RB-SR SYSTEMATICS OF VENDIAN CAMBRIAN CLAYSTONES FROM THE EAST EUROPEAN PLATFORM - IMPLICATIONS FOR A MULTISTAGE ILLITE EVOLUTION

A combined mineralogical, chemical, morphological and Rb-Sr isotopic s tudy of ''blue clays'' from the Lower Cambrian Lontova unit shows that these minerals contain three generations of illite. The oldest detrit al 2M illite particles are typically round with sharp borders sometime s rolled up and have a minimum isotopic age of 722 +/- 13 Ma. This gen eration of illite is mainly concentrated in the 0.6-2-mum size fractio ns of the samples. The second generation of illite is represented by w ell-crystallized lath-type particles of 0.1-mum size with an age of 53 3 +/- 8 Ma considered to be the sedimentation age. Faulting and/or reg ional emergence during the Ordovician to Early Silurian time probably induced extensive circulations of subsurface waters which precipitated an additional < 0.1-mum generation of low-temperature lath-type illit e particles at approximately 430-480 Ma ago. The Precambrian-Cambrian boundary may be set at approximately 530 +/- 10 Ma based on the data p resented here. Leaching of different clay subfractions by ammonium ace tate shows that illite particles of different generations crystallized in environments with different Sr-87/Sr-86 ratios. The varied environ mental Sr isotopic ratios were, at least in part, due to local interac tions between the interstitial waters and the sedimentary mass. Additi onally, the initial Sr-87/Sr-86 ratios of the clay particles from argi llaceous sedimentary rocks were mostly higher than that of marine Sr.