Precambrian muscovite–quartz (agalmatolite) palaeosols from Scotland and Canada

Abstract

Weathering, during late Precambrian times, of pink granitic gneisses and pegmatites of the Middle Proterozoic Lewisian Complex in northwest Scotland has produced a characteristic pea-green palaeosaprolite called agalmatolite, which occurs from centimetres to metres beneath the planar unconformity with the overlying quartz sandstones of the Lower Cambrian marine transgression. Agalmatolite consists predominantly of massive, felted, very fine-grained muscovite of the form called pinite together with variable amounts of relict quartz. Pyrophyllite may also occur in the most altered parts, close to the unconformity, and the proportions 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 present mineralogy reflects the burial of the saprolite subjected to conditions of anchimetamorphism.Palaeosols developed on both granitic and basaltic 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 palaeosols, muscovite, usually referred to as sericite, is the dominant mineral. In northwest Ontario weathering of the granitic basement beneath the Steep Rock Lake Group has produced a rock almost identical to the Scottish agalmatolite and consisting of muscovite, of the form pinite, relict quartz, and rutile. From published descriptions of other palaeosols, we conclude that the term agalmatolite may be applied to most Precambrian palaeosols, as it conveys more information than terms such as argillite and saprolith that have previously been in use.Compared with the less well-exposed alteration profiles of older palaeosols, the clear field relationships of agalmatolite in northwest Scotland allow more confidence to be placed on interpretations that such alteration profiles have been caused by surficial weathering, with subsequent changes due to burial and even low-grade metamorphism.