Stages in a Precambrian dolomitization, Scotland: cementing versus replacement textures

Dolomitization in member 1 of the Dalradian Bonahaven Formation of Islay was sequential. The sediments studied are siliciclastic tidal flat deposits, which originally contained pebbles of ferroan dolomicrite and sandy dolomicrite (type 1 dolomite). Following burial, both dolomicrite and clay-stone pebbles, and possibly other carbonates, were replaced by coarser, fluid-inclusion-bearing, chemically zoned (type 2) dolomite. Ferroan, inclusion-free, intergranular dolomite (type 3) in sandstones may be synchronous with late zones of type 2 dolomite which it closely resembles. Cathodoluminescence and microprobe analyses of type 2 dolomite characterize four ubiquitous growth stages within which there was considerable fluctuation in Fe and Mn content. There was much variation between and within samples in terms of timing of nucleation, absolute speed of growth and relative growth rate of crystals. Type 2 dolomite displays two types of intracrystalline truncation surfaces explicable by dissolution followed by overgrowth, or by replacement by a moving solution film. Type A is characterized by a single later zone cutting across earlier ones along the line of an irregular truncation surface which developed from the edges of the crystals. In type B truncations it is the interior, rather than the edges of crystals which show truncation, the truncations forming smooth closed surfaces which enclose areas filled centripetally with new zones. In general, dolomite has a pronounced tendency to form crystal faces, and thus compromise crystal boundaries, during replacement as well as during cementation. Thus, if nucleation of crystals is restricted to the margins of an area to be replaced, replacive dolomite is capable of completely mimicking a vug-filling texture.