Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Pavlína Radková, Milan Novák, Jan Cempírek, Stanislav Houzar, Radek Škoda

Amphibole and pyroxene as indicators of alkaline conditions in banded carbonatite-like marbles from Bližná, Český Krumlov Unit, Moldanubian Zone

Journal of Geosciences, volume 66 (2021), issue 4, 243 - 262

DOI: http://doi.org/10.3190/jgeosci.336

Amphiboles and diopside occur in several distinct mineral assemblages in banded carbonatite-like marbles from the Bližná graphite mine, Český Krumlov Unit, Moldanubian Zone. The carbonatite-like marble is interlayered with dominant ordinary metacarbonates as a single continuous stratabound layer, up to 3-4 m thick. It is enriched in elements typical for carbonatites (REE, Y, Th, Sr, Nb, Zr, and Mo) and contains accessory betafite-pyrochlore and uranothorite along with molybdenite, sulfides (e.g., pyrite, pyrrhotite) and oxides (e.g., magnetite, ilmenite). Detailed study of thin sections in optical microscope and BSE images as well as results of EMPA revealed three distinct mineral assemblages - richterite + forsterite (RF) assemblage (Rht + Fo + Cal ˃ Dol ˃ Phl + Di), Na rich actinolite + diopside (AD) assemblage (Di + Act + Rht + Cal ˃ Phl) and magnesio-hastingsite + diopside (HD) assemblage (Mhst + Di + Cal ˃ Ab + Phl). The distinct amphibole species found within the individual assemblages exhibit complex textural relations and crystallized in several prograde and retrograde stages. The following amphibole species were recognized in the individual assemblages: RF - (richterite > ferri-winchite > tremolite, edenite), AD - (actinolite > ferri-winchite; richterite > ferri-katophorite; magnesio-riebeckite), HD - (magnesio-hastingsite; ferri-winchite, actinolite). They differ especially in Mg/(Mg + Fe_tot) ratio and concentrations of F. Diopside contains up to 17 mol. % of the aegirine component and albite is Fe, Ba-enriched (≥ 0.69 wt. % BaO and ≥ 0.50 wt. % Fe₂O_3tot). The composition of amphiboles and pyroxenes indicates the alkaline environment of their formation; unusually high Fe₂O₃ contents in albite plagioclase have known counterparts in alkaline pegmatites and some carbonatite dykes. The following options of potential origin of the carbonatite-like marble from Bližná are discussed: (i) metacarbonates potentially with Na- and Na,Ca-carbonates and sulfate admixture; (ii) admixture of alkaline volcanodetritic component during carbonate sedimentation; (iii) metamorphosed carbonatites enclosed in ordinary sedimentary metacarbonate; (iv) Na-rich metasomatites characterized by mineral assemblages with Na,Fe,F-enriched amphiboles, diopside with high aegirine component and albite. With regard to the observed mineral assemblages, textures and composition of minerals, we can exclude (i) marine evaporates, whereas the other options (ii), (iii) and (iv) are, in general, possible. The amphiboles observed in the Bližná carbonatite-like marble are unique within the Bohemian Massif and distinctly different from amphiboles found in metacarbonates with evaporite component.