Variscan terrane boundaries in the Odenwald-Spessart basement, Mid-German Crystalline Zone: New evidence from ocean ridge, intraplate and arc-derived metabasaltic rocks
Variscan terrane boundaries in the Odenwald-Spessart basement, Mid-German Crystalline Zone: New evidence from ocean ridge, intraplate and arc-derived metabasaltic rocks
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DOI：
10.1016/j.lithos.2015.01.018
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发表时间：
2015-04
期刊：
影响因子：
3.5
通讯作者：
T. Will;S. Lee;E. Schmädicke;H. Frimmel;M. Okrusch
中科院分区：
文献类型：
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作者：
T. Will;S. Lee;E. Schmädicke;H. Frimmel;M. Okrusch
德国中部结晶区是大型瓦里斯坎缝合线的一部分，由暴露在德国中部的各种地下室复合体组成。对奥登瓦尔德-施佩萨特基底的各种角闪岩获得的新岩石地球化学和 Sr-Nd-Pb 同位素数据表明，它们的原岩形成于不同的构造环境中，并随后在瓦里斯坎大陆碰撞之前并入俯冲带。最北端的施佩萨特和奥登瓦尔德西部的变基性岩在地球化学上几乎相同，它们的原岩被解释为在拉张（可能是弧后）中形成。设置。拉斑斑玄武岩和钙碱性岩石具有中等 TiO2 浓度和高 Th/Nb 比，是典型的火山弧型和/或俯冲流体改性岩石。 Nd 初始比率已耗尽（εNd330 Ma= 5.0–5.8），Nd 模型年龄范围为 660 至 610 Ma，这表明新元古代末期有幼年地壳添加。样品在 206Pb/204Pb 与 207Pb/204Pb 空间中定义了线性阵列。相比之下，施佩萨尔特南部和中部的变基性岩的原岩要么形成于板内洋岛，要么形成于大陆弧环境中。施佩萨特基底南部的碱性板内岩石富含 TiO2，且 Th/Nb 比率非常低。这些岩石具有弱贫化的 Nd 初始值 (εNd330 Ma= 2.6–3.3)，Nd 模型年龄在 870 至 810 Ma 之间。相比之下，中部施佩萨尔板内岩石的 TiO2 浓度明显较低，但 Th/Nb 比率较高。此外，这些岩石同位素富集（εNd330 Ma= − 13.1 至 − 9.5），并具有古元古代 Nd 模型年龄。施佩萨特基底中部和南部的大陆弧岩石具有较低的 TiO2 浓度和可变的 Th/Nb 比值。大多数负 Nd 初始值 (εNd330 Ma= − 2.6 至 + 0.9) 和中元古代晚期 Nd 模型年龄表明，在大陆弧前体岩石的形成过程中，一定发生了旧地壳的再循环或不同年龄的地壳成分的混合。来自奥登瓦尔德东部的退变榴辉岩是拉斑玄武岩，具有球粒陨石归一化的扁平稀土元素模式，中等到高TiO2 浓度和非常低的 Th/Nb 比率。退缩最少的榴辉岩缺乏负 Nb-Ta 异常。此外，岩石具有贫化的 Nd 同位素组成（εNd360 Ma= 7.1–8.5），Nd 模型年龄为 470–425 Ma，超球粒状 147Sm/144Nd 比率和 87Sr/86Sr 初始比率与块体地球相似。这些特征与榴辉岩原岩起源于浅层、贫化的洋中脊地幔并位于洋中脊环境的观点一致。在地壳侵位过程中，这些岩石的成分因俯冲相关流体和/或地壳物质同化而发生不同程度的改变。施佩萨尔特基底南部和北部各种角闪岩的成分和成因至今未知的差异，需要对该地区现有的构造地层模型进行修订。各种角闪岩并不代表一个连续的单元，而是被解释为属于两个不同的地体，这些地体并列在施佩萨尔特-奥登瓦尔德基底中。这些地体被一条主断层（奥茨贝格-米歇尔巴赫断层带）分隔开，沿着这条断层，现在暴露在奥登瓦尔德西部和最北端施佩萨尔特的推断的近冈瓦纳大陆起源的岩石被推挤到可能源自波罗的海/阿瓦隆尼亚的岩石上，这些岩石作为构造窗口暴露在奥登瓦尔德东部和施佩萨尔特中部和南部。进一步建议奥茨伯格……
The Mid-German Crystalline Zone is part of a large Variscan suture and consists of various basement complexes that are exposed in central Germany. New lithogeochemical and Sr–Nd–Pb isotope data obtained on various amphibolites from the Odenwald–Spessart basement show that their protoliths formed in different tectonic settings and were subsequently incorporated into a subduction zone prior to Variscan continental collision.Metabasic rocks from the northernmost Spessart and the western Odenwald are geochemically almost identical and their protoliths are interpreted to have formed in an extensional, possibly, a back-arc setting. The tholeiitic and calc-alkaline rocks have intermediate TiO2concentrations and high Th/Nb ratios, typical of volcanic arc-type and/or subduction-fluid modified rocks. The Nd initial ratios are depleted (εNd330 Ma= 5.0–5.8) and Nd model ages range from 660 to 610 Ma, which points at juvenile crustal addition towards the end of the Neoproterozoic. The samples define a linear array in206Pb/204Pb versus207Pb/204Pb space.In contrast, the protoliths of the metabasic rocks from the southern and central Spessart formed either in an intraplate oceanic island or a continental arc setting. The alkaline intraplate rocks from the southern Spessart basement are very TiO2-rich and have very low Th/Nb ratios. The rocks have weakly depleted Nd initals (εNd330 Ma= 2.6–3.3) and Nd model ages between 870 and 810 Ma. In contrast, the central Spessart within-plate rocks have considerably lower TiO2concentrations but higher Th/Nb ratios. In addition, these rocks are isotopically enriched (εNd330 Ma= − 13.1 to − 9.5) and have Palaeoproterozoic Nd model ages. The continental arc rocks from the central and southern Spessart basement have low TiO2concentrations and variable Th/Nb ratios. Mostly negative Nd initials (εNd330 Ma= − 2.6 to + 0.9) and late Mesoproterozoic Nd model ages indicate that recycling of older crust or mixing of crustal components of different age must have occurred during the formation of the continental arc precursor rocks.Retrogressed eclogites from the eastern Odenwald are tholeiitic metabasalts with chondrite-normalised flat rare earth element patterns, intermediate to high TiO2concentrations and very low Th/Nb ratios. The least retrogressed eclogites lack negative Nb–Ta anomalies. In addition, the rocks have depleted Nd isotope compositions (εNd360 Ma= 7.1–8.5), Nd model ages of 470–425 Ma, superchondritic147Sm/144Nd ratios and87Sr/86Sr initial ratios similar to bulk Earth. These features are consistent with the notion that the eclogite protoliths originated in a shallow, depleted mid-ocean ridge mantle and were emplaced in a mid-ocean ridge setting. During crustal emplacement, the composition of these rocks was variably modified by subduction-related fluids and/or assimilation of crustal material.The hitherto unknown differences in the composition and genesis of the various amphibolites in the southern and northern Spessart basement require revision of existing tectonostratigraphic models for the area. Instead of representing one contiguous unit, the various amphibolites are interpreted to belong to two different terranes that became juxtaposed in the Spessart–Odenwald basement. These terranes are separated by a major fault (Otzberg–Michelbach Fault Zone), along which rocks of inferred peri-Gondwana origin, now exposed in the western Odenwald and the northernmost Spessart were thrust onto probable Baltica/Avalonia-derived rocks that are exposed as a tectonic window in the eastern Odenwald and the central and southern Spessart. It is further suggested that the Otzberg …