Devil’s Wall near Suletice (“Hibsch’s monchiquite”) ?- a dyke of Cenozoic alkaline lamprophyre in the České Středohoří Mts.


Simona Krmíčková, Lukáš Krmíček

Geoscience Research Reports 53, 2020, pages 35–45

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Published online: 25 May 2020

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A spectacular NE-SW-trending dyke of Cenozoic alkaline lamprofyre is located near Suletice in the České Středohoří Volcanic Complex (CSVC) and is spatially associated with a large magmatic centre of the Roztoky Intrusive Complex (Fig. 1). The dyke was exposed by denudation from surrounding less resistant basaltic rocks into the striking shape of the rock wall with distinct horizontal columnar jointing, which is ca. 1 m thick, 10 m long and 5 m high. A supposed continuation of the dyke was tracked SW from the rock wall in the distance of ca. 400 m (Figs 2 and 3). Due to its similarity with famous olivine nephelinite to melilitite dyke of the Great Devil’s Wall near Osečná in Northern Bohemia, it is called simply “Devil’s Wall” or “Hibsch’s monchiquite” according to J. E. Hibsch, the geologist who first documented the dyke. The mineral composition of the alkaline lamprophyre includes large phenocrysts of clinopyroxene, minor Ti-rich amphibole and accessory minerals such as magnetite and apatite, which are present in the form of inclusions in clinopyroxene. The fine-grained groundmass is formed by thin plagioclase laths and/or by basaltic volcanic glass. The proportion of plagioclase and glass in the groundmass varies within the intrusive body. Plagioclase-dominated groundmass is typical rather for the samples from the core represented by the rock wall, whereas hyaline groundmass of the rock forming the dyke continuation is exclusively composed of glass. Vesicular fillings or tiny veinlets of zeolite mineralisation also occur within the groundmass (Fig. 4). Alkaline lamprophyre of the Devil’s Wall is ultrabasic (SiO2 content ˜ 40 wt. %) and is enriched in TiO2, Al2O3, Fe2O3tot and K2O. In contrast, it is relatively poor in MgO with Mg-number varying from 44 to 58 (Table 3). The investigated samples display positive anomalies of Ti and Ta along with negative anomaly of Pb and show an enrichment of LREE over HREE (Fig. 5). Initial Sr-Nd isotope composition of the Devil’s Wall dyke yielded the least radiogenic 87Sr/86Sr ratios from the CSVC alkaline lamprophyres along with relatively high 143Nd/144Nd values (Fig. 6). Observed isotope signature in combination with the LILE/HFSE ratios similar to those of MORB indicates a more substantial contribution of the asthenospheric mantle component to the metasomatised lithospheric mantle source of the Devil’s Wall alkaline lamprophyre. Regarding the variable mineralogical and geochemical features of the studied dyke, the composition of the alkaline lamprophyre varies from camptonite to monchiquite, resembling a special variety of alkaline lamprophyre called “wesselite”. Considering the fact that an occurrence of such a spectacular rock wall formed by alkaline lamprophyre has not yet been documented in the Bohemian Massif and, moreover, any “wesselite”-like dyke has not yet been described in the CSVC, it makes the dyke of Devil’s Wall near Suletice a unique geoheritage not only in the České Středohoří Mts., but in the whole Bohemian Massif.