The drilling phase of the German Continental Deep Drilling Program (KTB) ended in late 1994 with a final depth of 9101 m. The drilled rocks belong to the Zone of Erbendorf-Vohenstrau (ZEV), a tectonometamorphic unit at the western margin of the Bohemian Massive where the drilling was located. They are mainly amphibolites, metagabbros, paragneisses and hornblende-gneisses. Minor rock types are calc-silicates, ultramafitites, lamprophyres and aplites. The major objective of the KTB was to reach a better understandig of the evolution of intracontinental crustal terrains. Our contribution was the characterisation of the drilled profile with respect to Sr and Nd isotopes in order to distinguish formations of different provenance and geologic history. A second topic was the dating of geological events which shaped the ZEV rocks by means of mineral ages.
Nd-model ages of paragneisses vary in a small range between 1000 and 1300 Ma, independently of depth. These data suggest a rather homogeneous source region of the sedimentary protoliths or, alternatively, constant mixing ratios of detritus derived from different source regions. Compared to model ages of Moldanubian gneisses (1200 - 2000 Ma; Liew and Hofmann, 1988) the source regions of the ZEV paragneisses appear to be younger on average.
Metabasites, on the other hand, exhibit a broad variation of model ages from 600 to 1400 Ma. Assuming a common value of 480 Ma for the emplacement of the metabasites (Hölzl and Köhler, 1994) the initial e-Nd-values range from +1.5 to +6.0 suggesting a contamination with crustal
material.
The paragneisses show slightly negative initial e-Nd values (-1.0 to -3.0) and initial 87Sr/86Sr ratios between 0.701 and 0.714, calculated for a time of 480 Ma before present. The relatively high e-Nd values and partly Sr-isotopic ratios are explained by high percentages of young crustal material incorporated in the paragneiss protoliths during sedimentation.
The mainly concordant and uniform U-Pb ages of metabasite zircons are interpreted as the time of mineral crystallisation during emplacement of the magmas about 480 Ma ago. We suppose that the metamorphic overprint these rocks obviously have suffered took place very shortly afterwards. Due to an overlap of error limits the rutile-cooling age is not distinguishable from the time of zircon crystallisation.
U-Pb age determinations of paragneiss titanit fractions yielded concordant figures around 400 Ma. We interpret this result as mineral growth at the peak of the ZEV medium pressure metamorphism.
However, discordant zircon fractions from these rocks exhibit upper concordia intercept age values of 900 and 1400 Ma indicating a detrital provenance. These age values enclose the range of Nd model ages portrayed above supporting the interpretation of short crustal residence times.
Hölzl, S. & Köhler, H., KTB-Report 94-2, B 35 (1994).
Liew, T.C. & Hofmann, A.W., Contrib. Mineral. Petrol. 98, 129-138 (1988).