Multiphase partial and selective dolomitization of Carnian reef limestone (Transdanubian Range, Hungary)
Partially dolomitized carbonate successions provide a good opportunity to understand the commonly multistage process of dolomitization. Petrographic methods, fluid inclusion microthermometry and stable isotope measurements were applied to reconstruct the diagenetic evolution and dolomitization of a partially dolomitized Carnian reef limestone from the Transdanubian Range, Hungary. The diagenetic history began with reef diagenesis and formation of dolomite micro aggregates in microbial fabric elements; this was followed by the development of euhedral porphyrotopic dolomite crystals through overgrowths around the previously formed dolomite micro aggregates during the earliest burial stage. Increasing burial resulted in the extension of the dolomite patches via formation of finely crystalline replacement dolomite. From the Late Norian, when the Carnian reef carbonates reached the depth of 1*0 to 1*8 km, the diagenetic evolution continued in an intermediate to deep burial setting. Contemporaneously, an extensional regime was established, leading to fracturing. The progressive burial resulted in the recrystallization of the pre existing dolomite with increasing temperature, while saddle dolomite cement was precipitated in fractures. In connection with the Alpine Orogeny, intense denudation took place during the Late Cretaceous, accompanied by fracturing. Similar tectonically controlled denudation and fracturing occurred in several stages during the Cenozoic. As a result of these processes, the studied Carnian carbonates were raised to a near surface position or became subaerially exposed, leading to dedolomitization of the last dolomite phase and precipitation of calcite cement in cavities and fractures. This study revealed that by investigating partially and selectively dolomitized rock types, it is possible to document and understand those stages of the multiple dolomitization process which can barely be detected in the completely dolomitized rock bodies. Recognition of the dolomitization phases could provide the basis for the analysis of their relations with the depositional, diagenetic and tectonic processes, and stages of basin evolution.