Ion micro-probe U¯Pb zircon geochronology of peak and retrograde stages of ultrahigh-pressure metamorphic rocks from the Kokchetav massif, northern Kazakhstan

The composition and the magnetic properties of the insoluble residue of Maastrichtian chalk from the North Sea have been investigated in order to get a better understanding of the cyclic signals in chalk carbonates and the depositional processes that have contributed to the pattern. The insoluble residue of the Maastrichtian North Sea chalk contains quartz, dolomite, pyrite, organic material, ferrimagnetic minerals and clay minerals (smectite¯illites). The concentration and in particular the type of clay minerals determine the variations in the magnetic properties of the insoluble residue and the bulk magnetic susceptibility of the chalk. The existence of a detrital component in the insoluble residue suggests that the cyclic variations in the bulk magnetic susceptibility and consequently in the composition of the chalk, are controlled by changes in the runoff from land into the basin.