Early Dinantian dolomites from East Fife: hydrothermal overprinting of early diagenetic fabrics?

Abstract

The dominantly siliciclastic early Dinantian succession of East Fife contains numerous thin carbonate horizons which have been extensively dolomitized. The carbonates are generally bioclastic wackestones with restricted molluscan faunas. The dolomite is fine-grained and homogeneous in backcattered SEM images. It generally contains 7–11 mol% FeCO 3 , and 49–52 mol% CaCO 3 , and has δ l8 O = –6 to + l% (PDB) and δ I3 C= –11.5 to 1.5% (PDB). Allochems within, and veins cutting, dolomite horizons are chemically indistinguishable from the matrix dolomite and there is little regional variation in dolomite composition. Sandstones in the study area also contain dolomite as cements which appear to be of similar age to abnormally abundant authigenic pyrite. The pyrite has replaced siliciclastic grains, filled intergranular porosity and also occurs as 2 cm thick crusts on bedding plane and joint surfaces. The pyritization is most pronounced within synclines. There is evidence elsewhere in Fife, that hydrothermal activity, related to Namurian volcanism, overprinted the chemistry of early mixing-zone dolomite in late Dinantian carbonate horizons proximal to exposed vents. It is much harder to prove a similar overprinting in the older carbonate horizons of East Fife, although the chemical similarity of vein and bulk rock dolomites and the massive pyritization of sandstones are compatible with a regional hydrothermal model.