Detrital zircon geochronology and provenance of the Torridonian, NW Scotland

Between 30 and 50 single detrital zircons from each of four specimens of the Steer Group and two specimens from the unconformably overlying Torridon Group were analysed on the GSC SHRIMP II ion probe. Pb-207/Pb-206 ages of zircons from the Steer Group range between 3.00 and 1.74 Ga with 95% concentrated between 2.93 and 2.48 Ga. The Bay of Steer Formation has a small mode c. 2.55 Ga, matching overgrowth ages on some older grains. A few grains between 1.92 and 1.74 Ga occur in each of the Steer Group specimens. These data are consistent with conventional provenance information and sedimentology, which indicate that the bulk of the Steer Group probably was derived from local basement of the Lewisian Gneiss Complex and that deposition occurred adjacent to basin-margin growth faults. The two Torridon Group samples have similar detrital zircon age profiles with distinctive modes at 1.80 Ga, 1.66 Ga and 1.10 Ga, interpreted to represent sources of Ketilidian, Labradorian and Grenvillian affinity, respectively. A less well defined cluster c. 2.85-2.55 Ga reflects reworking of the underlying Steer Group and possibly direct contribution from the Lewisian Gneiss Complex. The youngest concordant detrital zircon yields a maximum age of 1060 +/- 18 Ma for the Torridon Group (Applecross Formation). Our data, together with palaeocurrents from the Applecross Formation, suggest that the Torridon Group could have been deposited by a late to post-Grenvillian intermontane or foreland trunk river system flowing northeasterly, parallel to the Grenvillian orogenic belt.