Following the identification of grey quartz–albite–chlorite–calcite–muscovite rocks in Meso- to Neo-proterozoic sequences in Scotland as metamorphosed tuffs of intermediate composition, it has been shown that this lithology will generate calc-silicate rocks at higher metamorphic grades. Both rock types occur as thin beds with sharp contacts with their host, occur as multiple beds in isolated suites, and share chemical compositions suggestive of volcanic sources with tholeiitic andesite affinities. The failure to recognise calc-silicate rocks as tuffs might explain the apparent scarcity of volcanogenic material through c.220 million years of early Earth history in Scotland.
