Szekessy Vilmos (szerk.): A Magyar Természettudományi Múzeum évkönyve 57. (Budapest 1965)

Csalogovits, I. J.: Biotite dacite from bore log 20 at Esztergom

ANNALES HISTORICO-NATU RALES MUSEI NATIONALIS HUNGARICI Tomus 57. PARS MINERALOGICA ET PALAEONTOLOGICA 1965. Biotite Dacite from Bore Log 20 at Esztergom By I.J. CSALOGOVITS, Budapest Summary: At prospective boring E-20, biotite dacite was struck in the tract between 566.0 and 600.0 metres. This isolated rock, probably of laccolithic appearance, is similar in type to the biotite dacite described in the surroundings of Pilisszentkereszt. According to T. ZELENKA, these erupted at the Helvetian substage and represent the first eruptions of tertiary volcanism that was considered so far as being of Lower Tortonian origin. Laccolith demonstrated here proves that the described initial volcanic phase was not limited to the place mentioned. Prospective boring E-20, established near the Esztergom—Tát highway 500 metres from the Danube at the level of Körtvélyes holm, struck volcanic rock within the Oligocène series in the tract between 566.0 and 600.0 metres: preliminary descriptions defined this rock as biotite andésite. Macroscopically the rock was of light, whitish-grey matrix and coarse surface. Small feldspar slabs and larger biotite flakes (1—2 mm) can be found in it. Garnet is noted occasionally but is so rare that none of the six thin sections contained even a grain of it. It is visible macroscopically, too, that the biotites of porphyric facies have a longer diameter as they approach the border of the igneous body. Vulcanite touches the Oligocène country rock along the original contact line, no signs of tectonic effects are seen: this is proved by the presence of materials hardened and coloured red under the effect of heat, but is also readable from the well-logging curves (Figure 1). This 34.0-metre thick volcanic formation is in all probability in discordance with the Oligocène series of about 20° inclination, thus it must be by chance only that boring struck at it at the boundary of the Chattian and Rupelian-Lattorfian stages. The form of deposition cannot be determined with absolute certainty since no magmatic formation occurred in the borings in this basin, and the position cannot be ascertained on the basis of one boring. Microscopic investigations Mineral composition; 1) Quartz; porphyric crystals with round edges of about 60 u, magnitude, with small (1 p,) inclusions of zircon, apatite (?) are found in it. It occurs also among the 10 p. granules of the matrix. 2) Plagioclase; This occurs in three generations. Generation I; magnitude about 200—240 u. of the andesinic 4 Természettudományi Múzeumi Évk. 1965.