Origin of dolomites encrypted in their geochemical signatures

The Esino Limestone of the Western Southern Alps represents a differentiated Ladinian - (? ) Lower Carnian carbonate platform comprising of margin, slope and peritidal inner platform facies up to 1000m thick. A major regional subaerial exposure event led to a coverage by another peritidal Lower Carnian carbonate platform (Breno Formation). Multidolomitization events affected the carbonate sediments. Petrographic examinations identified at least three main generations of dolomites (D1, D2, and D3) that occur as both replacement and fracture-filling cements. These phases have crystalsize ranges of 3 to 35μm (dolomicrite D1), 40 to 600μm (eu- to subhedral crystals D2), and 200μm to 5mm (cavity- and fracture-filling anhedral to subhedral saddle dolomite D3), respectively. The fabric retentive near-micritic grainsize coupled with low mean Sr concentration (76 ± 37 ppm) and estimated δ¹⁸O value (~ -8 to +2.8‰ SMOW) of the parent dolomitizing fluids of D1 suggest formation in shallow burial setting at temperature ~ 45 - 50 ﾟC with possible contributions from volcanic-related fluids (basinal fluids circulated in volcaniclastics or related to volcanic activity), which is consistent with its abnormal high Fe (4438 ± 4393 ppm) and Mn (1219 ± 1418 ppm) contents. The larger crystal sizes, homogenization temperatures (D2, 108 ± 9 ﾟC; D3, 111 ± 14 ﾟC) of primary two-phase fluid inclusions, and calculated salinity estimates (D2, 23 ± 2 eq wt% NaCl; D3, 20 ± 4 eq wt% NaCl) of D2 and D3 suggest that they formed at later stages under mid- to deeper burial settings at higher temperatures from dolomitizing fluids of higher salinity, which is supported by higher estimated δ¹⁸O values of their parent dolomitizing fluids. This is also consistent with their high Fe (4462 ± 4888 ppm; and 1091 ± 1183 ppm, respectively) and Mn (556 ± 289 ppm and 1091 ± 1183 ppm) contents, and low Sr concentrations (53 ± 31 ppm and 57 ± 24 ppm, respectively). The similarity in shale-normalized REE patterns and Ce (Ce/Ce*)SN and La (Pr/Pr*)SN anomalies of the investigated carbonates support the genetic relationship between the dolomite generations and their calcite precursor. Positive Eu anomalies, coupled with fluid-inclusion gas ratios (N2/Ar, CO2/CH4, Ar/He) and high F/Cl molar ratios suggest an origin from diagenetic fluids associated with the Late Triassic (? Carnian) volcanic activities or circulated through volcanic rocks, which is consistent with the co-occurrence of volcaniclastic lenses in the investigated sequence. Earlier studies of the St. George Group carbonates in Western Newfoundland have also identified dolomite generations (D1, D2, and D3) with comparable crystal sizes and shapes but different cathodoluminescence characterisitics in D2 and D3 relative to their counterparts of the Esino and Breno formations. However, the comparison of the major and trace elements, O-isotopes, REEs, and fluid-inclusion gases of the dolomites from the Breno and Esino formations (Western South Alps) with their counterparts from the St. George Group carbonates shows clearly the influence of the co-occurrence of volcanic rocks on the dolomites from the Alps region.