The Chemical Composition of Sedimentary Deposits in the Rhenohercynian Belt of Central Europe

Abstract

About 600 samples from typical sections of eight stratigraphic units, exposed in an area of 25,000 square kilometres, have been investigated for the major and for 15 minor elements. Excepting the abundance of carbonate rocks and metavolcanic materials (spilites, keratophyres, quartz keratophyres) the average composition of the units is remarkably homogeneous throughout the Devonian and Carboniferous. Only the Lower Carboniferous of the SE region of the belt and the entire Givetian contain as much as 25 and 10 percent spilites respectively. Shales, siltstones and sandstones contribute about 34, 31 and 22 percent respectively to the bulk Rhenohercynian deposits. The average proportion of greywackes, carbonates and cherts is about 2, 6 and 1 percent respectively. Erosion of large source areas such as the Caledonides in the north and the Central German Rise in the south must have supplied major detritus. An average abundance of quartz (44 %), similar to that of clay minerals (41 %), and feldspar contents of less than 5 percent indicate effective rock decomposition and mineral separation during recycling and transport of the sediments. Geosynclines in general contain more calcareous and volcanic rocks (more Al, Na, P, Mg) and less clastic material than the Rhenohercynian Belt. Shales and sandstones from almost the total sequence contain twice to ten times the “normal” Cr and Ni concentration of clastic rocks. An accumulation of these elements is specific for ultramafic rocks and probably indicates decomposition of abundant ophiolites in the Caledonides. The lead isotopic composition of post-Carboniferous sediments and of Variscan granites reflects large contributions of Precambrian lead in the source and the basement of the Rhenohercynian deposits which rest on an about 25 km thick continental crust. Ophiolites and andesitic volcanics, which could indicate a plate boundary, are not exposed in the Rhenohercynian Belt. The change in abundance of albite, chlorite and of the potassium concentration in mica within the stratigraphic column can be used to trace the decomposition of Variscan meta-volcanic rocks which were mainly produced by reactions between magmatic rocks and hot sea water. Regional metamorphism had its climax during the Upper Carboniferous.