87Sr/86Sr chemostratigraphy of Neoproterozoic Dalradian limestones of Scotland and Ireland: constraints on depositional ages and time scales.

New calcite 87Sr/86Sr data for 47 limestones from the metamorphosed and deformed Neoproterozoic--Cambrian Dalradian Supergroup of Scotland and Ireland are used to identify secular trends in seawater 87Sr/86Sr through the Dalradian succession and to constrain its depositional age. Dalradian limestones commonly have Sr > 1000 ppm, indicating primary aragonite and marine diagenesis. Low Mn, Mn/Sr <0.6, δ18O and trace element data indicate that many 87Sr/86Sr ratios are unaltered since diagenesis despite greenschist- to amphibolite-facies metamorphism, consistent with the documented behaviour of Sr and O during metamorphic fluid-rock interaction. Thus, the 87Sr/86Sr data are interpreted largely to reflect 87Sr/86Sr of coeval seawater. Currently available data show that Neoproterozoic seawater 87Sr/86Sr rose from c. 0.7052 at 850-900 Ma to c. 0.7085 or higher in the latest Neoproterozoic. Temporal changes at c. 800 Ma and c. 600 Ma bracket the range in 87Sr/86Sr values of calcite in Grampian, Appin and lowest Argyll Group (c. 0.7064-0.7072) and middle and uppermost Argyll Group (c. 0.7082-0.7095) limestones, consistent with a rise in seawater 87Sr/86Sr around 600 Ma. 87Sr/86Sr data are consistent with the sedimentary affinity of the Islay Subgroup with the underlying Appin Group, and with a possible time interval between deposition of Islay and Easdale Subgroup rocks. They indicate that the Dalradian, as a whole, is younger than c. 800 Ma. [ABSTRACT FROM AUTHOR]