Accessing the Ediacaran-Cambrian boundary in SW Baltica based on SHRIMP and CA-ID-TIMS zircon (...) скачать в хорошем качестве

Accessing the Ediacaran-Cambrian boundary in SW Baltica based on SHRIMP and CA-ID-TIMS zircon (...)

P-T-t talk 33: Accessing the Ediacaran-Cambrian boundary in SW Baltica based on SHRIMP and CA-ID-TIMS zircon U-Pb dates. Speaker: Ion Francovschi (Geological Institute of Romania) Description: In recent years, using the youngest detrital zircon dates for sedimentary deposits, obtained with the LA-ICP-MS technique, has become a common method for constraining depositional ages. However, Pb loss from zircons can significantly affect LA-ICP-MS U–Pb zircon dates, potentially resulting in individual zircon U-Pb dates being younger than their crystallization ages. In the Moldova-Podillya basin (SW Baltica), recent U-Pb LA-ICP-MS analyses on zircons from sandstones of the lower Kanyliv Group yielded MDAs around 523-532 Ma. These dates conflict with established biostratigraphic evidence, which indicates a late Ediacaran age for the Kanyliv Group and places the E–C boundary (defined by the Phycodes pedum ichnofossil zone) above it. The LA-ICP-MS results suggest this boundary should be repositioned below the Kanyliv Group. To resolve this discrepancy, we analyzed zircons from the lower Kanyliv Group near Naslavcea, Moldova. Initially, U-Pb SHRIMP dating of 115 concordant zircons yielded an MDA of 530 ± 10 Ma. This age is younger than the accepted E–C boundary (538.8 ± 0.2 Ma) and would necessitate reconsidering the E–C boundary in SW Baltica. Subsequent CA-ID-TIMS dating of the same youngest zircons revealed a significantly older age of 553.20 ± 0.25 Ma, consistent with the biostratigraphic constraints. These results emphasize the importance of using CA-ID-TIMS for high-precision chronostratigraphic studies, as Pb loss can cause an apparently younger date than the depositional age of sediments.