Carbonate-rich dyke in Roztoky Intrusive Complex - an evidence for carbonatite magmatism in the Eger Rift?

[摘要] The possible presence of carbonatites in the Eger Rift (NW Bohemian Massif, Czech Republic) has been debated for several decades without any apparent resolution. Here, we document an almost 2 m thick dyke of a silicocarbonatite (23 wt. % SiO2, 27 wt. % CO2) in the R2 (Roztoky nad Labem) drilling at the depth of 152.9-154.8 m. Despite the fact that the silicocarbonatite is associated with alkaline intrusive complex, its content of alkalis is rather low (Na2O + K2O = 2.5 wt. %), as are REE ((?REE = 82.6 ppm). The stable-isotope signature (?18O = 7.43 \xe2\x80\xb0, (?13C = \xe2\x88\x922.46 \xe2\x80\xb0) of this rock is distinct from surrounding sedimentary rocks, while it can be compared with C-O isotope systematics of some carbonatites in the world which probably sourced carbonates from older subduction events. The Sr-Nd isotope composition (87Sr/86Sr30 ~ 0.7062; 143Nd/144Nd30 ~ 0.51205) points to an enriched reservoir without known counterparts among alkaline rocks from the Eger Rift, perhaps a lithospheric mantle modified in course of the Variscan subduction. The position of the R2 silicocarbonatite in the Sr-Nd space may indicate a continuum of enriched radiogenic isotope systematics in worldwide carbonatite occurrences.

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