Mineralogy, Mossbauer Characteristics, and K-Ar Isotopic Age of Glauconite from the Lower Cambrian Sediments, Western Lithuania

The paper presents results of lithological-mineralogical, Mossbauer, and isotope-geochronological study of globular phyllosilicates (glauconite and illite) from the Lower Cambrian Virbalis Formation in western Lithuania. K-Ar dates of these minerals (382 and 374 Ma) do not correspond to stratigraphic positions of the studied samples and indicate the loss of radiogenic 40Ar, probably, in Middle Devonian. Simulation of cation distribution in the mineral structure and comparison of the obtained data with Mossbauer spectral characteristics suggest that loss of 40Ar was caused by the structural rearrangement of globular phyllosilicates during the postdiagenetic substitution of Fe2+, Mg2+, and Fe3+ by Al3+, with the subsequent ordering of cation distribution in octahedral layers. These processes were presumably related to the same Caledonian tectonic events that were responsible for metasomatic dolomitization of Lower Paleozoic rocks in adjacent areas of the Baltic region at the terminal Silurian-Devonian. Diffraction analysis showed that globules consist of two micaceous phases with different Fe contents (illite and glauconite). Two alternative models are considered to explain the formation of two-phase globules: (1) disequilibrium conditions of diagenetic mineral formation; (2) transformation of glauconite into illite. Although the available data are insufficient to make an unambiguous choice between these models, the second mechanism seems to be more preferable.