Multistage carbonation of the Variscan ophiolite? Insights from geochemical and isotopic diversity of ophiolite-hosted carbonates

The mantle section of the ophiolite sequence is composed of ultramafic rocks, abundant in magnesium silicates. When exposed to crustal depths, these minerals become thermodynamically unstable and often interact with fluids of different provenance and chemistry. Ophiolite-hosted carbonates are one of the most widespread secondary phases resulting from interactions between silicates and CO₂-rich fluids. There is ample evidence that carbonation of ultramafic rocks can occur in various geological settings and across a broad range of P-T conditions (Plümper & Matter, 2023). Therefore, accurately identifying the source of the fluids responsible for carbonation could contribute to a more comprehensive understanding of the long and complex evolution of the ophiolites.

In this study, we investigate carbonates hosted by the two ultramafic massifs (Szklary and Braszowice) of the Central Sudetic Ophiolite (CSO) (NE Bohemian Massif). This ophiolitic complex represents a late Devonian oceanic lithosphere formed in the slow-spreading regime. The complex story of the CSO includes prograde metamorphism, reaching its climax within the amphibolite facies, emplacement of syn- and post-Variscan magmas, as well as tropical weathering event(s) in the Cenozoic. Carbonate mineralization appears mostly as extensive veins and vein-like structures within partially serpentinized peridotites and serpentinites. The vast majority of veins in both localities exhibit a high modal abundance of cryptocrystalline magnesite accompanied by chalcedony or quartz. Field investigations revealed that carbonate veins containing magnesite-dolomite and calcite-dolomite are comparatively less common occurrences and they are mostly seen in the Braszowice ophiolitic fragment. In some samples, hydrous magnesium silicates co-occur with the studied carbonates. Based on both bulk and single-spot chemical composition (ICP-MS/ES and LA-ICP-MS) of carbonates, discrepancies and similarities have been observed between two ultramafic massifs. The concentrations of several trace elements (Ni, Al, Mn, Sr, Ba, Fe) noticeably vary between Szklary and Braszowice. Moreover, varied chemical compositions have been pinpointed among veins sampled at different depths. Strontium isotope composition was analyzed for fraction dissolved in HCl. At least two groups of carbonate veins can be distinguished based on their ⁸⁷Sr/⁸⁶Sr ratios. Veins sampled from the Braszowice pit floor, exhibiting Mgs or Mgs ± Dol or Cal ± Dol paragenesis, consistently display 0.7064 - 0.7065 values. Carbonate veins located at shallow depths in Braszowice and Szklary, primarily composed of cryptocrystalline Mgs or Mgs ± Qz, show ⁸⁷Sr/⁸⁶Sr values ranging from 0.7070 to 0.7113 suggesting input from fluids derived from the continental crust. Our research indicates a level of complexity in the formation of ophiolite-hosted carbonates including several stages of their formation as well as several sources of carbonating fluids.

Plümper, O., & Matter, J. (2023). Olivine—the alteration rock star. Elements, 19(3), 165-172.

Funding: Research financially supported by NCN PRELUDIUM project 2022/45/N/ST10/00879