ISOTOPIC DATINGS OF THE LOWER CAMBRIAN CLAYS FROM ESTONIA AND THEIR INTERPRETATION

Lower Cambrian ''blue clays'' from the Lontova: Formation of northern Estonia, in which previous investigators distinguished three illite ge nerations of different ages, were subjected to a complex study using t he methods of X-ray diffraction, electron microscopy, and Rb-Sr and K- Ar isotopic geochronology. The study of the Rb-Sr isotopic system incl uded sample leaching with a solution of ammonium acetate, which was ap plied to clay fractions with a grain size of < 2.0, 0.6 - 1.0, < 0.6, and 0.1 - 0.4 mu m, each containing no less than two illite generation s. Despite the linear correlation of data points in the Rb-Sr evolutio n diagrams, which was revealed for equi-sized fractions of a series of samples, the estimated age values and initial Sr-87/Sr-86 ratios are meaningless in a geological sense. The distinguished relations corresp ond to mixing lines, suggesting a uniform contribution of different il lite generations into equi-sized fractions of clay samples.