ALBERT

Description: The K-Ar dates and oxygen isotope data of illite-to-mica mineral phases in Permian to Triassic sandstones from Germany and Libya have been compared to previously published results of similar minerals of Liassic ages taken from lithologically similar rocks in several sites in western Europe and northern Africa. The widespread Liassic age for the micaceous mineral phases can be related to abnormal heat pulses induced by pre-rifting conditions. The temperatures in most cases appeared to be in the 150–220°C range. Some of these hydrothermal activities were responsible for both gas generation and U and Pb-Zn concentrations.An approach combining K-Ar isotope dates and oxygen isotope signatures provides a framework for the characterization of the clay separates used in isotopic investigations and for depicting any crystallization conditions. The trend in decreases in δ18O values relative to increases in K-Ar dates, while the size of the clay fractions in a single sample increases, is indicative of detrital contamination. Varied trends in the δ18O values at constant K-Ar dates for clay fractions with increase in size might be indicative of changes in the pore volume of the host rocks.

Description: The thermal history of the Buntsandstein (Lower Triassic) sedimentary rocks that crop out in the Vosges Mountains, or are deeply buried in the Rhine Graben, was evaluated on the basis of a combined mineralogical, morphological, oxygen isotope and K-Ar-dating study of illite separates from five locations over a limited 120 km2 area. The results indicate a complex pre-rift evolution characterized by the following features: (1) in the northern Vosges, no record of diagenetic illite formation could be detected; (2) in the nearby Saverne rifting fracture-field, tectonic activity favoured the formation of faults in the crystalline basement ~210 Ma ago. This induced fluid migrations into the porous sandstone cover with illite precipitation occurring either repetitively over a period of ~25 Ma or as two separate events, the latter at ~185 Ma, affecting rocks to the east in the deeper part of the post-Eocene Upper Rhine Graben system; this latter stage of tectonic activity and fluid migration continued until 160 Ma, unless overprinted by another tectonic pulse at that time; (3) afterwards, the main tectonic activity moved to the Soultz-sous-Forêts area where two episodes could be identified at ~95 and 70 Ma by dating mineral crystallization; the latter illite age was also detected in more deeply buried Buntsandstein lithologies of the rift basin, west of Strasbourg, reflecting diverse crystallization under different physical and/or chemical conditions; (4) further illitization which at present is poorly constrained may have restarted ~55 My ago in the same deep part of the rift graben.The youngest detected illitization age is clearly older than the main Miocene rifting episode of the Rhine Graben, suggesting that no illite crystallization occurred in the Buntsandstein sandstones during this tectonic event. This is explained by the fact that these rocks, as well as most of the underlying crystalline basement, were already sealed by fluid migration and mineralization during late Mesozoic rifting, and were therefore almost impermeable to the circulation of Tertiary fluids. The overall evolution of the region between the Vosges Mountains and the central Rhine Graben appears to be more complex than that of equivalent rock lithologies elsewhere in Western Europe. Several episodes identified here have been detected in other regions, but no single location exists which was affected by all events.