Abstract

Isotopic data are used to determine a new chronology for the Torridonian of NW Scotland. The calcite fraction of the Stoer Group Limestone Member yields a Pb–Pb age of 1199 ± 70 (2σ) Ma (MSWD = 1.1). Rb–Sr whole-rock regressions from siltstones of the Diabaig and Applecross Formations (both Torridon Group) produce ages of 994 ± 48 (2σ) Ma (MSWD = 17) and 977 ± 39 (2σ) Ma (MSWD = 10) respectively. These dates, which agree well with palaeomagnetic estimates, are interpreted as the time of early diagenesis which is probably within error of the time of deposition. Previous isotopic age determinations are shown to be in error.

Size fractionsfrom shales of the Stoer Group and Applecross Formation have been analysed for Rb–Sr and produce ages of 689 ± 87 (2 σ) Ma (MSWD = 14) and 667 ± 66 (2 σ) Ma (MSWD = 55) respectively. They are used, together with Sm-Nd isotopic data, to suggest a thermal event affecting the entire Torridonian at this time.

Our new chronology for the Torridonian is consistent with the Proterozoic geology of Scotland and Laurentia. It suggests that deposition of the Stoer Group is contemporaneous with uplift of the Lewisian and the Torridon Group may be associated with the extensional collapse of the Grenville orogen. The c. 700Ma diagenetic event is possibly related to Neoproterozoic rifting of the Laurentian margin.

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