PRECAMBRIAN MUSCOVITE QUARTZ (AGALMATOLITE) PALEOSOLS FROM SCOTLAND AND CANADA

Weathering, during late Precambrian times, of pink granitic gneisses a nd pegmatites of the Middle Proterozoic Lewisian Complex in northwest Scotland has produced a characteristic pea-green palaeosaprolite calle d agalmatolite, which occurs from centimetres to metres beneath the pl anar unconformity with the overlying quartz sandstones of the Lower Ca mbrian marine transgression. Agalmatolite consists predominantly of ma ssive, felted, very fine-grained muscovite of the form called pinite t ogether with variable amounts of relict quartz. Pyrophyllite may also occur in the most altered parts, close to the unconformity, and the pr oportions of precursor minerals, mainly quartz and feldspar, increase as the parent gneisses are approached. Trends in whole-rock elemental variations are consistent with a weathering hypothesis, and the presen t mineralogy reflects the burial of the saprolite subjected to conditi ons of anchimetamorphism. Palaeosols developed on both granitic and ba saltic basement rocks, with ages ranging up to about 3.0 Ga, have been described from a number of areas throughout the world. In all these p alaeosols, muscovite, usually referred to as sericite, is the dominant mineral. In northwest Ontario weathering of the granitic basement ben eath the Steep Rock Lake Group has produced a rock almost identical to the Scottish agalmatolite and consisting of muscovite, of the form pi nite, relict quartz, and rutile. From published descriptions of other palaeosols, we conclude that the term agalmatolite may be applied to m ost Precambrian palaeosols, as it conveys more information than terms such as argillite and saprolith that have previously been in use. Comp ared with the less well-exposed alteration profiles of older palaeosol s, the clear field relationships of agalmatolite in northwest Scotland allow more confidence to be placed on interpretations that such alter ation profiles have been caused by surficial weathering, with subseque nt changes due to burial and even low-grade metamorphism.