Sr/Sr chemostratigraphy of Neoproterozoic Dalradian limestones of Scotland and Ireland: constraints on depositional ages and time scales
نویسندگان
چکیده
New calcite Sr/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 Sr/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, O and trace element data indicate that many Sr/Sr ratios are unaltered since diagenesis despite greenschistto amphibolite-facies metamorphism, consistent with the documented behaviour of Sr and O during metamorphic fluid–rock interaction. Thus, the Sr/Sr data are interpreted largely to reflect Sr/Sr of coeval seawater. Currently available data show that Neoproterozoic seawater Sr/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 Sr/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 Sr/Sr around 600 Ma. Sr/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.
منابع مشابه
Title: Re-Os geochronology of the Neoproterozoic – Cambrian Dalradian Supergroup of Scotland and Ireland: Implications for Neoproterozoic stratigraphy, glaciations and Re-Os systematics
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