Early and late dolomites in the carbonate platform: An example from Middle Devonian carbonates of the Taurus Mountains, south-central Turkey

摘要

This paper describes the occurrence of dolomite and the mechanism of dolomitization of the Middle Devonian Safaktepe Formation in the autochthonous Geyikdagi unit of the East Taurus Mountains in south eastern Turkey. The early dolomite (Group A) tends to occur in SW of the region reflecting the extensive dolomitization of Amphipora bioherms and laminated mi-crobial. These dolomites have planar crystal boundaries and are fine to moderately crystallized, taking place as replacive dolomite consisting of the different petrographic types such as dirty, clear and limpid crystals. Carbon and oxygen isotopic compositions of these dolomites are delta~(18)O = -3.60 to 0.04 per thousand indicating that the dolomitization fluids were normal or slightly modified (evaporated) sea-water. The late dolomites (Greup B), generally coarser than the earlier ones, predominate in the NE region. They consist of massive and zebroid dolomites. Massive dolomites are characterized by a mottled structure including white spots on the coarse dirty dolomite matrix. Zebroid dolomites are formed by an alternation of dark and light coloured bands or sheets, which locally host ore deposits. In addition, some moderately or coarse crystalline dolomite beds are individually seen within the peripheral limestone fields. The late dolomites consist of replacive, cementing (void-filling)dolomites including drusy mosaic, fibrous or saddle crystals. They are more depleted in ~(18)O (delta~(18)O = -10.75 10 to -5.69 per thousnad) relative to the early dolomites, suggesting that dolomitization occurred at higher temperatures during late diagenesis (40 to 72 deg C). It is difficult to determine the origin of the late dolomites, several possibilities exist e. g. 1) deep burial diagenesis, 2) hydrothermal alteration, and 3) thermal effects from tectonic shearing.