MACRO- AND MICROVARIATIONS IN THE CARBON AND OXYGEN ISOTOPIC COMPOSITION OF PRECAMBRIAN CARBONATES FROM THE ALDAN SHIELD

Abstract

The isotopic composition of carbon and oxygen was determined by two techniques in whole-rock samples and single grains of carbonates from rocks of various morphogenetic types from the Aldan Shield. The isotopic values obtained by different methods in carbonates of the same types are generally similar. The values of δ13C and δ 18O for the marbles, calciphyres, and apatite-carbonate rocks reflect their primary values and are close to those in sedimentary Precambrian carbonate rocks elsewhere. Among the rocks examined, high δ13C values (up to 5.45‰, PDB) were shown by apatite-carbonate rocks, whose protolith was produced chemogenically in shallow evaporitic basins with significantly oxidizing conditions and a limited influx of terrigenous material. The subsequent transformations of the primary rocks by meteoric waters and metasomatic fluids can be identified using low δ18O and negative δ 13C values.