The major and trace element contents of metasedimentary rocks are influenced by provenance, weathering, transport, sedimentation and metamorphism. Easily soluble elements (Ca, Na, Sr) or the CIW (chemical index of weathering) may indicate climatic conditions. Other parameters like K2O/Na2O and Al2O3/SiO2 reflect the composition of the former sediments. Because of their relative low mobility during sedimentary processes, their low residence time in sea water and relatively stable behavior during metamorphism, trace elements such as Ti, Zr, Sc, Cr and REE are particularly useful to constrain the tectonic setting of sedimentation. All these processes lead to typical chemical signatures in metasediments. This signature have been used to correlate metasediments from different metamorphic units in the Erzgebirge with the weakly metamorphosed early Palaeozoic section in Thuringia.
The Erzgebirge is situated on the northern border of the Bohemian Massif in the Saxothuringian zone of the European Variscan Belt. It is composed of a crystalline complex in the core and marginal units of gneisses, mica schists and phyllites. These rocks have traditionally been divided in a lithostratigraphic sense, with decreasing age and metamorphic grade from the central part to the margin. This concept has been challenged by recent petrological studies on metasediments and leucocratic orthogneisses (Willner et al., 1994) which subdivided the Erzgebirge into five metamorphic units. The main metamorphic units are, from the deepest to the highest structural level: 1. the MP-MT gneiss unit (~ 6 kbar, 650°C), 2. the HP-HT gneiss-eclogite unit
(~ 20 kbar, 750°C), 3. the HP-LT mica schist-eclogite unit
(~ 12 kbar, 550°C), 4. the MP-LT garnet-phyllite unit
(~ 8 kbar, 480°C), and 5. the LP-LT phyllite unit (< 2kbar, 400 °C).
The lowermost amphibolite facies unit (1) consists of para and orthogneisses with granodioritic composition. The overlying HP-HT unit (2) is a sequence composed mainly of felsic orthogneisses intercalated with eclogites, marbles, metagreywackes and highly mature metapelites. Nearly the same lithologies, including felsic orthogneisses, eclogites, marbles, paragneisses and interbedded, highly mature psammitic and metapelitic schists have also been found in the succeeding mica schist-eclogite unit. Mica schists represent the main rock type of this unit, and they can be subdivided into feldspar-free, feldspar-bearing and graphite-bearing schists. The lithological sequence of the mica schists is repeated in the garnet-phyllite unit (4) as well as in the phyllite unit (5).
A correlation of the phyllite unit with early Palaeozoic rocks of Thuringian facies is accepted because of the accordance in lithological composition and has been partly proved biostratigraphically. However, the correlation has not previously been extended to the higher grade units. Maaß et al. (1984) have found marked breaks in the mineralogical and geochemical compositions of the upper Proterozoic to the Devonian sequences in the Schwarzburger antiform (Thuringia). In the Cambrian sediments the Na, Sr, Ba contents decrease while the K, Rb, Zr contents increase. This trend culminates in an extremely high K2O/Na2O (>20) ratio in the Tremadoc (Frauenbach formation). With the base of the Phycoden formation the contents of Na, Sr, Li und Ni increase. The Gräfenthaler group (Llanvirn - Ashgill) has the same geochemical composition as the Phycoden formation but higher organic carbon contents. The whole Thuringian Ordovician section is characterised by a high maturity and consequently a distinctive chemical composition. The overlying Silurian sediments are typical graphite slates and lydites.
The Thuringian sedimentary sequence is repeated in variably metamorphosed units of the Erzgebirge. Compositional indices such as extremly high K2O/Na2O-ratios, high K and Rb contents and an extreme depletion of Ca, Na, Sr in the feldspar-free phyllites and in mica schists of the HP-LT and HP-HT units are similar to the Thuringian Frauenbach formation. This assumption has been verified by geochemical correlations (major and trace elements, REE) of the Thuringian Phycoden formation with the feldspar-bearing schists and the Thuringian Gräfenthaler group with the graphite-bearing schists of different metamorphic units in the Erzgebirge. The same lithology and the major and trace element data suggest that the weakly metamorphosed rocks of the Ordovician Thuringian facies reappear in the higher-grade units of the Erzgebirge.
Maaß, I. & Hahne, K., Loos, G., Vogler, P., ZfI - Mitt. 85, 107-118 (1986).
Willner, A., Rötzler, K., Krohe, A., Maresch, W.V. & Schuhmacher, R., Terra Nostra 3, 104-106 (1994).