Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Jaromír Ulrych, Edvín Pivec, Langrová, Emil Jelínek, Erzsébet Árva-Sós, Höhndorf, Jiří Bendl, Řanda

Geochemically anomalous olivine-poor nephelinite of Říp Hill, Czech Republic

Journal of the Czech Geological Society, volume 43 (1998), issue 4, 299 - 311

Sodalite olivine-poor nephelinite of Říp Hill substantially differs both from olivine nephelinite and olivine-free nephelinite of the Cenozoic volcanic province of the Bohemian Massif. Říp Hill represents an erosion-resistant neck 25.6 Ma in age located in the Teplá-Barrandian terrane block in contrast to the České středohoří Mts. volcanic complex in the Saxothuringian terrane. The very low Mg-value (48.5), low contents of compatible elements as Cr (79 ppm), Ni (31), Co (32), Sc (21) and lack of mantle-type xenoliths gives evidence of differentiation of primary mantle magma. Anomalous enrichment in incompatible elements particularly in ∑REE (650 ppm), U (3.6), Th (15.6), Nb (154), Ta (12.1), is associated with late magmatic crystallization (mostly of apatite, Ti-magnetite ± hauyne). ⁸⁷Sr/⁸⁶Sr (0.7036-0.7038) and ¹⁴³Nd/¹⁴⁴Nd (0.51278) ratios of the olivine-poor nephelinite are consistent with HIMU OIB from a sublithospheric source; ε^t_Nd (+3.4) value implies an depleted mantle source. Olivine-poor nephelinite could be derived from carbonated nephelinite magma with high CO₂/H₂O + CO₂ volatile fraction resulting in high viscosity and thus stopping of magma associated with differentiation and contamination in crustal reservoir during its ascent to the surface. Such magma could be associated with high explosive pyroclast-rich volcano of maar or stratovolcano type. The olivine-poor nephelinite of Říp Hill reveals common features in mineral paragenesis and chemical composition of minerals such as clinopyroxene, (Mg, Al, Ti)-magnetite, barite, ankerite with carbonate-bearing alkaline ultramafic lamprophyres in the vicinity. This fact suggests their mutual genetic association within the Říp Hill volcanic centre.