GEOCHEMISTRY AND PROVENANCE OF THE ALTENDORF FELDSPATHIC SANDSTONE INTHE MIDDLE BUNTER OF THE THURINGIAN BASIN (GERMANY)

An integrated mineralogical and geochemical study of the Altendorf fel dspathic sandstone in the Middle Bunter of the Thuringian basin (Germa ny) was carried out to obtain more information on its provenance, sedi mentological history and geochemistry. The fine- to coarse-grained san dstone has an average composition of 66% (63-71%) quartz, 30% (27-35%) feldspar, and a fine-grained illitic matrix. The heavy-mineral assemb lage consists mainly of tourmaline, zircon, rutile, apatite and opaque Fe-Ti minerals. It is inferred that this terrigenous material was lai d down at the margin of the Bunter basin. High abundances of igneous q uartz grains determined by cathodoluminescence (CL), the lack of metam orphic heavy minerals as well as alkali feldspar with an average K2O/N a2O ratio of 14.9 point to material of mostly granitic origin. Regiona l geologic and paleogeographic observations suggest that granites of t he Older and Younger Intrusive Complex of the western Erzgebirge are p ossible source rocks of the feldspathic sandstone. The geochemical sig nature 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 sed imentation 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 l ed to the development of a positive Eu anomaly. The immobile trace ele ments (e.g., Ta, Th, Zr, Sc, Co, Ti) are best suited for provenance de termination because of their relatively low mobility during sedimentar y processes. Elemental ratios such as La/Sc, Th/Sc, Co/Th and Th/Ta ha ve inherited the signature of the parent rocks and thus, may be partic ularly useful for provenance studies. To evaluate the control of speci fic minerals and grain-size fractions on the distribution of trace ele ments in the sandstone, separated fractions of quartz, feldspar, the < 20-mu m fraction and heavy minerals were additionally analyzed. The c oncentration densities (elemental concentration in the fraction/fracti on weight) reveal that, with exception of Kf, the influence of the hea vy-mineral fraction on the whole-rock geochemistry is insignificant al though this fraction generally plays the most important role as carrie r 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 E u anomaly). The influence of the < 20-mu m fraction on the distributio n of the REE and the elements U, Th, Sc, Ta strongly depends on the ab undance of this fraction in the sediment. Alkali feldspar contains mos t 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 w hole-rock samples can be related to changes in the mineralogical compo sition (quartz/feldspar ratio) of the sediment. (C) 1998 Elsevier Scie nce B.V. All rights reserved.