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 xe2x80xb0, (?13C = xe2x88x922.46 xe2x80xb0) 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.