Boros István (szerk.): A Magyar Természettudományi Múzeum évkönyve 52. (Budapest 1960)
Ravasz, Cs.: Petrographical study of the biotitic pyroxene andesite of Gyöngyöstarján
In the hydroandesite contacting endoxyandesite, one third of the feldspars is still fresh ; the rest has turned into clay minerals. A single still determinable grain indicates the dark ingredients, of relatively small abundance, to have been hypersthenes. The magnetite inclusions of the not too thoroughly decomposed hypersthenes are still fresh, whereas in the matrix the one-time presence of magnetite is indicated by no more than some limonite specks. The porphyric ingredients are surrounded by broad wreaths of limonite. The joints corresponding to the main joint orientations are filled by jasper and opal. The hydroandesite zone, its rocks decomposed to a clayey stage, is accompanied throughout bv occurences of opal, milky or light blue chalcedony and flesh-red to ochre-coloured jasper. Precipitation of the liberated silica has turned the rock of a small height west of Lógi Creek into banded jasper, the banding consisting in an alternation of mainly red and yellow, less frequently almost black dark and light green as well as white layers. The orientation of the banding is identical with that of the original rock, nearly horizontal. On the top and on the southern and eastern flanks of the height there occur huge blocks of red jasper. Abundant occurrences of jasper are found in the valley of Lógi Creek and in the southwest, in joints on Mulató Hill. On the hill flank between Más Creek and Bolya Farm, chalcedony is most frequent in a hydroandesite which is in a more elevated situation than the previouslv described ones. The mentioned occurrences suggest the existence of a silicified belt of E —W or SW — NE strike. Most of secondary silica occur in and around the metavolcanitic formations. Furthermore, veins of jasper and opal are frequent in the area of dark pyroxene andésite as well. I consider that these are likewise of metavolcanitic origin, and am of the opinion that they were formed by the precipitation of the silica leached out of the kaolinitized andésite situated north of our area, in the Világos Hill and Bánya Hill environment. A further outcrop of hydroandesite lies at the entrance of Más Creek into Gyöngyöstarján village, on the west end of the latter. This sickle-shaped valley section of the creek is of tectonic origin, delineating the position of the ancient solutionbearing fissure. Further good disclosures of hydroandesite are found near the south-western wall of the church and in the deeper caves of the wine cellars of the State Wine Enterprise. On the walls of the latter, the dip of the banking is 4 h /12°. This disclosure is about 12 metres below surface, about 200 metres above sea level. The horizontal extension of this thick hydroandesite mass cannot be traced continuously. On the surface, there is no disclosure in the direction of Lógi Creek, so that, according to my observations, the altered andésite forms no uniform layer, which would bisect the biotitic andésite body in an almost horizontal plane, but is rather due to alteration along a tectonically pre-formed vertical fissure. The gently dipping lava banks were altered to varying dephts along their contact with the mentioned fissure, in at least two levels situated one above the other. The relative age of biotitic pyroxene andésite and dark hypersthene-augite andésite cannot be established with certainty in the present state of disclosure. Considering the structural relationships of the Mátra Mountains, as well as the observations made in the excellent disclosures of the Gyöngyösoroszi ore mine, close to our area, and also the situation at the symmetrically built Kis Hill at Gyöngyössolymos, E. Szádeczky-Kardoss and A. V i d a c s have drawn the conclusion that biotitic andésite is younger, having broken through dark hypersthene-augite andésite. However, the emplacement observed in the Gereg quarry (unfortunately no contiguous outcrop, having a hiatus of about
