Geochronological studies on the Torridonian sediments of north-west Scotland

Synopsis K-Ar and/or Rb-Sr measurements on detrital felspars, micas and on pebbles of acid porphyry and micaceous metasediment from the Torridonian succession yield rather well-defined groups of ages in the range c. 1700–1150 million years. The Torridonian sediments were derived from a geologically and geochronologically complex source area, probably to the west, in which a gneissic basement complex was overlain by a now vanished 1800–1500 million year-old (Laxfordian?) supracrustal series which contained metasediments and acid igneous rocks. Rocks affected by an isotopic event about 1150–1100 million years ago also contributed detrital material. A further minimum age of c. 1150 million years is reported for the Lewisian basement rocks of the Torridon area. Total-rock Rb-Sr measurements on three Upper and two Lower Torridonian shales with rather high Rb/Sr ratios strongly suggest that the Torridonian sediments were deposited less than about 850–800 million years ago. Comparisons with the scanty published geochronological data do not prove or disprove the widely accepted Torridonian-Moine equivalence, but rule out any possible time correlation between the Torridonian sediments and the Gardar sediments of south-west Greenland, the Lower and Middle Keweenawan sediments of Canada and the Jotnian sediments of Scandinavia.