Original paper

Ijolitic segregations in melilite nephelinite of Podhorni vrch volcano, Western Bohemia

Ulrych, J.; Pivec, E.; Lang, M.; Lloyd, F. E.

Neues Jahrbuch für Mineralogie - Abhandlungen Band 175 Heft 3 (2000), p. 317 - 348

49 references

published: Sep 1, 2000

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ArtNo. ESP154017503004, Price: 29.00 €

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Abstract

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.

Keywords

ijolitesegregationsmelilite nepheliniteCenozoic volcanoW Bohemia