The lithology, geochemistry, and metamorphic gradation of the crystalline basement of the Cheb (Eger) Tertiary Basin, Saxothuringian Unit


Authors: Fiala J, Vejnar Z

Published in: Bulletin of Geosciences, volume 79, issue 1; pages: 41 - 51; Received 5 June 2002; Accepted in revised form 5 May 2003;

Keywords: crystalline basement, Cheb Tertiary Basin, Saxothuringian, lithology, geochemistry, metamorphism,

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The crystalline basement of the Cheb Tertiary Basin is comprised of muscovite granite of the Smrčiny Pluton and crystalline schists of the Saxothuringian Unit. With increasing depth (as seen from the 1190mdeep drill hole HV-18) this crystalline schist exhibits rapid metamorphic gradation, with the characteristic development of garnet, staurolite, and andalusite zones of subhorizontal arrangement. The dynamic MP-MT and static LP-MT crystallization phases were followed by local retrograde metamorphism. The moderately dipping to subhorizontal S2 foliation, which predominates in the homogeneous segments, is followed by subvertical S3 cleavage.
The vertical succession of psammo-pelitic, carbonitic, and volcanogenic rock sequences, together with geochemical data from the metabasites, indicates a rock complex representing an extensional, passive continental margin setting, which probably originated in the Late Cambrian to Early Ordovician. On the contrary, the geochemistry of the silicic igneous rocks and of the limestone non-carbonate components point to the compressional setting of a continental island arc. This disparity can be partly explained by the inheritance of geochemical characteristics from Late Proterozoic rocks in the source region.