Ijolitic segregations in melilite nephelinite of Podhorni vrch volcano, Western Bohemia
Ulrych, J.; Pivec, E.; Lang, M.; Lloyd, F. E.
published: Sep 1, 2000
ArtNo. ESP154017503004, Price: 29.00 €
The Miocene Podhorní vrch volcano (15-12 Ma), situated near Mariánské Lázně in western Bohemia, is associated with a tectonic zone striking NNW - SSE and pertains to the Neogene Cheb-Domažlice Graben. The volcanic products represent undifferentiated melts of mantle origin of olivine nephelinite composition: Mg # (70-72); Cr (410-570 ppm), Ni (240-380), Co (50-55), Sc (24-29); the presence of harzburgite xenoliths. 87Sr/86Sr (0.7035) and 143Nd/l44Nd (0.51286) isotope ratios (Wilson et al. 1994) correspond to HIMU OIB from a sublithospheric source. Enhanced contents of incompatible elements such as U, Th, Nb, Ta, (REE) with high (La/Yb)N are characteristic. Coarse- to medium- and fine-grained segregations of ijolite to turjaite composition in the parental olivine nephelinite show the following mineral paragenesis: nepheline + diopside (with fassaite-aegirine rims) + melilite/(leucite, sanidine), titanian magnetite, hydroxylapatite, olivine, sodalite. The pegmatoid segregations are characterised by low contents of Si, Al and K and higher concentrations of Na and Ca in comparison with those from classic localities (Meiches and Löbauer Berge - Germany). This difference in chemistry is reflected in the crystallisation of nepheline + melilite instead of, more commonly, nepheline + leucite and sanidine. Origin of the pegmatoid segregations is associated with post-eruptive low-pressure fractional crystallisation, which resulted in the formation of segregations of restricted volume. However, independently (?) sourced late-magmatic fluids may have introduced Na and selected trace (REE, U, Th, Nb, Ta, P) and volatile elements (F, Cl, SO3) that contributed to pegmatoid crystallisation.