Detrital chromian spinels in Rhenohercynian greywackes and sandstones (Givetian-Visean, Variscides, Germany) as indicators of ultramafic source rocks

More than 75% of heavy mineral spectra of Rhenohercynian greywackes from th e Hart Mountains (Famennian-Visean), the Werra greywacke (Frasnian-Famennia n) and the Rhenish Slate Mountains (Frasnian-Visean) contain small amounts of detrital chromian spinels. Similar grains have also been proven in sands tones of the Givetian Stiege beds (East Hart nappe). These sandstones are d erived from the area of the Mid German Crystalline Rise, which is commonly interpreted as the source area of the Rhenohercynian greywackes. Microprobe analyses have revealed mainly a picotite composition. Presumable source ro cks were chiefly Alpine-type peridotites. Due to the onset of spinel sedime ntation in Givetian times, these peridotites must be at least partly of pre -Givetian age, and are assumed to be linked to Cadomian subduction. Potenti al source rocks are not exposed in the present outcrops of the Mid German C rystalline Rise nor in the Rhenohercynian zone or the Northern Phyllite zon e, which implies that the presumed mafic to ultramafic lithologies were com pletely eroded and/or downfaulted during later stages of the Variscan oroge ny.