U–Pb zircon and Sr–Nd–Pb whole-rock investigations from the continental deep drilling (KTB)

Abstract

 Among the large number of gabbroic intrusions within the profile of the continental deep drilling (KTB), several types can be distinguished. In this study petrographic, geochemical investigations, conventional U–Pb zircon data and results of Sr–Nd–Pb isotope characteristics of meta-gabbros, amphibolites and felsic rocks are reported. The main results are: U–Pb zircon age data reveal a magmatic Cambro-Ordovician evolution of the Erbendorf-Vohenstrauss zone (ZEV). The range of the ages of magmatic intrusions varies from 496 to 476 Ma, probably as a result of overlap of magmatic and metamorphic processes. Cambro-Ordovician magmatic ages were observed within different mafic (b unit) as well as in variegated units (v unit) of the KTB profile. The Silurian-Devonian metamorphism is dated at around 398 Ma. The Silurian as well as the Variscan metamorphic events apparently did not influence the U–Pb system. The Cambro-Ordovician mafic rocks from the KTB scatter on a ɛ-Nd evolution diagram. Several meta-gabbros cluster around the ɛ-Nd_T- ⁵⁰⁰ value of +5, but others vary from +8.6 to +0.65 and suggest variable degrees of contamination with continental crust material. A contamination of only 5–10% is required to reduce ɛ-Nd from +8.6 to approximately +5. According to the REE patterns and the Sr–Nd–Pb data, the mafic rocks could either derive from an enriched mantle source or else their protolith magma originated from a depleted mantle and was contaminated by continental crust. The data for the mafic rocks of the ZEV are in accord with observation by other authors regarding the evolution of the Late-Proterozoic to Early Palaeozoic magmatism in the Bohemian massif.