Testing tectonic models with geochemical provenance parameters in greywacke

Abstract

A variety of models has been proposed for the origin of late Devonian low-grade greywacke, juxtaposed with the high-grade Variscan basement of the Black Forest. Here we interpret detrital zircon ages, and geochemical and Sm–Nd isotopic data of greywacke samples as evidence for their sources and tectonic setting of deposition. Detrital zircon ages of 371–384 Ma reveal igneous activity shortly before the deposition of the greywacke sequence at c . 365 Ma. ε Nd values of c . +3 to −10 in the greywacke samples indicate mixing of juvenile and older crustal material. Samples with low MgO concentrations and ε Nd values of −10 to −4 are interpreted as mostly derived from older granite–gneiss basement. Involvement of a juvenile mafic source is documented by samples with up to 7 wt% MgO and ε Nd values up to +3. Assessment of the composition of the juvenile component shows high concentrations of light REE, and low Sm/Nd and Nb/La ratios, all consistent with a subduction-related origin. We conclude that the greywacke sequence was derived from a late Devonian Andean-type continental margin, implying a cryptic suture in the internal high-grade Variscan basement of the southern Black Forest.

Consequences of open-system melting in tectonics

“…In the case of greywackes, the major element compositions vary significantly within and across different tectonics regimes (Barnes 1990;Floyd et al 1990;Korsch et al 1993;Banerjee & Bhattacharya 1994;Gill et al 1994;Gu 1994;McDaniel et al 1994;Duller & Floyd 1995;Roser et al 1996;Hayashi et al 1997;Holail & Moghazi 1998;Kalsbeek et al 1998;Hegner et al 2005;Wanas & Abdel-Maguid 2006;Kiminami & Fujii 2007;Dostal & Keppie 2009). Collisional orogenesis typically involves passive margin turbidites and subordinate amounts of arc-derived greywackes.…”

“…Collisional orogenesis typically involves passive margin turbidites and subordinate amounts of arc-derived greywackes. Thus, for the modelling in this study an average chemical composition of 33 passive margin greywackes that have SiO 2 of 69-76 wt% and Na 2 O >1.5 wt% was chosen (McDaniel et al 1994;Roser et al 1996;Kalsbeek et al 1998;Hegner et al 2005;Dostal & Keppie 2009; Table 1) and 15% of the iron was assumed to be ferric.…”

“…The model P-T path (Ague 1991). †Average of 33 passive margin greywacke samples that contain 69-76 wt% SiO 2 and Na 2 O >1.5 wt% (McDaniel et al 1994;Roser et al 1996;Kalsbeek et al 1998;Hegner et al 2005;Dostal & Keppie 2009). comprises two segments, a prograde heating segment from the solidus at 1.2 GPa up to peak P-T of 860 °C at 1.8 GPa followed by an isothermal decompression segment to 0.4 GPa.…”

Provenance of Pleistocene Rhine River Middle Terrace sands between the Swiss–German border and Cologne based on U–Pb detrital zircon ages

“…These ages can be linked to several very different sources. One potential source may be the Black Forest, and related complexes, where Variscan and Caledonian ages are equally abundant in late Devonian greywackes (Hegner et al 2005). Another option is a derivation from the mid-German crystalline rise.…”

Re-assessment of ammonoid specimens from the Early Carboniferous Protocanites Beds of the Badenweiler–Lenzkirch Zone (Schwarzwald, Central Variscan Belt): age constraints for a lithostratigraphic key bed

“…They concluded an undifferentiated Devonian time for the deposition of the Protocanites Beds, whilst the Ordovician and Silurian fossils were regarded as recycled (Hann and Sawatzki 1998, p. 186, 188). In subsequent papers, Hann et al (2003), Vaida et al (2004) and Hegner et al (2005) regarded the Protocanites greywackes as Late Devonian to Early Carboniferous in age, while Brockamp et al (2015), without providing evidence, suggested an early Viséan age. Already earlier, Schaltegger (2000) Kalt et al 2000).…”