Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 66. (Budapest 1974)

Örkényi-Bondor, L.: The plagioclase crystals of the andesite agglomerate from the Csattogó Hill in Börzsöny Mountains (Hungary)

ANNALES HISTORICO-NAÏURALES MUSEI NATIONALIS HUNGARICI Turnus 66. Budapest 1974. The Plagioclase Crystals of the Andésite Agglomerate from the Csattogó Hill in Börzsöny Mountains (Hungary) By L. ÖRKÉNYI — BOND OR, Budapest Abstract — Description of the plagioclase phenocrysts of the Csattogó hill from the pyroclastite of the South-Börzsöny Mountains. Avarage anortite content in the andésite bomb and lapilli types is 66 percent. Beside the predominating Albite, Carlsbad and Roc-Tourne twinning there are 2 — 3 percent twinning according to the Börzsöny and Visegrád twin laws. — 9 figures. The work reported in this paper forms part of a continuous investigation of the andésite agglomerate in the Börzsöny Mountains. After a series of papers on the various localities the final genetic conclusions will be given. The Börzsöny Mountains is the most western part of the Intra-Carpathian volcanic arc in Hungary. The South-Börzsöny Mts. lie south of Márianosztra, Kóspallag, Szokolya and Szendehely villages reaching down to the Danube. While the central part of the Börzsöny Mts. has been included in to the national research programme as a possible field fc r ores, on the other hand, the South-Börzsöny Mts. have not much interest to offer the geologists, accordingly respective literature is only scanty on this southern field. STÄCHE (1866) was the first to study the andésite of this area, but he divided andésite types on the basis of colour. SZABÓ (1872 — 1894) published several works about the Börzsöny Mts. and divided five trachyte types on the basis of plagioclase, biotite, amphibole and pyroxene occurence. He assumed that these andésite types are the results of five different eruptions. He supposed these eruptions to take place from the Upper Eocene to the end of the Sarmatian. His last papers discussed the mixing of the andésite types. For his hypothesis the resorbed biotite, hypersthene and amphibole cyrstals served as bases. He assumed that the earlier andésite melt again. A survey paper on the geology of the South-Börzsöny Mts. was prepared by BÖCKH (1899 — 1902) with a geological map still being the best geological review of this area available. For want of deep-boring the investigation depends on the material of the quarries and natural exposures. Andésite lava flow is exploited in two quarries, but the pyroclastics are exposed in some old trial quarries. The pyroclastic specimens of the present work derive from these old quarries, from couloir and barren hillsides. This fact makes it even more difficult to interpret the alternation of the different volcanic rocks. In many cases the andésite lava flow agglomerate and tuff alternate within a short distance. Owing to lack of fresh rocks investigations on tuff and cement of the agglomerate would not have repaid X-ray, DTA, chemical and misroscopic investigation. We had to be content with the study of volcanic bombs and lapillis. In spite of these difficulties valuable results from the optical research of the plagioclase and dark silicate have been expected. The optical investigation began six years ago and it has yielded evidence to the interpretation of the genesis of this volcanic area. This research helped to recognize several new plagioclase twin laws