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

Abstract

New calcite 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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, δ 18 O and trace element data indicate that many 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr data are interpreted largely to reflect 87 Sr/ 86 Sr of coeval seawater. Currently available data show that Neoproterozoic seawater 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr around 600 Ma. 87 Sr/ 86 Sr 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.