Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 72. (Budapest 1980)

Örkényi-Bondor, L.: Andesite agglomerate from Zebegény village, Börzsöny Mountains (Hungary)

ANNALES HISTORICO-NATURALES MUSEI NATIONALIS HUNGARICI Tomus 72. Budapesté 1980. Andésite Agglomerate from Zebegény Village, Börzsöny Mountains (Hungary) By L. ÖRKÉNYI-BONDOR, Budapest Abstract — Andésite bombs are described from outcrops of Miocene andésite agglomerate at Zebegény village. The An-content of plagioclase phenocrysts ranges between 75-93% depending on zoning. The Albite-Carlsbad-Roc Tourné twin laws predominate, but the Börzsöny, Visegrád and Bánát (Baveno) types combined make up more than 10%. Most of the amphiboles are basaltic hornblendes. Pyroxene occurs in every sample collected at the Kerekdomb outcrop. The studied rocks are characterized by uniformly high A1

2

0

3 content and by a narrow variation range of Si0

2 content. The iron content is lower, the alkali content higher than in average andésites

. With 4 figures and 2 photoplates. Introduction — The mineralogical-petrographical examination was carried out by using a Leitz polarisation microscope and a five-axial U-stage. In the chemical laboratory of the Mineralogical-Petrographical Department of the Hungarian Museum of Natural History 17 silicate analyses have been made by DR. GY. PITTER. 19 minor and trace elements were determined by emission spectrography in the Hungarian Geological Institute, for 12 samples. X-ray tests and photomicrographs are the courtesy of the Mineralogical Department of L. Eötvös University. Description of the rock samples The Miocene andésite agglomerates of the Southern Börzsöny Mountains are well exposed in natural outcrops at Zebegény village. Out of the 28 samples two originate from older lava flows. The other 26 display crystallinity degrees indicating subvolcanic andésites. The samples have been collected in two outcrops: I. Western slope of Kerekhegy (hegy =«= hill) II. „Vizesárok" valley at Zebegény. The two groups of samples differ considerably. I I. Western slope of Kerekhegy hill Andésite bombs and lapillis embedded in tufaceous cement can be collected. The volcanic bombs range from 2 to 50 cm in diameter; they are of different colour and hardness. The studied 28 samples may be divided into 11 types. Type 1. — One single sample, sample 1 /71, rather massive, light-grey coloured, represents this type. The groundmass is brown-spotted by limonite and other opaque substances. The plagioclase phenocrysts vary largely in size. The zoned plagioclases contain numerous inclusions and much of volcanic glass (brown-coloured). The average An-content deduced from measurements by the U-stage is about 75-78 %. Albite-Carlsbad-Roc Tourné twinning is common, Börzsöny, and Visegrád twins occur but rarely. Further porphyric crystals are amphiboles and pyroxenes. The amphibole crystals (0.1-20 mm in length) have almost completely been altered, and replaced by magnetite, pyroxene and plagioclase. In the groundmass tridimite crystals of characteristic twinning are common, accompanied by limonite. Type 2. — Reddish-grey, rather porous bombs of 10-15 cm diameter belong to this type. (Samples 2/71, 7/71,8/71 and 1/75). There are some differences as regards the groundmass and secondary mineral formation. The texture is microholocrystalline-porphyric. At some places the microlites are very tiny. Most of the plagioclase phenocrystas are polysynthetically twinned according to the Albite-Carlsbad-Roc Tourné laws; the Bánát type of Baveno, Börzsöny and Visegrád twins occur up to 10%. The An-content varies in function of zoning. The lowest An-content measured was 73%. The samples contain scarce amphibole crystals. These are relatively fresh, with only narrow opaque corrosion rims. Ann. Hist.-nat. Mus. Nat. Hung., 72, 1980 3 Természettudományi Múzeum Évkönyve 1980