Isotopic and thermochronologic evidence of extremely cold lithosphere associated with a slab flattening in the Central Andes of Argentina
Isotopic and thermochronologic evidence of extremely cold lithosphere associated with a slab flattening in the Central Andes of Argentina
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阿根廷安第斯山脉中部板块扁平化与极冷岩石圈相关的同位素和热年代学证据
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发表时间：
2017
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通讯作者：
A. Carter
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作者：
G. Collo;F. Dávila;W. Teixeira;J. Nóbile;Lucy G. Sant′ Anna;A. Carter
我们对阿根廷中部安第斯山脉的前陆沉积中心Bermejo和Vinchina盆地的砂泥岩和凝灰质层进行了矿物学、同位素和热年代学分析，确定了地壳的低温状态，类似于板块变浅和变平过程。伊利石/蒙皂石间层（I/S）中的伊利石含量显示出向盆地深部进行的伊利化作用。I/S的分布与理论模拟和预测的热流值的ca。Vinchina盆地8-3.4 Ma间隔的26 mW m−2，约为1.5 mW m−2。Bermejo盆地自9 Ma以来为42 mW m−2。后者显示的热流值与大地电磁分析报告的热流值（36-40 mW m−2）相当，与先前发表的现代安第斯前陆沿着热流计算结果一致。砂泥岩（<2 μm）的Rb-Sr等时线年龄为125 ~ 165 Ma，而K-Ar年龄随粒度的减小而减小（1-2 μm为136-224 Ma，0.2 ~ 1 μm为112-159 Ma，<0.2 μm为76-116 Ma，<0.1 μm为39.3 ~ 42 Ma）。这些年龄比盆地中的沉积年龄要大得多（约1000年）。Vinchina盆地为16 Ma; U-Pb年龄），并可以解释为存在大量的碎屑成分，主要是伊利石，即使在较细的部分。碎屑岩年龄的保存与此处为盆地提出的与低温状态相关的浅层成岩作用一致。基底凝灰质层的K-Ar年龄较年轻（21.3-12 Ma）。Vinchina盆地最深处的粘土矿物学和R 0排序，以及I/S随深度的演化模式，表明埋藏温度不会超过约100 ℃。100°C，与对从两个凝灰质单元中提取的磷灰石进行的（U-Th）/He分析一致。来自两个研究盆地的热指标证实了在平坦俯冲期间存在低温状态。
We present mineralogic, isotopic and thermochronologic analyses on psammopelitic and tuffaceous levels from the Bermejo and Vinchina basins – both foreland depocentres of the Central Andes of Argentina – that define a low‐temperature regime for the crust akin to a slab shallowing and flattening process. The contents of illite in illite/smectite interstratified (I/S) show a progressive illitization into the deeper parts of both basins. The distribution of I/S is compatible with theoretical simulations and predicted heat flow values of ca. 26 mW m−2 in the 8–3.4 Ma interval for the Vinchina Basin and ca. 42 mW m−2 since 9 Ma for the Bermejo Basin. The latter shows heat flow values that are comparable to those reported by magnetotelluric analysis (36–40 mW m−2) in agreement with previously published heat flow calculations along the modern Andean foreland. The Rb–Sr isochrones in psammopelites (<2 μm fractions) show ages between 125 and 165 Ma, whereas the K–Ar ages decrease as the grain size is smaller (136–224 Ma for 1–2 μm, 112–159 Ma for 0.2–1 μm, 76–116 Ma for <0.2 μ and 39.3–42 Ma for <0.1 μm). These ages are significantly older than the sedimentation in the basins (ca. 16 Ma for the Vinchina Basin; U–Pb age), and can be explained by the presence of a significant amount of detrital components, mainly illite, even in the finer fractions. The preservation of detrital ages is consistent with the shallow diagenesis related to a low‐temperature regime, proposed here for the basins. Younger K–Ar ages (21.3–12 Ma) were obtained for a basal tuffaceous level. Clay mineralogy and R0 ordering in the deepest part of the Vinchina Basin, together with the evolution model of I/S with depth, suggest that the burial temperatures would have not exceeded ca. 100°C in agreement with (U–Th)/He analyses performed on apatite extracted from two tuffaceous units. Thermal indicators from both studied basins confirm the existence of a low‐temperature regime during flat subduction.