Nalez kenozoicke bazanitove zily u Dvora Kralove nad Labem

In the area of Dvůr Králové nad Labem, only compositionally exotic rocks (ouachite, melilitic rocks), similar to rock association of the Osečná Intrusive Complex, were described in the past. The age at Cretaceous/Paleogene boundary was also suggested for the Dvůr Králové Complex based on petrological similarities with the Osečná Complex. The occurrence of common Cenozoic alkaline rocks (basanites, nephelinites), rather common in other regions of the northern part of the Bohemian Massif, was never before reported from the surroundings of Dvůr Králové nad Labem. Recent construction works on the eastern outskirts of Dvůr Králové nad Labem (Fig. 1) have exposed a 35 cm thick dyke of basanite, which also explains the presence of morphological ridge in soft Turonian marlstones. The documented dyke, running in approximately east-west direction, shows also a significant magnetic pattern exceeding the surrounding magnetic field by some 850 nT (Fig. 6). The dyke consists of two, texturally distinct facies. The darker facies represents a chilled first portion of the magma containing abundant xenoliths of country-rock sediments. The darker colour is due to oxidized groundmass, originally probably glassy. The lighter-coloured facies with better crystallized groundmass corresponds to the second phase, when a new portion of magma healed fractures in sharply cooled first batch. Both facies contain abundant phenocrysts of altered olivine (Figs 7 and 8). Pseudomorphs after amphibole were also scarcely identified. The age of the observed dyke has not been determined, as the altered and heterogeneous character of the rock with numerous xenoliths prevents geochronological determination. On the other hand, the nearby volcanic rocks of the Jičín Volcanic Field yield Miocene ages of around 17 Ma, and this age can also be presumably assumed for the new dyke at Dvůr Králové nad Labem. Defining the geometry and position of the volcanic dyke, which forms an important hydrogeological barrier, plays a crucial role in numerical modelling of subsurface-water dynamics.

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