Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 72. (Budapest 1980)

Noske-Fazekas, G.: Statistical investigations of plagioclase twin laws in lavas erupted between the Danube and the Central Börzsöny Mountains (Hungary)

ANNALES HISTORICO-NATURALES MUSEI NATIONALIS HUNGARICI Tomus 72. Budapest 1980. Statistical Investigations of Plagioclase Twin Laws in Lavas Erupted between the Danube and the Central Börzsöny Mountains (Hungary) by G. NOSKE-FAZEKAS, Budapest Abstract — Statistical studies of plagioclase twin laws have shown that the frequency of single twin laws is interrelated with rock chemistry. The higher the An content of the plagioclases in the rock, the greater the value of Gorai's C/A twin ratio. With 3 figures and 4 tables. Since 1970 the author has been measuring plagioclases of lava products from the southern Börzsöny Mountains with the aid of a universal stage (NOSKE-FAZEKAS 1972, 1974, 1977, 1979). The first summarizing paper on the hundreds of measurements, both early and quite recent could not however be published until now. These results may be of interest even from two points of view: 1. They have brought the knowledge of the Na/Ca ratio for a great number of plagioclases, the most frequent constituents of different andésite and dacite varieties; 2. They have allowed us to shed light on the relationship between plagioclase twin laws and parent rock. The first may be helpful in enhancing relevant research work mainly in Hungary, since in addition to the microprobe determination (KUBOVICS & PANTO 1970 and NAGY 1975) of the chemical composition of a few rockforming minerals, these are the only data available. The second appears to have attracted attention even abroad (SMITH 1974). and quite recently, in some xenoliths a rather peculiar frequency of twin laws could be revealed. Area of sampling and methods of determination The Börzsöny Mountains form the western part of the intra-Carpathian volcanic range which had its main eruption cycle in Tortonian time. The volcanism yielded both lava and pyroclastic agglomerate products in considerable quantities. The pyroclastics occur in the marginal zone of the Börzsöny, while the lava rocks are situated mainly in the central area. The author made her field traverses and samplings by proceeding north of the Danube's line. In doing so, she sampled the andésite and dacite occurrences (Fig. 1). During elaboration of this work, she made a qualitative description of the mineralogical composition and carried out observations on the textures in thin sections cut in three different directions perpendicular to one another. The quantitative mineralogical composition of the andésites and dacites was determined by using a Leitz integration stage, and the detailed optical examination of feldspars was performed on a Leitz UT-5 universal stage. Mineralogical composition The Börzsöny is made up mainly of andésites. In addition, on the eastern and western borders of the mountain (the Várhegy at Nógrád and the Csákhegy at Szob), the effusive rocks are also represented by dacite. The individual andésite and dacite varieties were classified according to their mafic silicate content, like it had been reported in earlier geological and pétrographie works devoted to the studied area (CSESZKÓ 1958, JUGOVICS 1935, KUBOVICS & PANTO 1970, PANTO 1949, GY. PANTO 1966). By the way, the author avoided distinguishing an excessively great number of transitional rock varieties, as this would have handicapped the statistical analysis of the results. As it is obvious from Table 1, if the quantity of the eponymous mafic silicate in each andésite variety is about 10%, so the share of the other mafic constituents is only a fraction of that figure. Exception to the rule is the hornblende-bearing pyroxene andésite, in which a considerable amount of hornblende is associating. Hist.-nat. Mus. Nat. Hung., 72, 1980