德国的厄尔士山脉——北冈瓦纳的俯冲部分:在变质逆冲单元中重现早古生代变质沉积层序的地球化学证据

The Erzgebirge, Germany, a subducted part of northern Gondwana: geochemical evidence for repetition of early Palaeozoic metasedimentary sequences in metamorphic thrust units

作为波希米亚地块的一个主要变质地体 ,厄尔士山脉记录了冈瓦纳古生代边缘的俯冲部分。低绿片岩到变粒岩相变质作用的不同变质单元的非钙质沉积物的地球化学研究支持了近来建立的逆冲模式。厄尔士山脉变质沉积物的地球化学识别和对比表明在变质逆冲单元中重现了早古生代变质沉积层序。这个新发现与变质流纹岩夹层的最新放射性测年结果相一致 ,放射性测年值为 4 80Ma左右。此外 ,与图林根低级变质标准剖面的对比也支持了这种观点。图林根剖面具有较高成熟度的沉积物 ,代表从造山带过渡到被动边缘环境。还识别出至少在厄尔士山脉的三个变质单元中再现的三种不同的岩石类型的重要地球化学特征。通过使用平均值的简单对比和统计学方法 ,建立了这些单元的地球化学对比模式。变质岩组不同岩类的重要地球化学特征对地形分析和古构造背景重建具有重要意义。前进变质单元中的岩性恢复及其不同的化学成分可用来研究在巴罗型变质作用中元素的活动性。统计对比显示 ,从绿片岩到角闪岩相Li逐渐减少 ,而Ca则少量富集 ,其它被研究的元素均是不活动的。

One of the major metamorphic terranes of the Bohemian Massif,the Erzgebirge, is interpreted to record a subducted part of a Palaeozoic margin of Gondwana, A geochemical study on noncalcareous metasediments from the various metamorphic units from lower greenschist to granulite facies metamorphism supports a recently established thrust model. Geochemical discrimination and correlation from the metasediments of the Erzgebirge suggest repetition of an early Palaeozoic metasedimentary sequence in metamorphic thrust units. This new finding is in line with recent radiometric dating of intercalated metarhyolitic rocks, which yielded ages of around 480 Ma.It is furthermore supported by correlation with a low-grade standard section in Thuringia, which represents the transition from an orogenic belt to a passive margin setting, with highly mature sediments. Significant geochemical signatures have been identified in three different lithotypes, which reappear in at least three metamorphic units of the Erzgebirge. Geochemical correlation of these units was established using simple comparison of averages and with statistical techniques.The identification of significant geochemical signatures from different lithotypes in metamorphic suites has important implications for terane analysis and reconstruction of ancient tectonic settings. The repetition of lithologies and their distinct chemical compositions in progressively metamorphosed units is useful for examining element mobility during Barrovian metamorphism. Statistical comparison implies that Li is progressively depleted from the greenschist to amphibolite facies,whereas Ca exhibits some enrichment. All the other elements studied are considered to be immobile.