Quartz-spessartine metasediments (coticules) and their protoliths in North Wales

Abstract

AbstractThin beds and small pods of quartz-spessartine metasediments (coticules) occur in pelites of the Gamlan Formation (Cambrian) of the Harlech Dome. The original microfabric of these sediments has been largely destroyed by recrystallization under low greenschist facies conditions. The quartz-spessartine pods occur throughout the pelites and appear to have formed by the metamorphism of diagenetic Mn-carbonate concretions. The thin beds of quartz-spessartine rock appear to have formed by the metamorphism of a protolith which was deposited rapidly and repeatedly over a wide area. Although there is no direct evidence of the nature of this protolith, the presence of altered Mn-carbonate-rich tuff beds in anchizonal mudstones of similar age in the St Tudwal's Peninsula suggests that the bedded quartz-spessartine rocks are, in fact, metamorphosed Mn-carbonate-rich tuff units. Both the bedded and podiform quartz-spessartine metasediments appear to have formed by reaction between clay minerals and Mn carbonates of earlier diagenetic origin.