Mineral and geochemistry of clay-sized fractions from sediments of the Oligocene maar near Baruth (Saxony, Germany)

Twenty seven clay-sized (<2 μm) separates from drill core samples of the Upper Oligocene maar near Baruth, Saxony, were mineralogically and chemically characterized using X-ray diffraction analysis coupled with the Rietveld method, X-ray fluorescence analysis, scanning and transmission electron microscopy, and stable isotope analysis. The predominant minerals in the clay fractions are kaolin minerals (kaolinite and halloysite) and opal-A. Abundant Fe- and smectite-rich illite-smectite mixed-layer minerals occur exclusively in the lower turbidites and debris flows. Illite and beidellites are minor components. Accessory phases are anatase, siderite, sanidine, quartz, albite-rich plagioclase, and pyrite. Four main detrital components were detected in the Upper Oligocene maar sediments: 1) biogenic opal-A as diatom fragments, 2) disordered kaolinite (and halloysite) + illite + beidellite + Zr-rich anatase from soils on granodioritic material, 3) Fe- and smectite-rich I-S from weathered basaltic volcanic material and 4) sporadic eolian input of sanidine-bearing, probably phonolithic, ashes. The Mid-Miocene kaolinite-rich clays that cover the maar sediments were derived from eroded saprolites on the Lausitz granodiorite. The δD and δ18O values of the kaolinite-rich clay fractions from various lithologies indicate kaolinization at temperatures of ~15 ± 5 °C from meteoric waters that were isotopically heavier than present-day meteoric waters. There is no evidence for hydrothermal clay formation in the maar