K-feldspar sand-grain provenance in the Triassic, west of Shetland: distinguishing first-cycle and recycled sediment sources?



Sandstone provenance studies can help constrain palaeogeographic reconstructions and ancient drainage system scales and pathways. However, these insights can be obscured by difficulties in geochemically distinguishing or adequately characterizing potential sourcelands, or by failure to identify sedimentary recycling. Triassic basins west of Shetland accumulated ∼2.5 km of sand-rich sediment. The Middle-Upper Triassic Foula Formation represents fluvial, aeolian and sabkha facies deposited in the northern interior of the Pangaean supercontinent. Published U-Pb zircon geochronology and heavy mineral analysis suggest that these sandstones were derived from East Greenland. They contain significant fresh K-feldspar which is likely to be first cycle and derived directly from its source. Pb isotopic analyses of individual K-feldspar sand-grains show a single, unradiogenic Pb population, consistent with the provenance indicated by U-Pb zircon geochronology. Archaean and Palaeo-Mesoproterozoic rocks—the Nagssugtoqidian Mobile Belt, the Lewisian Complex or equivalents—are the likely source, with terranes south of the Moine Thrust (Grampian, Caledonian and Variscan) ruled out by both the Pb and U-Pb data. However, it is not possible to distinguish between rift flank sources to the east and west, as both areas have similar crustal affinity and/or share the same tectonic history. It is possible that the sediment was derived from the West Shetland Platform and not from Greenland. The comparison of provenance signals from robust and less stable mineral phases provides a means of recognizing sedimentary recycling. Robust zircon populations and less stable feldspar in Foula Formation sandstones concur in indicating the same source, suggesting that they are likely to be first cycle. The Triassic sand supply can be contrasted with that in Upper Carboniferous (Namurian) basins in the north of England where a significant zircon population has no corresponding K-feldspar component. This zircon population is likely to have been recycled from Lower Palaeozoic greywackes from the Southern Uplands Belt or it's along strike extension. Copyright © 2009 John Wiley & Sons, Ltd.