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首页> 外文期刊>Geochimica et Cosmochimica Acta: Journal of the Geochemical Society and the Meteoritical Society >The Upper Neoproterozoic-Lower Cambrian of the Central Iberian Zone, Spain: Chemical and isotopic (Sm-Nd) evidence that the sedimentary succession records an inverted stratigraphy of its source
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The Upper Neoproterozoic-Lower Cambrian of the Central Iberian Zone, Spain: Chemical and isotopic (Sm-Nd) evidence that the sedimentary succession records an inverted stratigraphy of its source

机译：西班牙伊比利亚中部地区的上新元古代-下寒武统：化学和同位素（Sm-Nd）证据表明沉积演替记录了其地层的倒置地层

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The Upper Neoproterozoic-Lower Cambrian sedimentary succession in the Central Iberian Zone has recently been divided into 12 sedimentary units (I to XII). Units I to IV are of Late Neoproterozoic age, and units V to XII are Early Cambrian. Throughout the stratigraphic section, shales with similar ranges of SiO2 and Al2O3 have TiO2 and Zr contents that are coupled and vary gradually from the bottom (unit I, TiO2 = 1.0-1.1%, Zr = 245-287 ppm) to the top (unit XII, TiO2 = 0.74-0.86%, Zr = 141-192 ppm). These two parameters clearly distinguish Upper Neoproterozoic from Lower Cambrian compositions, as do certain elemental ratios (Al2O3/TiO2, Rb/Zr, Nb/Ti), which vary from low values at the bottom of the section to high at the top. However, other elements (the rare earth elements [REEs], Y) do not fit this general trend, with heavy REEs and Y showing extreme concentrations in some Lower Cambrian units, which also have negative Cc anomalies. Units I to III and units XI to XII have uniform T-DM neodymium model ages (similar to1.1 and 1.7 Ga, respectively), but the units between these two groups have erratic values between 1.4 Ga (unit VII) and 3.4 Ga (unit VIII). Thus, the bottom and top units have neodymium isotope ratios that indicate the presence of two source compositions: a composition that records a juvenile contribution younger than 1.1 Ga and a composition mainly derived from an old basement. Some samples from Lower Cambrian units have disturbed REE abundances and neodymium isotope ratios that are attributed to extensive chemical alteration that resulted in REE and Y redistribution. This alteration probably affected the material now preserved as unit IV during a period of relative sea level fall at the end of the Late Neoproterozoic. Both elemental and isotopic results support the suggestion that the Upper Neoproterozoic-Lower Cambrian stratigraphic section records an inverse stratigraphy of a source hinterland composed of a cover sequence of relatively juvenile crustal materials underlain by an older basement. Through time, the deeper crust gradually increased its contribution to the sediments in the basin from Late Neoproterozoic to Lower Cambrian times. These results are compatible with data from many other European zones. Copyright (C) 2003 Elsevier Science Ltd. [References: 69]
机译：最近，伊比利亚中部地区的上新元古代—下寒武统沉积相被划分为12个沉积单元（I至XII）。 I至IV单元为新元古代晚期，V至XII单元为早寒武纪。在整个地层剖面中，SiO2和Al2O3范围相似的页岩的TiO2和Zr含量是耦合的，并且从底部（单位I，TiO2 = 1.0-1.1％，Zr = 245-287 ppm）到顶部（单位）逐渐变化XII，TiO 2 ＝ 0.74-0.86％，Zr ＝ 141-192ppm）。这两个参数清楚地将上新元古代与下寒武统成分区分开，某些元素比（Al2O3 / TiO2，Rb / Zr，Nb / Ti）也是如此，这些元素比从截面底部的低值到顶部的高值变化。但是，其他元素（稀土元素[REE]，Y）不符合这种总体趋势，重稀土元素和Y在某些下寒武统单元中显示出极高的浓度，这些元素也具有负Cc异常。 I至III单元和XI至XII单元具有统一的T-DM钕模型年龄（分别类似于1.1和1.7 Ga），但两组之间的单元值在1.4 Ga（VII单元）和3.4 Ga（ VIII）。因此，底部和顶部单元的钕同位素比表明存在两种源成分：记录不到1.1 Ga的未成年人贡献的成分和主要来源于旧地下室的成分。下寒武纪单元的一些样品扰乱了稀土元素的丰度和钕同位素比，这归因于广泛的化学变化，导致了稀土元素和Y的重新分布。这种变化可能影响了新元古代晚期末相对海平面下降期间现在被保存为第四单元的物质。元素和同位素结果都支持这样的建议，即新元古代—下寒武统地层剖面记录了一个原始内陆的逆地层，该原始内陆由一个相对较幼的地壳物质覆盖在一个较旧的地下室之下构成。随着时间的流逝，从新元古代晚期到下寒武统时期，深层地壳对盆地沉积物的贡献逐渐增加。这些结果与来自其他许多欧洲区域的数据兼容。版权所有（C）2003 Elsevier Science Ltd. [参考：69]

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