Geochemistry and provenance of the Altendorf feldspathic sandstone in the Middle Bunter of the Thuringian basin (Germany)

An integrated mineralogical and geochemical study of the Altendorf feldspathic sandstone in the Middle Bunter of the Thuringian basin (Germany) was carried out to obtain more information on its provenance, sedimentological history and geochemistry. The fine- to coarse-grained sandstone has an average composition of 66% (63-71%) quartz, 30% (27-35%) feldspar, and a fine-grained illitic matrix. The heavy-mineral assemblage consists mainly of tourmaline, zircon, rutile, apatite and opaque Fe-Ti minerals. It is inferred that this terrigenous material was laid down at the margin of the Bunter basin. High abundances of igneous quartz grains determined by cathodoluminescence (CL), the lack of metamorphic heavy minerals as well as alkali feldspar with an average K2O/Na2O ratio of 14.9 point to material of mostly granitic origin. Regional geologic and paleogeographic observations suggest that granites of the Older and Younger Intrusive Complex of the western Erzgebirge are possible source rocks of the feldspathic sandstone. The geochemical signature of the alkali feldspar (K2O/Na2O ratio, BaO content) points to the Bergen and Kirchberg Granites as possible parent rocks. Comparing the mineralogical and geochemical data of the presumed source granites with those of the Altendorf sandstone provides information on the sedimentation processes. Weathering, fluvial transport and sedimentation under arid climatic conditions have caused complete decomposition and removal of plagioclase and biotite, lowered the bulk REE content and led to the development of a positive Eu anomaly. The immobile trace elements (e.g., Ta, Th, Zr, Sc, Co, Ti) are best suited for provenance determination because of their relatively low mobility during sedimentary processes. Elemental ratios such as La/Sc, Th/Sc, Co/Th and Th/Ta have inherited the signature of the parent rocks and thus, may be particularly useful for provenance studies. To evaluate the control of specific minerals and grain-size fractions on the distribution of trace elements in the sandstone, separated fractions of quartz, feldspar, the < 20-mu m fraction and heavy minerals were additionally analyzed. The concentration densities (elemental concentration in the fraction/fraction weight) reveal that, with exception of Kf, the influence of the heavy-mineral fraction on the whole-rock geochemistry is insignificant although this fraction generally plays the most important role as carrier of trace elements. Quartz, feldspar and the < 20-mu m fraction have similar concentration densities (between 20 and 40%) for the REE with exception of Eu, which is mainly hosted in feldspar (strong positive Eu anomaly). The influence of the < 20-mu m fraction on the distribution of the REE and the elements U, Th, Sc, Ta strongly depends on the abundance of this fraction in the sediment. Alkali feldspar contains most of the Ba, Sr, Rb and Cs in the samples (concentration densities of up to 97%). Thus, differences in the abundances of these elements in whole-rock samples can be related to changes in the mineralogical composition (quartz/feldspar ratio) of the sediment. (C) 1998 Elsevier Science B.V. All rights reserved.