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| Titel |
Petrogenesis of Variscan lamproites of the Bohemian Massif |
| VerfasserIn |
Lukáš Krmíček, Rolf L. Romer, Johannes Glodny |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250104683
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| Publikation (Nr.) |
 EGU/EGU2015-4111.pdf |
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| Zusammenfassung |
| Paleozoic convergence of Laurussia and Gondwana-derived terranes with subduction of
oceanic and continental crust of various compositions metasomatized the local mantle, a
process which eventually led to a highly heterogeneous lithospheric mantle beneath the
European Variscides. The eastern termination of the European Variscides (Moldanubian and
Saxo-Thuringian zones of the Bohemian Massif in the Czech Republic, Austria, Germany,
and Poland) is unusual as within a small area, the mantle had been modified by
material from several subduction zones. Along the eastern border of the Bohemian
Massif, mantle-derived dyke intrusions of peralkaline, perpotassic, and ultrapotassic
compositions occur. The rocks are distinguished by mineral associations with K-amphibole
and Fe-microcline and correspond mineralogically to a new variety of silica-rich
lamproites. Lamproites from the Moldanubian Zone contain characteristic Ba-Ti-Zr
accessory minerals (e.g., baotite, benitoite, hollandite), whereas lamproites from the
Saxo-Thuringian Zone lack these minerals. Variscan lamproites from the Bohemian Massif
sampled lithospheric mantle, whose chemical signature reflects extreme depletion (low
CaO and Al2O3 contents) followed by strong metasomatic enrichment by material
released from the subducted rocks, giving rise to crust-like trace element pattern,
variably radiogenic Sr and unradiogenic Nd isotopic compositions, crustal Pb isotopic
compositions, and a wide range of δ7Li ratios ranging from markedly positive to
highly negative values. The metasomatic component is variably prominent in the
lamproites, depending on the extent of partial melting, and the nature of the source of the
metasomatic component. Preferential melting of the metasomatically enriched lithospheric
mantle with stable K-amphibole resulted in lamproitic melts with very negative
δ7Li values, which correlate positively with the peralkalinity, HFSE contents, and
lower ÉNd of these rocks. Both, higher degree of melting and consumption of the
metasomatic component reduce the chemical and isotopic impact of the metasomatic
contribution to the lamproites. The very positive δ7Li values of some lamproites
together with the depleted geochemical signature of the whole rocks indicate that
subducted altered oceanic crust also may have modified the source of these lamproites. |
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