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Knut Hahne, GeoForschungsZentrum Potsdam (Germany)
Rudolf Naumann, GeoForschungsZentrum Potsdam (Germany)
Heike Rothe, GeoForschungsZentrum Potsdam (Germany)
Sabine Tonn, GeoForschungsZentrum Potsdam (Germany)
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The chemical composition of sedimentary rocks provides an important record of the geological evolution of the continental crust through time. Hereby, the REE are the most useful elements as their distribution is not affected by secondary processes and the REE pattern of pelitic sediments is assumed to reflect the source area.
The current study is focused on the rare-earth element (REE) distribution over a long time. Two regions in Germany, both with a different geodynamic history were investigated. The first area (Saxothuringian) includes the Schwarzburg Anticline, the Thuringian Forest and the Thuringian Basin. This section consists of low metamorphic metasediments from the Neoproterozoic up to Triassic. The second region is located in NE-Germany. Samples were collected from deep boreholes and include sedimentary units from the Ordovician up to the Quarternary. This area was part of East-Avalonia. During the Mesozoic, both regions underwent a similar evolution.
The selection of the samples for the REE analyses is based on XRF- and XRD- data. REE analyses (∼1000 samples) were carried out by ICP-AES and ICP-MS. The REE-patterns are interpreted by applying mean values for 35 stratigraphic units. The parameters: ÓREE, ÓLREE/ ÓHREE, La/Yb, Er/Yb, EuCN/EuCN* and CeCN/CeCN are discussed.
The average of ÓREE varies between 95 and 475 ppm. During a long time span, the REE accumulation in pelitic sediments was triggered by mode of the clay minerals and zircon.
The chondrite- normalized ratio ÓLREE/ ÓHREE ranges from 6 to 13. A flat pattern indicates the influence of magmatic material, a steep ratio correlates with a distal sedimentary environment. Additionally, the Er/Yb- ratio is useful for the understanding of extreme sedimentary conditions during Silurian. The Eu anomaly is an important tool for the characterisation of source areas (Mc Lennan and Taylor, 1991). The variation for 35 stratigraphic units is:
EuCN/EuCN*= 0.45-0.82. This broad range is usually interpreted as being inherited from igneous source rocks. In the Saxothuringian zone the latest Neoproterozoic, the early Ordovician and the Late Devonian are times with volcanic inputs. In the East-Avalonia region, the Early Carboniferous is characterized by a pronounced negative Eu anomaly and enrichments of HFS- elements.
The geochemical proxis of the Mesozoic samples, including the REE pattern, are different from that of the older units.
Mc Lennan; S. M. and Taylor, S. R. (1991): J. Geol. 99, 1-21.
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