Environmental influences and timing constraints on early marine dolomitisation of a giant carbonate platform (Dolomia Principale, Northern Italy)

Although early marine (replacement) diagenetic dolomitisation is a well-documented process in ancient carbonate rocks, the factors driving its occurrence and timing remain less clearly understood. In the Lombardy Basin (Northern Italy), the Norian Dolomia Principale platform underwent pervasive, mimetic dolomitisation soon after sediment deposition, making it an exemplary case study of early marine dolomitisation. To identify the environmental parameters influencing the occurrence and intensity of early dolomitisation in Triassic shallow marine settings, a comprehensive multi-proxy analysis was conducted. Samples were collected from various depositional environments, including restricted lagoon, inner platform, outer platform/margin, slope, and intraplatform basin. To unravel the diagenetic history and distinguish early diagenetic features from later alterations, a paragenetic sequence was established based on petrographic observations, cathodoluminescence, fluid inclusion microthermometry, U-Pb dating, and stable isotope measurements (δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr). Although the intensity of these events varied according to depositional environment, four main diagenetic events were identified: (i) early marine dolomitisation, replacing precursor sediment and marine cement; (ii) a succession of late dolomite cement precipitations due to burial or local hydrothermalism; (iii) burial calcite cement precipitation; and (iv) late meteoric calcite precipitation. Radiometric dating of the now-stoichiometric "early marine-diagenetic dolomite" indicates that replacement occurred within the first few million years after deposition, likely followed by a prolonged period of "ripening." Fluid inclusion analysis reveals that the dolomitising fluid was a modified seawater mixed with a halite-dissolution brine, supporting a reflux-type dolomitisation model. Geochemical data reveal a progressive depletion in δ¹³C and δ¹⁸O throughout diagenetic evolution, while also highlighting discrepancies among "early dolomites" from different depositional environments. Bulk isotope data of Dolomia Principale dolostones are dominated by the fabric-retentive replacement dolomite phase and have, within limitations, value as archives of past seawater and altered marine porewater data.