Geochemistry, provenance and palaeoclimate of the Sleat and Torridon groups in Skye

Abstract

Synopsis Arkoses of the Sleat and Torridon groups in south-east Skye form a continuous succession 4500 m thick. Palaeocurrents came from the west. The source rocks were hornblende-biotite-gneisses, granite or granite-gneisses, quartzose sediments and acid volcanics, all with upper crustal geochemical signatures. Detritus from the basic gneisses predominates in the lowest sediments, but is gradually replaced by granitic material stratigraphically up the succession. The lowest sediments accumulated in palaeovalleys with a relief of <200 m. They retain the geochemistry of the parental basic gneisses which formed the local basement. Subsequent rifting brought sediments tens of kilometres from the west, some from as far as the flanking highlands, where acid rocks were common. The climate was initially arid. But the development of rift highlands attracted rainstorms, and the higher runoff leached Ca from hornblende as it weathered to chlorite. Small amounts of Na may also have been lost from plagioclase during weathering. The ratio of material transported in solid form to that in solution is estimated to lie in the range 15–20 for the lowest formation, but to have been <10 higher in the succession. By analogy with very large modern catchments, this suggests latitudes of 22–40 deg. Palaeolatitudes of 30–45 deg. have been deduced for the Torridon Group on the mainland of Scotland, using palaeomagnetism.