Rodingites in the Western Carpathians: mineralogy and geochemistry

Soutenance de thèse de Juraj Butek

Et on-line

Université Matej Bel, Banskà Bystrica, Slovaquie

https://univ-tlse3-fr.zoom.us/j/98325059103?pwd=QWNmWDhib0FQQjM3V24yanhhcXAzQT09

Rodingite represents a particular metasomatic rock type that occurs as dykes or lenses in association with serpentinized ultramafic rocks and typically consists of Ca-rich, commonly hydrated silicate minerals such as garnet, diopside, vesuvianite, chlorite, epidote-zoisite, prehnite and others. It is chemically characterized by high CaO and low SiO2, Na2O and K2O contents. The formation of rodingites is directly connected to the serpentinization process and takes place in ocean floor or subduction setting. Rodingites have been recognized as a source of information, complementary to serpentinites, on fluid compositions and their circulation in both ocean floor and subduction environments. Yet, the nature of rodingite protoliths, the chemistry and the origin of hydrothermal fluids inducing the rodingite formation, the details of fluid-rock interactions, and the metamorphic contexts remain often obscure. The aim of this work is to investigate these questions in the case of rodingites from the Western Carpathians, which have not yet been investigated in detail. In the Western Carpathians, rodingite occurrences are associated with tectonically dismembered bodies of serpentinized ultramafic rocks. These serpentinites are typically associated with Triassic carbonate sediments or Carboniferous metasediments, and their age and tectonic provenance are commonly unclear.
Detailed petrographic, mineralogical and geochemical investigation of two rodingite occurrences (Bôrček and Dobšiná) are presented to shed light on their genesis and evolution. The specific mineral association and composition of rodingites from Bôrček characterized by dominant presence of vesuvianite, diopside, garnet and chlorite with minor epidote, calcite and Fe-Ti oxides, together with whole rock chemistry characterized by high CaO and low SiO2 contents, reflect a high extent of hydrothermal alteration. Different stages of mineral crystallization were recognized: 1) diopside, vesuvianite and garnet; 2) titanite and second generations of vesuvianite and garnet; and 3) epidote, chlorite and calcite. The whole rock and mineral REE contents marked by prevailing positive Eu anomaly indicate a plagioclase-rich gabbroic protolith. Moreover, Fe-Ti rich samples presumably result from alteration of a Fe-Ti rich gabbro. Fe-Ti oxides, namely rutile and ilmenite, document complex hydrothermal-metamorphic evolution. In situ U-Pb dating of rutile yields a crystallization age of 102.6 ± 19.9 Ma, which presumably indicates the main rodingitization (and serpentinization) event occurring during the alpine peak metamorphism in the Cretaceous. Rodingites from Dobšiná are characterized by a similar mineral evolution and composition dominated by hydrated garnet, vesuvianite, diopside and chlorite. Chlorite thermometry models suggest a crystallization temperature around 265°C. In conclusion, rodingites from the Western Carpathians represent vesuvianite-rich intensively metasomatized mafic rocks formed during the Cretaceous in the active setting of the Alpine orogen, either in the ocean floor or in the accretionary prism.
Geochemical modelling indicates that an open hydrothermal system with high fluid influx expressed by high water-rock ratio is required for the transformation of gabbro into vesuvianite-rich rodingite. The progressive transformation is characterized by significant decrease in Si-Ca ratio in the rock. The variable extent of transformation represents the main control on mineral diversity in rodingites worldwide. The rodingitization process is driven by hyperalkaline and reducing serpentinization fluids, which have not to be particularly rich in Ca, but they are necessarily devoid of Si, Mg and S.