Boros István (szerk.): A Magyar Természettudományi Múzeum évkönyve 52. (Budapest 1960)

Ravasz, Cs.: Petrographical study of the biotitic pyroxene andesite of Gyöngyöstarján

basic and acid media. The light striae are rich in tridymite (15 per cent), with a proportion of 35 per cent of the feldspar laths, while in the darker striae magnetite and biotite are more abundant, feldspar microliths (25 per cent) and tridymite (5 per cent) being scarcer. The striation of the rock was subsequently emphasized by metavolcanitic alteration. Metavolcanitic opacitization rendered the dark striae a reddish brown, while the light striae of smaller biotite and magnetite content have become even paler. On the other hand, the dark ingredients and feldpars were, in the lack of an opacitic coating, altered more thoroughly in the light striae. The transvaporisatory origin of the striation is also corroborated by the circumstance that in the parts of the rock situated more deeply beneath the surface and farther off the assumed sedimentary contact the mentioned mineralogical features and the effects of subsequent alteration are seen to grow weaker. Composition and external appearance of the rock become gradually similar to those of dark hypersthene-augite andésite. This transitory member of striated andésite is characterized by a decreasing content of biotite, by an enrichment of microporphyric feldspars and dark silicate grains (60 to 100 fi) as well as by the appearance of two generations of different grain size of feldspar microliths (5 to 10 and 40 to 50 /i) respectively. Striated biotitic andésite may be traced from the southern side of Gyöngyöstarján village ; it occurs north of the road connecting Gyöngyöspata with the state highway, on Hosszú Hill and Gereg Hill. Bólyatető is the western limit of its extension. It occurs in a contiguous, large, well-disclosed mass on Mulató Hill and especially on the northern peak of the same. Its eastern limit seems to be Lógi Creek. The author was unable to find massive biotitic andésite east of the same. Considering the circumstances at Más Creek, it is improbable that biotitic andésite should occur north of the pseudotuff disclosures in the Csege Creek valley. A thorough survey of the area has yielded no occurrence of biotitic andésite there. The rock type of smaller biotite content occurs at Cserepes Farm, in the Gereg quarry, and in the Mérőkő outcrop. Dark grey hypersthene-augite andésite Macroscopically, this is a medium to dark grey, locally black, massive rock. It exhibits to the naked eye feldspars of 2—3 and pyroxenes of 1—2 millimetres length. In a weathered state, its colour is a quite light grey ; it is limonite-speckled, chloritic. On the surface, it occurs in thin-banked masses. The mean mineralogical composition of the rock, as determined by analyzing eight slides, is as follows : porphyric feldspar, 30,3 per cent, augite and hypersthene, 11,2 per cent, matrix, 58,5 per cent. The average grain size distribution curve of dark pyroxene andésite differs from that of biotitic pyroxene andésite only by a slight shift of the peaks. In the case of the first peak, this is due to the presence of two generations of different size of feldspar microliths. In the case of the second peak it is seen that dark pyroxene andésite contains bigger and more frequent porphyric ingredients than biotitic andésite. The detailed description of the rock-forming minerals, as based upon measurements and observations on 15 slides, is given as follows. As regards their size, the porphyric feldspars belong to the 120—180, 400 and 600 /j, groups. Their extreme sizes are 2000 to 2200 fi. The average constitu-