Senckenberg Forschung

Basalt 2013
field TRip 1 (24th april  2013):

Osecna and the Ploučnice area – Ultramafic pre-rift volcanism in the Eger-Graben, the typus localities of Polzenite

Guided by: Antonin Paluska, Jaromír Ulrych, Pavel Veselý, Vladislav Rapprich, Jiří Adamovič

In the northern part of the Bohemian Massif occur Late Cretaceous to Palaeocene aged (68 to 59 Ma) ultramafic melilitic and melilite-bearing rocks concentrated in the Devil’s Dyke swarm and associated Osečná Complex. The NNE–SSW trending Devil’s Dyke swarm consists predominantly of melilite-bearing olivine nephelinite to olivine melilitite. All primitive melilitic rocks of this area have a primary olivine + melilite + spinel  nepheline, clinopyroxene association. The dykes are intruded into Upper Cretaceous sandstones and due to erosion they were sculptured as more or less dominant ridges. The most imposing of these dyke ridges is the “Great Devil’s Wall” which is protected since 1964. During the excursion the complex internal structures can be observed in the up to 9 m high wall. These melilitic suites occur in the outer parts of the rift zone (Eger Graben) of the Bohemian Massif. Here in the Ploučnice River area (Polzengebiet) K.H. Scheumann defined 1913 the group of olivine- and melilite-bearing rocks including no clinopyroxene as polzenites. The field trip leads to the type locality of vesecite near of Vesec in the north-eastern part of the polzenite-complex. Finally the drill cores and additional documentation of the melilitolite intrusion of the Osečná Complex will be presented. The Osečná Complex is a laccolite-like subvolcanic intrusion (sill), composed mainly of olivine melilitolite with rare pegmatoids, ijolites and glimmerites, with numerous apophyses of olivine micromelilitolite composition, accompanied by numerous dykes of polzenites (Type Vesec, Modlibohov and Luhov) of lamprophyre character.