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

[摘要] Amphiboles and diopside occur in several distinct mineral assemblages in banded carbonatite-like marbles from theBližná 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 elementstypical for carbonatites (REE, Y, Th, Sr, Nb, Zr, and Mo) and contains accessory betafite-pyrochlore and uranothoritealong with molybdenite, sulfides (e.g., pyrite, pyrrhotite) and oxides (e.g., magnetite, ilmenite). Detailed study of thinsections 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). Thedistinct 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 individualassemblages: RF – (richterite > ferri-winchite > tremolite, edenite), AD – (actinolite > ferri-winchite; richterite > ferri--katophorite; magnesio-riebeckite), HD – (magnesio-hastingsite; ferri-winchite, actinolite). They differ especially inMg/(Mg+Fetot) ratio and concentrations of F. Diopside contains up to 17 mol. % of the aegirine component and albite isFe, Ba-enriched (≤ 0.69 wt. % BaO and ≤ 0.50 wt. % Fe2O3tot). The composition of amphiboles and pyroxenes indicatesthe alkaline environment of their formation; unusually high Fe2O3 contents in albite plagioclase have known counterpartsin alkaline pegmatites and some carbonatite dykes. The following options of potential origin of the carbonatite-likemarble 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 carbonatitesenclosed in ordinary sedimentary metacarbonate; (iv) Na-rich metasomatites characterized by mineral assemblages withNa,Fe,F-enriched amphiboles, diopside with high aegirine component and albite. With regard to the observed mineralassemblages, 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 withinthe Bohemian Massif and distinctly different from amphiboles found in metacarbonates with evaporite component.

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[关键词] alkali amphibole;pyroxene;carbonatite-like marble;Český Krumlov unit;Moldanubian Zone;Bohemian Massif