Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Newly drilling-detected phonolite intrusions in Lužické hory Mts.
Published online: 18 December 2017Abstract
The core drilling within the framework of the Reassessment of the ground-water resources project has discovered two new bodies of differentiated alkaline rocks in the area of Lužické hory Mts. (Lausitz Mts.), belonging to the Lausitz Volcanic Field. In this contribution, we provide petrological constraints on two rocks discovered by boreholes 4650_X and 4640_C, classified as phonolites, as well as their intrusive position in the stratigraphic sequence of Upper Cretaceous marine sediments. Whereas the phonolite penetrated by borehole 4650_X was emplaced into the lowermost part of the Březno Formation, the phonolite reached by the borehole 4640_C terminated its ascent much lower, at the level of the Jizera Formation. The phonolite from the 4650_X drill core has trachytic texture with preferentially oriented phenocrysts of sanidine (1 mm) enclosed in fine-grained matrix of feldspar, sanidine and amphibole. On the other hand, phonolite from the 4640_C drill core is generally finer-grained, slightly porphyritic. Scarce sanidine (up to 1 mm) and amphibole (up to 0.5 mm) phenocrysts are enclosed in very fine-grained matrix. Compared to the phonolites exposed on the surface, both rocks are depleted in alkalis, namely in sodium which was probably leached out as the rocks were permanently buried by water saturated sedimentary deposits. The concentrations of less-mobile trace elements (Zr, Ti, Nb and Y) point towards phonolitic composition, as well as the pronounced U-shaped chondrite-normalized REE pattern. Both newly discovered phonolites differ in concentrations of less-mobile trace elements (namely higher Sr, lower Cr and Rb, and Nb in one case higher in other case lower) from nearest outcropping rocks, suggesting the drilled bodies represent independent intrusions at different levels. The investigation has proved, the trachytic and phonolitic intrusions were emplaced at various stratigraphic levels of the Cretaceous sequence, without any preferred stratigraphic position.
