Geologie, petrologie a stari pohrbenych vulkanickych teles v prostoru Dolu Nastup Tusimice

The area between the České středohoří Mts. and Doupovské hory Mts. within the Eger (Ohře) Rift was filled with Miocene sediments of the Most Basin (cf. Rajchl et al. 2009). Tens to hundreds of metres thick sediments of this basin buried numerous local volcanic bodies. Nevertheless, several of them were found and documented below the Miocene sedimentary sequence due to exploratory drilling and extraction of the coal seam. The present study provides data on three volcanic bodies occurring in the northern part of the Tušimice mining area owned by the North Bohemian Mines Co. The investigated volcanic bodies consist of one maar-diatreme volcano associated with a lava flow (Prunéřov diatreme and Prunéřov lava -see Fig. 1), two smaller possible conduits connected with another lava flow (Merkur lava), and a larger lava flow without documented vent (Spořice lava in Figs 1 and 2). These volcanic bodies are arranged in E-W direction, which may follow a fault zone. The Spořice lava was sampled for further petrological and geochronological investigations. The rock is classified as analcimized olivine leucitite with minor nepheline (Fig. 4). The alteration of the rock is demonstrated by decomposed olivine and by analcimization of leucite. Steep chondrite-normalized REE-pattern (Fig. 4c, La_N/Yb_N = 24.31) indicates low degree of partial melting in mantle source. Negative K and Rb anomalies in the primitive mantle-normalized pattern (Fig. 4d) are interpreted in terms of leucite analcimization and mobilization of liberated K and Rb ions. The alteration of the rock is also implied from the positive Pb anomaly. The study of weathering profile starting with the relatively fresh rock up to the argillized rims of the examined volcanic body revealed a significant decrease in Ba and Sr, and slight decrease in Zr (Fig. 5, Table 2). On the other hand, the Nb contents remain unaffected by the weathering processes. Based on the K-Ar geochronological data (30.57 ± 3.23 Ma) it may be concluded that the lava was emplaced in Early Oligocene (at any time in 33.8-30.6 Ma range).

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