U-Pb dating of calcite in ancient carbonates for age estimates of syn-to post-depositional processes: A case study from the upper Ediacaran strata of Finnmark, Arctic Norway

Abstract

Results of in situ U-Pb dating of calcite spherulites, cone-in-cone (CIC) calcite and calcite fibres from a calcareous concretion of the upper Ediacaran of Finnmark, Arctic Norway, are reported. Calcite spherulites from the innermost layers of the concretion yielded a lower intercept age of 563A ±A 70A Ma, which, although imprecise, is within uncertainty of the age of sedimentation based on fossil assemblages. Non-deformed CIC calcite from the bottom part of the concretion yielded an age of 475A ±A 25A Ma, which is interpreted as the age of CIC calcite formation during a period of fluid overpressure induced during burial of the sediments. Deformed CIC calcite from the top part of the concretion yielded an age of 418A ±A 23A Ma, which overlaps with a known Caledonian tectono-metamorphic event, and indicates a potential post-depositional overprint at this time. Calcite fibres that grew in small fissures along spherulite rims, which are interpreted as a recrystallization feature during deformation and formation of a cleavage, gave an imprecise age of 486A ±A 161A Ma. Our results show that U-Pb dating of calcite can provide age constraints for ancient carbonates and syn-to post-depositional processes that operated during burial and metamorphic overprinting.