Conference Agenda

The Mid-German Crystalline Zone (MGCZ) is defined as the northwestern part of the Saxo-Thuringian Zone (STZ), comprising metamorphic and magmatic rocks exposed in the Odenwald, Spessart, Ruhla, and Kyffhäuser crystalline complexes in Germany. Based on field relationships, as well as petrological and geochronological data, these rocks form a highly heterogeneous crystalline wedge. In addition to surface exposures and borehole data, gravity and magnetic data have been used to constrain the lateral extent of the MGCZ.

Here, we apply an integrated geophysical approach combining joint inversion of magnetotelluric and gravity data with seismic reflection and field magnetic intensity datasets to map regional-scale lateral heterogeneities along the MGCZ and the northwestern STZ. The joint inversion results reveal spatial heterogeneities in geophysical properties across the study area. A major electrical resistivity contrast within the central STZ is interpreted as the tectonic boundary between the MGCZ and STZ. At depth, however, the MGCZ between the Ruhla crystalline complex and the Black Forest shows a gradual decrease in resistivity.

Seismic reflection data indicate a layered crust characterized by high-amplitude, continuous, subhorizontal reflectors in the southeastern and northwestern domains, separated by a central zone of lower amplitude and more transparent character. This central zone contains oppositely dipping, high-amplitude reflections and corresponds to low-resistivity regions in both the southwest and northeast of the study area. It exhibits high density in the southwest but lower density in the northeast. Overall, our results demonstrate that geologically defined tectonic boundaries correlate with regional-scale geophysical contrasts.

Presently, little is known about the zircon age-Hf isotope record of (meta)sedimentary rocks exposed in the southernmost part of the Mid-German Crystalline Zone (MGCZ) and adjacent Rhenohercynian Domain, preventing detailed reconstructions of the geodynamic evolution in Central Europe. In this study, such data are presented from Late Devonian to Viséan metagreywackes, metapelites and granite gneisses of the Palatinate Forest (MGCZ) and the Harz Mountains (Rhenohercynian). These provide evidence that sediments in both realms were sourced from a so far unrecognized, juvenile oceanic arc terrane, which initially was formed at 490-570 Ma (εHft up to +13.0), and internally reworked at ca. 500, 400, 370 and 335 Ma (εHft = +2.0 to +8.4), prior to Variscan collision and post-kinematic magmatism at 330 Ma. Late Devonian arc reworking is reflected by the ca. 370 Ma Albersweiler granite gneiss showing superchondritic εHft of +3.6 to +7.7, and older reworking by the detrital zircon record. Results of data compilation further suggest that the oceanic arc terrane remained widely isolated from continental input within the Prototethys-Rheic ocean realm until its collision with Avalonia at 335 Ma, but also that relics of the arc system became widely dispersed within the Saxothuringian, Rhenohercynian and Moldanubian domains during Variscan collision and collapse.

Present models suggest that the European Variscides host several terranes, formed originally along the N-Gondwana margin during the Cadomian orogeny (750-530 Ma). Subsequently, after a period of drift (500-360 Ma) these terranes became assembled in the Variscan Orogen during the collision with Baltica-Avalonia (360-320 Ma). However, details about amalgamation, drift and collision remains enigmatic, preventing the establishment of a coherent model for the pre- to syn-Variscan evolution.

Results of this study indicate that many increments of this evolution are recorded by detrital zircon populations of the Odenwald Crystalline Complex, forming part of the Mid-German Crystalline Zone in Central Europe. Uranium-Pb ages provide evidence for periodic sediment deposition during the Neoproterozoic (560-540 Ma), Cambro-Ordovician (500-480 Ma), Silurian-Early Devonian (440-400 Ma), and late Devonian (380-360 Ma). The Hf isotope record additionally indicates a prolonged period of crust formation from the depleted mantle between 750 and 500 Ma (perhaps until 400 Ma), and for the reworking of older crust. The most juvenile Hf isotope data can be interpreted to reflect prolonged oceanic spreading between 750 and 400 Ma. This spreading was periodically accompanied by juvenile arc formation and reworking at 660-570 Ma, 550-530 Ma, 500-480 Ma, 410-390 Ma and 375-355 Ma, reflecting successive ocean closure. Zircon grains with more negative Hf signatures probably were added to the juvenile spectrum either during Cadomian or Variscan amalgamation. The results of this study suggest that the Prototethys-Rheic formation lasted for at least 350 Ma until the collision between N-Gondwana with Avalonia-Baltica at 350 Ma.