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首页> 外文期刊>Contributions to Mineralogy and Petrology >An 40Ar/39Ar, Rb/Sr, and stable isotope study of micas in low-grade fold-and-thrust belt: an example from the Swiss Helvetic Alps
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An 40Ar/39Ar, Rb/Sr, and stable isotope study of micas in low-grade fold-and-thrust belt: an example from the Swiss Helvetic Alps

机译：低品位俯冲冲断带中云母的40 Ar / 39 Ar，Rb / Sr和稳定同位素研究：以瑞士Helvetic Alps为例

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White micas in carbonate-rich tectonites and a few other rock types of large thrusts in the Swiss Helvetic fold-and-thrust belt have been analyzed by 40Ar/39Ar and Rb/Sr techniques to better constrain the timing of Alpine deformation for this region. Incremental 40Ar/39Ar heating experiments of 25 weakly metamorphosed (anchizone to low greenschist) samples yield plateau and staircase spectra. We interpret most of the staircase release spectra result from variable mixtures of syntectonic (neoformed) and detrital micas. The range in dates obtained within individual spectra depends primarily on the duration of mica nucleation and growth, and relative proportions of neoformed and detrital mica. Rb/Sr analyses of 12 samples yield dates of ca. 10–39 Ma (excluding one anomalously young sample). These dates are slightly younger than the 40Ar/39Ar total gas dates obtained for the same samples. The Rb/Sr dates were calculated using initial 87Sr/86Sr ratios obtained from the carbonate-dominated host rocks, which are higher than normal Mesozoic carbonate values due to exchange with fluids of higher 87Sr/86Sr ratios (and lower 18O/16O ratios). Model dates calculated using 87Sr/86Sr values typical of Mesozoic marine carbonates more closely approximate the 40Ar/39Ar total gas dates for most of the samples. The similarities of Rb/Sr and 40Ar/39Ar total gas dates are consistent with limited amounts of detrital mica in the samples. The δ18O values range from 24–15‰ (VSMOW) for 2–6 μm micas and 27–16‰ for the carbonate host rocks. The carbonate values are significantly lower than their protolith values due to localized fluid-rock interaction and fluid flow along most thrust surfaces. Although most calcite-mica pairs are not in oxygen isotope equilibrium at temperatures of ca. 200–400 °C, their isotopic fractionations are indicative of either 1) partial exchange between the minerals and a common external fluid, or 2) growth or isotopic exchange of the mica with the carbonate after the carbonate had isotopically exchanged with an external fluid. The geological significance of these results is not easily or uniquely determined, and exemplifies the difficulties inherent in dating very fine-grained micas of highly deformed tectonites in low-grade metamorphic terranes. Two generalizations can be made regarding the dates obtained from the Helvetic thrusts: 1) samples from the two highest thrusts (Mt. Gond and Sublage) have all of their 40Ar/39Ar steps above 20 Ma, and 2) most samples from the deepest Helvetic thrusts have steps (often accounting for more than 80% of 39Ar release) between 15 and 25 Ma. These dates are consistent with the order of thrusting in the foreland-imbricating system and increase proportions of neoformed to detrital mica in the more metamorphosed hinterland and deeply buried portions of the nappe pile. Individual thrusts accommodated the majority of their displacement during their initial incorporation into the foreland-imbricating system, and some thrusts remained active or were reactivated down to 15 Ma.
机译：用40 Ar / 39 Ar和Rb / Sr技术分析了富含碳酸盐的构造体中的白色云母和瑞士Helvetic褶皱-冲断带中其他一些大推力的岩石类型，以更好地分析限制了该地区高山变形的时间。在25个弱变质（从兰花到低绿片岩）样品上进行40 Ar / 39 Ar增量加热实验，得出高原光谱和阶梯光谱。我们解释了大多数阶梯释放光谱是由构造（新形态）和碎屑云母的可变混合物产生的。在单个光谱中获得的日期范围主要取决于云母成核和生长的持续时间，以及新形成和碎屑云母的相对比例。 12个样品的Rb / Sr分析得出的日期约为10-39 Ma（不包括一个异常的年轻样本）。这些日期比相同样品获得的40 Ar / 39 Ar总气体日期稍早。 Rb / Sr日期是使用从碳酸盐为主的宿主岩中获得的初始87 Sr / 86 Sr比率计算得出的，该比率高于正常的中生碳酸盐值，因为与较高的87流体进行了交换 Sr / 86 Sr比率（以及更低的18 O / 16 O比率）。使用大多数中生代海相碳酸盐典型值87 Sr / 86 Sr值计算的模型日期更接近于大多数样品的40 Ar / 39 Ar总气体日期。 Rb / Sr和40 Ar / 39 Ar总气体日期的相似性与样品中少量的碎屑云母一致。对于2–6μm云母，δ18 O值的范围为24-15‰（VSMOW），对于碳酸盐岩宿主岩，δ18 O值的范围为27-16‰。由于局部的流体-岩石相互作用和沿着大多数推力面的流体流动，碳酸盐值明显低于其原生岩值。尽管大多数方解石-云母对在约200℃的温度下都不处于氧同位素平衡。在200–400°C下，它们的同位素分馏表明1）矿物与常见的外部流体之间的部分交换，或2）在碳酸盐与外部流体进行同位素交换之后，云母与碳酸盐的生长或同位素交换。这些结果的地质意义不容易或无法唯一确定，并且举例说明了在低品位变质地层中对高度变形的构造体的非常细粒度的云母进行测年的固有困难。关于从Helvetic逆冲中获得的日期，可以得出两种概括：1）来自两个最高逆冲（冈德山和Sublage）的样品的所有40 Ar / 39 Ar台阶均高于20 Ma和2）大多数来自最深的Helvetic推力的样品在15至25 Ma之间具有阶跃（通常占39 Ars释放的80％以上）。这些日期与前陆抚育系统中的逆冲顺序一致，并且在变质的腹地和深层推覆土层中，新形成的云母与碎屑云母的比例增加。单个推力在最初合并到前陆抚育系统中时能容纳大部分位移，有些推力仍保持活动状态或重新激活至15 Ma。

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