Tem and Aem Study of Pervasive, Multi-Step Dolomitization of the Upper Triassic Dolomia Principale (Northern Italy)

Abstract

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 µm), planar-e dolomite and polymodal (20-200 µm), 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 18O values. Matr x dolomite crystals with more abundant slightly calcian and stoichiometric areas have negative 18O 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°C. Regionally extensive dolomitization was almost completed by the end of the Triassic. Late hydrothermal dolomitization did not cause neomorphism of early replacive dolomites.