ORIGIN OF PORE-LINING CHLORITE IN THE AEOLIAN ROTLIEGEND OF NORTHERN GERMANY

A B S TRACT: Pore-lining chlorite is common in Rotliegend lake shoreline aeolian sandstones from northern Germany and preserves abnormally high primary intergranular porosity. In the north of the study area, in a basinward irection, the chlorites are Mg-rich while towards the south they become Fe-rich over a distance of about 15 kin. All are unusually rich in Mn. Magnesium-rich examples tend to be more abundant han illite, while Fe-rich examples cover framework grains less continuously and are admixed with more abundant illite. Oxygen isotope analysis of 30 chlorites in the 2-6 lam fraction gave filSo (SMOW) values of 7 to 12%o (mean 9.8%0). These data show no obvious trend across the study area, nor in relation to changes in chlorite composition, or burial depth. The honeycomb arrangement of chlorite crystals suggests chlorite formation via the sequence smectite-corrensite-chlorite. Chlorite distribution and the systematic hanges in its composition suggest that formation of a precursor was related to lateral migration of Mg-rich fluids from basinal shales and/or evaporites during shallow burial. Interaction of these fluids with early formed oxy-hydroxide coatings on the aeolian sand grains provided a source of Fe and the Mn. The isotope data suggest that the eventual formation of chlorite during deep burial occurred from waters with positive filSo values, comparable to those present during the deep burial formation of illite. Most studies concerned with clay minerals in the Permian Rotliegend of Western Europe have focused on the various forms of the illite that is so common in these rocks (e.g. Rossel, 1982; Lee et al., 1989; Warren & Curtis 1989; Robinson et al.