NEW ISOTOPIC AGE-DETERMINATIONS FOR THE TORRIDONIAN, NW SCOTLAND

Isotopic data are used to determine a new chronology for the Torridoni an of NW Scotland. The calcite fraction of the Steer Group Limestone M ember yields a Pb-Pb age of 1199 +/- 70 (2 sigma) Ma (MSWD = 1.1). Rb- Sr whole-rock regressions from siltstones of the Diabaig and Applecros s Formations (both Torridon Group) produce ages of 994 +/- 48 (2 sigma ) Ma (MSWD = 17) and 977 +/- 39 (2 sigma) Ma (MSWD = 10) respectively. These dates, which agree well with palaeomagnetic estimates, are inte rpreted as the time of early diagenesis which is probably within error of the time of deposition. Previous isotopic age determinations are s hown to be in error. Size fractions from shales of the Steer Group and Applecross Formation have been analysed for Rb-Sr and produce ages of 689 +/- 87 (2 sigma) Ma (MSWD = 14) and 667 +/- 66 (2 sigma) Ma (MSWD = 55) respectively. They are used, together with Sm-Nd isotopic data, to suggest a thermal event affecting the entire Torridonian at this t ime. Our new chronology for the Torridonian is consistent with the Pro terozoic geology of Scotland and Laurentia. It suggests that depositio n of the Steer Group is contemporaneous with uplift of the Lewisian an d the Torridon Group may be associated with the extensional collapse o f the Grenville orogen. The c. 700 Ma diagenetic event is possibly rel ated to Neoproterozoic rifting of the Laurentian margin.