Comparative study of dolomites of different genesis (Raikküla Formation, Silurian; Estonia)

Dolomites of the Upper Subformation of the Raikküla Formation (middle Llandovery), representing the deposits of the northern marginal part of carbonate shelf consisting of cyclically laminated limestone and lagoonal dolomite were studied. The formation of dolomite was investigated using X-ray diffraction, X-ray fluorescence, and titration analyses of the rocks of the same primary origin in nondolomitized sections and those affected by massive pervasive dolomitization. The CaCO3/MgCO3 ratio and lattice parameters of dolomites are in good agreement with the genesis. The primary lagoonal dolomite near the contact with limestone and dolomite in limestone have the most expanded lattice; the primary dolomite near the contact with pervasively dolomitized limestone has the parameters equal to the minimal for primary dolomites, registered in the centre of the layer; the most altered secondary dolomite is close to stoichiometric, The bimodal frequency distribution of the Ca content in dolomites of different genesis reflects the preferred levels of Ca uptake. The highest stoichiometry of the secondary (replacive) dolomite points to the role of recrystallization and crystallization rate. Compared to limestones dolomites are depleted in Sr and enriched in Mn. Major, minor, and trace element concentrations suggest that seawater (or modified seawater) was the dominant dolomitizing fluid. No evidence of hypersalinity or inflow of outside fluids has been observed. The direct relationship of dolomitization to the migrating inner shelf facies and regressive stages of the evolution of the Silurian Baltic Palaeobasin suggests early dolomitization.