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

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

nized, but the possibility of drawing conclusions is rather restricted due to the agglomerate nature of the rock. The opaque minerals are mostly limonite. Type 9. — Sample 1/75 represents this type. This is a slate-grey, massive rock of microcrystalline porphyric texture. The groundmass consists of tiny plagioclase laths and pyroxene crystals smaller than 20 micron. Fluidal texture is common. The phenocrysts consist of twinned and untwinned plagioclase crystals and pyroxenes, the latter occurring mostly in patches. The amphiboles are completely altered, only the magnetite "dust" recalls the original shape of the amphibole crystals. The orthorhombic pyroxene grains are strongly pleochroic in green and apricot colours. They may be identified as hypersthenes. One part of the monoclinic pyroxene crystals is augite; a smaller portion may be determined as Mg-rich clinohypersthene. Most of the opaque mineral constituents are limonite spots. Several hexagonal cross-shaped quartz crystals occur, with numerous inclusions of apatite. Type 10. — Most of the volcanic bombs belong to this group: samples 2/75, 3/75, 5/75, 8/75,. 12/75 and 13/75. They are light grey or reddish-grey coloured, porous rocks containing dark chain silicates of about 1 mm in length. Phenocrysts of varying size are embedded in microholocrystalline to cryptocrystalline groundmass. The porphyric plagioclases are mostly isometric, finely zoned. Untwinned crystals or those with only 1-2 narrow twin members are common. Some orientation is detectable as for the feldspars. Inclusions of glass are common. The An-content varies largely with zoning. The Albite- CarlsbadRoc Tourné twin laws predominate, the Bánát type of Baveno, Börzsöny and Visegrád twins are rare. All samples contain pyroxene phenocrysts in the range of 3 to 4.5%. Amphibole is rare, mostly altered, only the dust of magnetite and other pseudomorphic minerals are visible. (E. g. in the case of sample 4/75). In the same rock haematite and serpentine have arisen secondarily. In sample 5/75 there are numerous haematite pseudomorphs after amphibole, and the amount of cristobalite is remarkable. Amphibole generally does not form twin crystals, with the exception of sample 8/75, where (100) and (210) twin laws occur. This rock is of semiholocrystalline vitrophyric texture. In sample 9/75 the amphiboles and pyroxenes are strongly altered. The sample 12/75 differs from the others by its pyroxene and amphibole crystals being fresh. Moreover, the pyroxenes are richer in iron than those of the other rocks. Type 11. — Sample 14/75 is the single representative of this geochemically highly interesting type. It is an andésite of brownish-red colour and trachytic texture; the microlites lie among flowlines. Most phenocrysts are plagioclase, but pyroxene also occurs. Shape, twin laws and An-content of the plagioclase crystals are similar to those of the other volcanic bombs. Originally, the rock had contained a lot of amphibole phenocrysts, but those became altered, and only the decomposition products mark their previous location and shape. There are many pyroxene crystals in the groundmass. These as well as the phenocrysts may be identified as Mg-rich orthopyroxene and clinopyroxene. Among the pyroxenes of the groundmass a few augite grains occur, too. They seem to have crystallized subsequently to the hypersthene. (enstatite and clinoenstatite). Numerous opaque microlites are to be found in the groundmass; these are supposed to be responsible for the minor element content, which is higher than usual. II. „Vizesárok" valley In the Vizesárok valley, rocks could be collected from small outcrops only, due to rather bad exposure. These samples differ remarkably from those of Kerekhegy. Unfortunately, the geological relationship between the two localities is unknown, the contact is not exposed. Sample 1/69 is a silicified andésite. The original texture can not be determined; nor can the plagioclase crystals optically measured by means of U-stage. On the contrary, alteration of the earlier crystallized amphiboles is not common. The second-generation amphiboles are green coloured, fresh crystals without domes or sphenoids, approximately 0.1 mm long. There are also some biotite crystals with rutile inclusions. Beside the sagenite nets, apatite needles are common in the biotites. The latter occur also in plagioclase grains. The ends of the orthorhombic pyroxenes have usually been altered to serpentine. Sample 3/69 is of vitrophyric texture. The phenocrysts embedded in the groundmass are tiny. The mass consist of green and brown amphibole, some feldspar, and biotite. The phenocrystas are smaller than those of the other rocks. This sample most likely is a cemented crystal tuff. Sample 4/69 is of trachytic texture. The plagioclase laths are very small, almost microcrystals. This rock includes fresh amphiboles, strongly altered plagioclases, orthorhombic pyroxenes, clinoenstatite and augite. The orthorhombic pyroxene is often intergrown with augite. Sample 6/69 is of microholocrystalline porphyric texture. Zoned, polysynthetically twinned plagioclase crystals with many inclusions, and fresh amphibole constitute the phenocrysts. Haematite dust is responsible for the red colour of the rock. This is the only sample in which a potash feldspar has been identified, one single crystal, 2 mm in diameter.