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

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

ANNALES HISTORICO-NATURALES MUSEI NATIONALIS HUNGARICI Tomus 52. PARS MINERALOGICA ET PALAEONTOLOGICA 1960. Petrographical study of the biotitic pyroxene andésite of Gyöngyöstarján By Cs. RAVASZ, Budapest The Mátra Mountains are members of the Tertiary so-called inner Carpathian volcano belt. The striated, biotitic pyroxene andésite of transvaporisatory origin, mentioned in the title, occurs in the southwestern part of the stratovolcanic mountain group, in a superficial extension of about 4 sq. kilometres. By its composition and other properties, this rock differs significantly from the rest of the andésites constituting the Mátra Mountains. The striatum and biotite content is due to transvaporisatory effects. Out of the magma enriched in volatiles, striate of differing mineralogical composition were formed by rhythmic precipitation. Subsequent to solidification, the rock was partly broken up by tectonical processes and later also metavolcanitized ; the joint result of these processes was the formation of oxy- and hydroandesites, pseudotuffs, and a number of varieties of chalcedony and jasper. In the autumn of 19S8 the author has, in co-ordination with the work of the State Geological Survey, prepared a detailed geological map of an area of about 12 sq. kilometres north and west of the village of Gyöngyöstarján (Heves County, North Eastern Hungary) and carried out measurements and collected samples in the natural disclosures of the mentioned area. The striated biotitic pyroxene andésite is situated in the southern part of the area. Its striation emanates from a northern center and spreads in a half circle in a more or less radial pattern. The endometavolcanitic products occur in the striated andésite and on the northern rim of the same, in a relatively small superficial extension. West, east and north of the striated andésite, there occurs dark grey pyroxene andésite, on an identical and partly on a more elevated level. Up to now, no detailed geological description and petrographical investigation of the area was made. The greatest bulk of data is contained in a petrographical description byB. Mauritz (1). A mention of the biotitic facies and of the banded jasper was made by Gy. V i g h (2). The area was dealt with in general by J. N o s z k y sen. (3). The greatest help in attacking the problems presented by the area was Academician Szádeczky-Kardoss's paper (4) on the new system of altered rocks and on the laws governing the interaction of magma and water. In surveying work, too, the present author has applied the methods outlined by SzádeczkyKardoss. The detailed residts of the microscopical study of the diverse kinds of andésite will be described as follows. Biotitic pyroxene andésite This is the most conspicuous rock of the Gyöngyöstarján environment. It is generally of a greyish brown, violent-brown colour, porous, raw of surface. It exhibits a light-to-dark striation. The light striae are greyish, the dark ones of a reddish-brown tone. The striation of the weathered rock is different, blurred, the colour of the rock being a more general reddish to violet brown. The rock exhibits to the naked eye a content of fresh and altered feldspars of 2—3 millimetre size, biotite flakes of 1—2 millimetre diameter and hexagonal cross section, columnar and platy pyroxenes and also tridvmite crystals in cavity linings.