Hydrothermal alteration of Britain’s oldest palaeosols: saddle dolomite and smectite at the Lewisian–Torridon Group (early Neoproterozoic) unconformity, NW Scotland

Abstract

Weathering profiles 2–3 m thick mantle a palaeoplain formed on the Lewisian Complex and are overlain unconformably by fluvial conglomerates of the c . 980 Ma Applecross Formation (Torridon Group) in NW Scotland. Geological and palaeomagnetic data indicate that the profiles formed penecontemporaneously with deposition of the Applecross Formation and constitute the oldest known palaeosols in Britain. However, their degree of later alteration and burial depth are contentious. The weathering profiles are characterized by the presence of mixed-layer illite–smectite and smectite, which is anomalous for Precambrian palaeosols, raising doubts regarding a Precambrian age for the clay suite. Estimates of burial depth range from <1.5 km, based on the apparently limited burial alteration with the preservation of smectite, to c . 6 km and possibly c . 12 km following Caledonian tectonism. Thin (1–10 mm) veins of saddle (baroque) dolomite are present in weathered amphibolite ≤1 m below the Lewisian–Torridon Group unconformity and also in overlying conglomerate 15 km SSW of Cape Wrath, indicating the flow of heated ( c . 90–160 °C) saline fluids. The anomalous occurrence of mixed-layer illite–smectite and smectite in these Precambrian palaeosols implies ‘retrograde diagenesis’ of diagenetic illite and chlorite likewise induced by hydrothermal fluids. Alteration of the palaeosols is attributed to hydrothermal fluid flow along the Lewisian–Torridon Group unconformity that was related to fluid movement in the Moine Thrust zone during Silurian or Permian regional tectonism. Radiometric dating of the illite may illuminate the timing of alteration of the palaeosols and their burial history.