A geochemical investigation of palaeosols developed on Lewisian rocks beneath the Torridonian Applecross Formation, NW Scotland

Alteration zones are developed on biotite gneiss and amphibolite: of the Le wisian Basement Complex beneath Neoproterozoic rocks of the Applecross Form ation of the Torridon Group in northwestern Scotland. The age and origin of these altered rocks has been disputed but systematic sampling and chemical analyses reveal patterns in Al2O3 - CaO + Na2O - K2O space that are identi cal to weathering trends in palaeosols elsewhere. The Lewisian palaeosols, like other ancient weathering profiles, display significant loss of Ca and Na and exhibit strong K-metasomatism. K/Rb ratios in the profiles exhibit s ystematic variations that are thought to be related in part to the nature o f metasomatizing fluids, rather than reflecting their provenance. Chondrite -normalized REE patterns for the host rocks and palaeosols demonstrate the conservative nature of these elements. There are, however, some differences between REE patterns obtained from the fresh and altered basement rocks an d those of mudstones of the Applecross Formation. These differences suggest that the provenance area for the Applecross sediments included rock types other than those of the immediately underlying Lewisian basement.