Original paper

Melilitic rocks from northern Bohemia: geochemistry and mineralogy

Pivec, E; Ulrych, J; Höhndorf. A., ; Rutsek, J

Neues Jahrbuch für Mineralogie - Abhandlungen Band 173 Heft 2 (1998), p. 119 - 154

80 references

published: Jul 7, 1998

DOI: 10.1127/njma/173/1998/119

BibTeX file

ArtNo. ESP154017302001, Price: 29.00 €

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Abstract

Late Cretaceous to Early Tertiary (79-52 Ma) melilititic and melilite-bearing volcanic rocks from the Upper Ploucnice River Region (Bohemian Massif) are concentrated in the alkaline ultrabasic Osecmi Ring Complex. The lopolith-like intrusion, composed of olivine melilitolite with sporadic metasomatic pegmatoids, ijolites and phlogopitites is accompanied by numerous cone-sheets and dykes of micromelilitolite, polzenite and clinopyroxene polzenite, a NNE-SSW trending dyke system of the Devil's Wall formed by melilite-bearing olivine nephelinite and radial dykes of tephritebasanite composition. All rocks of the Osecmi Complex are strongly influenced by postmagmatic fluids enriched in Na, K, Ti, P, Rb, Cs, Sr, Ba, Y, REE, U, Th, Zr, Hf, Nb, Ta and volatiles (CO2, F, CI, H2O). The early high-temperature olivine + spinel + melilite + clinopyroxene association became unstable under hydrothermal conditions. Reaction with fluids resulted in the crystallization of a late-magmatic association represented by monticellite, phlogopite, carbonate, Zr-bearing melanite, Al- or Na-rich diopside, wollastonite, perovskite, calzirtite, titanian magnetite and nepheline. (F, OH)-bearing titanian andradite, (Ba, Ti)-rich phlogopite, pectolite, sodalite, hauyne, fluorapatite, fluorite and zeolites were formed under post-magmatic conditions. Both primary magmatic and hydrothermal (calcites and dolomites), as well as secondary carbonates are common. The mineral assemblage and the isotopic composition of these carbonates (δ13 C PDB = -2 to -9% and δ18O SMOW = +14 to + 23%) demonstrate that whereas the rocks under investigation may be genetically related to carbonated ultramafic magmas, they have clearly interacted with meteoric fluids. The parental magma of the Osecmi Complex is inferred to have originated by low-degree partial melting (about 1%) of metasomatized mantle peridotite and/or in association with a zone refining process.

Keywords

Northern Bohemia • Ohfe Rift • Osecna Ring Complex • melilite-bearing rocks • rock-forming minerals • Nd and Sr isotopes • geochronology