Detrital zircon geochronology and provenance of the Torridonian, NW Scotland

Between 30 and 50 single detrital zircons from each of four specimens of th e Steer Group and two specimens from the unconformably overlying Torridon G roup were analysed on the GSC SHRIMP II ion probe. Pb-207/Pb-206 ages of zi rcons from the Steer Group range between 3.00 and 1.74 Ga with 95% concentr ated 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 bet ween 1.92 and 1.74 Ga occur in each of the Steer Group specimens. These dat a 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 hav e 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, Labrado rian 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 concord ant 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 d eposited by a late to post-Grenvillian intermontane or foreland trunk river system flowing northeasterly, parallel to the Grenvillian orogenic belt.