Abstract

Microstructures, textures, and composition of dolomites from different facies and stratigraphic level of a carbonate platform sequence 1000-2000 m thick in the Brenta Dolomites and Eastern Lombardy (Italy) are interrelated. Mimetic, unimodal (4 mu m), planar-e dolomite and polymodal (20-200 mu m), planar-s to nonplanar matrix dolomite show calcian and stoichiometric areas in the same crystals. Ca-rich areas (53-56 mole % Ca) have pervasive modulated microstructure. As stoichiometric composition is approached (51-52 mole % Ca), modulations are coarser and less pervasive. Stoichiometric zones show few dislocations. Dolomites with excess calcium and modulated microstructures appear to have positive delta 18 O values. Matrix dolomite crystals with more abundant slightly calcian and stoichiometric areas have negative delta 18 O values. Void-filling dolomites are stoichiometric and contain only a few dislocations. The onset of solid-state diffusion is recorded in some void-filling dolomites formed at temperatures above 60 degrees C. Regionally extensive dolomitization was almost completed by the end of the Triassic. Late hydrothermal dolomitization did not cause neomorphism of early replacive dolomites.

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