Abstract

Detrital and inherited zircons from rocks of the Moine Supergroup from structural and stratigraphic positions above and below the Sgurr Beag and Naver thrusts have been dated by ion microprobe. ²⁰⁷Pb/²⁰⁶Pb ages for 65 detrital zircons from the Moine Nappe (Morar Group) range between 2707 and 947 Ma, with a bimodal distribution with clusters at c. 1650 and 1400 Ma. Five grains (8% of the analyses) were Archaean. Analyses of 97 inherited zircons from two Caledonian migmatites and two granites from the Naver Nappe yield ²⁰⁷Pb/²⁰⁶Pb ages between c. 2940 and 926 Ma. They have a dominant cluster at c. 1650 Ma with significant clusters at c. 1400 and c. 1050 Ma. Eight Archaean grains were discovered (8.5% of analyses). Three rocks above the Sgurr Beag Thrust have been examined and a total of 42 analyses obtained. ²⁰⁷Pb/²⁰⁶Pb ages for 35 inherited zircons from two outcrops of the essentially in situ West Highland granite gneiss within the Glenfinnan and Loch Eil Groups range from c. 1889 to 947 Ma. A sample of the Lochailort pelite provided a further seven detrital analyses. These data have a distribution with clusters at c. 1500 and 1100 Ma. The combined datasets indicate that the Moine Supergroup was deposited in a post-Grenvillian basin(s) with detritus derived predominantly from a late Palaeoproterozoic source, e.g. Gothian or Labradorian with only a very small proportion from older sources. The data indicate that the Naver and Sgurr Beag nappes had different sediment sources and so may not correlate. The Morar Group is dominated by late Palaeoproterozoic detritus with lesser amounts of Grenville-aged detritus. The Glenfinnan and Loch Eil Groups appear to have roughly equal proportions of detritus from the two sources. The few Archaean grains indicate that the source comprised comparatively little material of this age, precluding the Lewisian Complex or sub-Moine basement as a significant source and arguing against correlation of the Moine with the Torridonian.