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

Thomas, C.W., Graham, C.M., Ellam, R.M. and Fallick, A.E. (2004) ⁸⁷Sr/⁸⁶Sr chemostratigraphy of Neoproterozoic Dalradian limestones of Scotland and Ireland: constraints on depositional ages and time scales. Journal of the Geological Society, 161(2), 229 -242. (doi: 10.1144/0016-764903-001)

Abstract

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 greater than 1000 ppm, indicating primary aragonite and marine diagenesis. Low Mn, Mn/Sr less than 0.6, ë¹⁸O 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.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Fallick, Professor Anthony and Ellam, Professor Rob
• Authors: Thomas, C.W., Graham, C.M., Ellam, R.M., and Fallick, A.E.
• Subjects: Q Science > QE Geology
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Journal of the Geological Society
• Publisher: The Geological Society of London
• ISSN: 0016-7649
• Copyright Holders: © 2000 The Geological Society of London
• First Published: First published in the Journal of the Geological Society 161(2):229-242
• Publisher Policy: Reproduced in accordance with the copyright policy of the publisher.