A Petrological Study of Mimetic Dolomites from the Triassic Khaneh Kat Fm., South-Central Iran

Despite the abundance of mimetic dolomites in the Precambrian rock record, large-scale fabric-preserving dolomites are seemingly rarer in the Phanerozoic. Why this is remains unknown. This study examines the pervasive mimetic dolomites in the Khaneh Kat Fm., a collection of Triassic-age, inner ramp carbonates that outcrop in the High-Zagros structural zone in south-central Iran. The Khaneh Kat is overlain by a regionally extensive, fabric destructive dolomite layer called the Sefidar Dolomite Mbr. Preservation of depositional facies, relatively shallow burial depths, and an absence of late diagenetic overprinting in the Khaneh Kat provide an attractive example to study the formational conditions of mimetic dolomites. Although the Khaneh Kat section is nearly 100% dolomite, petrographic data from a wide range of facies indicate that depositional components, including mud, microbialites, ooids, peloids, intraclasts, and skeletal grains, were initially CaCO3. Thin-section petrography shows fenestrae filled with dolomite with a similar uniform cathodoluminescence signature as the matrix dolomites. X-ray diffraction mineralogy indicates that the Khaneh Kat dolomites are generally stoichiometric and moderately ordered with values that range from 50.9 to 52.0 mole%MgCO3, and 0.3 and 0.6, respectively. δ18O values for the Khaneh Kat dolomites range from -3.16 to -0.68 ‰ VPDB, similar to the range reported for Triassic marine brachiopods. These data, coupled with low [Na], [Sr], Fe], [Mn], and a lack of evaporites suggest that the replacive, mimetic dolomites of the Khaneh Kat Fm. formed early in the diagenetic sequence, and likely in marine-derived fluids.