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)

quency is the Carlsbad twin law (12-43%). Looking at the diagram based on the frequencies of the two different twins (Fig. 2), we can see that the above twin laws are antagonists of each other. Although less compared to the above simple or parallel twins, the number of plagioclase feldspars crystallized according to the complex albite-Carlsbad twin law is considerable (Fig. 3). Their frequency in the individual rock varieties shows much smaller scatter compared to the preceding cases. In the more acid andésite varieties, the role of acline twins is remarkable, the greatest frequency being associated with oxy-hornblende andésite (29%). Even higher is the relative quantity of these twins in the xenoliths of the hornblende-bearing pyroxene andésite in the vicinity of Nagyirtáspuszta. Here the plagioclases are crystallized according to a total of only four twin laws, but almost the half of the twins obey the acline law (Table 2). In her earlier twin law studies the author could not observe so high a frequency of acline twins either in* volcanic or in intrusive igneous rocks. In addition to the above, she identified twinnings obeying the Manebach, Ala and albiteAla laws generally in a few per cent ratio. In only a few cases did she observe complexes intergrown according to the Baveno law, though the absence of cleavage informative on morphological orientations or the poor recognizability of its track had rendered the determination uncertain. It is evident already from Diagram 4 that the frequency of twins obeying the seven identified twin laws varies from one rock variety to the other. Even more conspicuous is the relationship between the chemical composition of the rock and the twin law frequency, when the identified twins are grouped as proposed by GORAI (1951), into so-called C- and A-twins (A-twins: albite and acline twins, C-twins: the other five twins identified: Manebach, Carlsbad, Ala, albite-Carlsbad and albite-Ala). Accordingly, there must be a distinct difference even between single varieties distinguished within the andésite. Pyrosene andésite, the variety regarded as the most basic in composition, is characterized by the greatest number of C-twins. And this feature shows a gradual decrease, as one proceeds through the hornblende andésites up to the biotite andésite. This series is then joined by the dacites, with their rather low (35:65) C/A twin ratio. Plagioclase studies in the Dunazug Mountains by VINCZE-SZEBERÉNYï (1977) proved a C:A twin ratio of 66:34 in pyroxene andésites. Regarding the proximity of the two areas and their similar genetic circumstances, the twin laws could well be expected to agree statistically. While examining the carbonate-bearing hornblende andésite and the spherical xenolith; of the Róka-hegy, we found that, owing to heavy carbonatization, the chemical analyses could not be taken into consideration in the genetic interpretation of the above formations. By measuring the plagioclases with the universal stage, however, we did reveal that the feldspars of the inclusions had crystallized from an earlier melt that must have been a little more acidic (C/A = 52:48) than the enclosing rock (C:A = 58/42). Having calculated the weighted mathematical average of the An% measured on twin individuals, we see a surprising agreement between the chemical composition of lava rocks and the An content of their plagioclases. The basicity shows a gradual decrease from 71% An measured for the most basic variety (pyroxene andésite) to 54% An obtained for dacite. Correlating these data with the C/A ratio (Table 4), we can observe a good relationship between the two sets of values. In other words, there are distinct and regular correlations between the albite-anortite molecule ratio measured with the universal stage and the frequency of the single twin laws on the one hand the genetic conditions of the rocks on the. other.