ISOTOPIC SIGNATURES OF DEPOSITION AND TRANSFORMATION OF LOWER CAMBRIAN SALIFEROUS ROCKS IN THE IRKUTSK AMPHITHEATER: COMMUNICATION 2. STRONTIUM ISOTOPIC COMPOSITION

Abstract

Rb-Sr data were obtained for different Lower Cambrian rocks from boreholes Bel'sk and Zhigalovo drilled in the Irkutsk amphitheater. Shales in the lower part and insoluble residue of marls throughout the entire (up to 2 km thick) sequence define the first transformation stage of silicate rocks dated at 580 Ma with a large uncertainty. Sulfate and carbonate rocks are the main Sr carriers in the Lower Cambrian rocks. Owing to a negligible amount of silicate impurities in them, the input of radiogenic Sr exerted no significant influence on the Rb-Sr isotope system. However, almost all carbonate and sulfate rocks studied are contaminated by radiogenic Sr relative to its initial isotopic composition in the sedimentation basin. Hence, the rocks experienced epigenetic (probably, repeated) recrystallization and one or several stages of dissolution. © Pleiades Publishing, Inc., 2006.