> Ca ≥ Mg > Sr > K. Differences in sand versus mud generation potential in the two regions are prominent. Namibian rivers draining the dry Etendeka Plateau carry purely volcaniclastic sand, whereas streams draining even exclusively the Paraná lava field carry sediments containing invariably significant and even overwhelming amounts of recycled quartz, testifying to the much greater sand-generation potential of the very-locally-exposed underlying, interbedded or overlying quartz-rich sandstones and loess deposits. Smectite is dominant throughout the Uruguay River catchment, even in the lower course where sand is pure quartzose and pyroxene negligible. A comparison with basaltic provinces worldwide indicates that, especially in low-relief wet regions where weathering is intense, basalt generates gravel, very minor sand and abundant smectite. Kaolinite forms where chemical leaching is extreme.

中文翻译：

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对比景观和气候条件下大陆洪水玄武岩产生的沙子和泥浆（巴拉那-埃滕德卡共轭火成岩省、乌拉圭和纳米比亚）

本文详细介绍了巴拉那-埃滕德卡火成岩省产生的粘土、粉砂、沙子和细砾石的详细岩石学、矿物学和地球化学研究，该省曾经统一，如今暴露在大西洋共轭两侧对比鲜明的气候和地形条件下。低地势湿润的乌拉圭和更陡峭的超干燥纳米比亚之间的比较允许研究风化对玄武岩衍生沉积物的影响，粒度范围从 <2 μm 到 >2000 μm。在纳米比亚，就像在乌拉圭一样，对排出玄武岩熔岩的溪流中的泥浆样本进行 X 射线衍射分析，结果只显示了蒙脱石粘土矿物。宏观和微观观察表明，沙子以不同比例的玻璃状到板状火山岩碎片、斜长石和带有少量变辉石的正斜辉石组成，砾石主要由火山碎屑组成，局部存在玉髓或石英晶洞。地球化学数据突出了风化强度对粒度的强烈依赖性。而细砾石密切反映纳米比亚和乌拉圭的烃源岩成分，而砂中的移动元素适度枯竭，α^铝E 指数在粉土中显着增加，在粘土中达到最大值，流动性顺序为 Na >> Ca ≥ Mg > Sr > K。两个区域的砂泥生成潜力差异突出。排出干燥的埃滕德卡高原的纳米比亚河流中含有纯粹的火山碎屑砂，而排出甚至完全排出巴拉那熔岩场的河流携带的沉积物含有大量甚至压倒性的再生石英，证明了非常局部暴露的更大的产砂潜力下伏、互层或上覆富含石英的砂岩和黄土矿床。蒙脱石在整个乌拉圭河流域占主导地位，即使在沙子是纯石英和辉石可忽略不计的下游也是如此。与全世界玄武岩省份的比较表明，特别是在风化强烈的低地势潮湿地区，玄武岩会生成砾石、极少量的沙子和丰富的蒙脱石。高岭石形成在化学浸出极端的地方。