Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Vladislav Rapprich, Yulia V. Kochergina, Tomáš Magna, František Laufek, Patricie Halodová, František Bůzek

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

Journal of Geosciences, volume 62 (2017), issue 2, 121 - 136

DOI: http://doi.org/10.3190/jgeosci.238

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 ((ΣREE = 82.6 ppm). The stable-isotope signature (δ¹⁸O = 7.43 ‰, (δ¹³C = −2.46 ‰) 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.