Vinařická hora Hill Cenozoic Composite Volcano, Central Bohemia: Geochemical Constraints

Olivine-poor nephelinites, sometimes analcimized and con- taining minerals of the sodalite group, occur only in the central part of the Bohemian minerals of the sodalite group ± (OH)-bear- ing mineral phase (amphibole/biotite) are characteristic only for the central part of the Bohemian Cretaceous Basin between Mělnik and Doksy. Chemical composition of (olivine-poor) nephelinites of central Bohemia is substantially different from both (i) olivine nephelinite to basanite widespread in the Ceske středohoři Mts., N Bohemia etc. (Ulrych and Pivec 1997) and (ii) olivine-free to olivine-poor Ti-rich nephelinites (TiO2 max. 5.8 wt.%) in the Krusne hory Mts. (Shrbený 1980). ABSTRACT: Olivine-poor nephelinite of Vinařicka hora Hill substantially differs from both olivine nephelinite and olivine-free nephe- linite of the Cenozoic volcanic province of the Bohemian Massif. Vinařicka hora Hill represents a volcanic relict 31.0-25.5 Ma in age located in the Bohemian Cretaceous Basin in central Bohemian area. The very low Mg-values (46.6-49.7), low contents of compatible elements such as Cr (4.9-23.3), Ni (3-55), Co (39-43), Sc (17.0-18.6) (all data in µg g -1 ) and the lack of mantle-type xenoliths evi- dence differentiation of primary mantle magma. Anomalous enrichment in incompatible elements, particularly in ΣREE (523-589), Zr (601-842), U (2.1-3.1), Th (11.6-13.0), Nb (176-188), Ta (10.5-11.2), is associated with magmatic differentiation manifested in crystallization of apatite, Ti-magnetite ± hauyne(?). Olivine-poor nephelinite could be derived from carbonated nephelinite magma with high CO2/H2O + CO2 volatile fraction resulting in high viscosity and consequent stopping of magma associated with differentia- tion and contamination in a crustal reservoir during its ascent to the surface. Such magma could be associated with a highly explosive pyroclast-rich complex volcano of stratovolcanic type. The olivine-poor nephelinite of Vinařicka hora Hill belongs to a specific group of nephelinites of the Bohemian Massif characteristically developed in central Bohemia.