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)
The commonest andésite variety in the investigated area is hornblende andésite, widely varying in macroscopic habit. Most frequent is the dark grey,compact variety in which the porphyric constittuents are less distinct from the groundmass, though light grey, hornblende-lathed or greyish brown, porphyry-speckled varieties may also occur. The groundmass is also variable, though most frequently microholocrystalline, constituting, as a rule, 50% or so of the total volume. The volume percentage of porphyric feldspars is nearly three times greater than the figure of the other prophyric •constituents, of which hornblende is predominant, accompanied by some hyperstene and biotite. This variety occurs throughout the investigated area. The transition between the above two commonest andésite varieties of the southern Börzsöny Mountains is represented by hornblende-bearing pyroxene andésite agreeing both macro- and microscopically with the two previous andésites. According to plagioclase twin examinations, biotite andésite is next to follow in terms of chemisstry. It has not yet been observed in the central area of the mountain, being most frequent in the Szob—Márianosztra—Királyrét zone. Macroscopically, the brownish grey colour prédominantes, while microscopically a greenish stain due to the frequent chloritic decomposition is observable. The ratio of the porphyric constituents to the groundmass is similar to the case of hornblende andésites. Of the porphyric components, feldspars in terms of frequency are followed by biotite which is often heavily opacitized. Besides, some hornblende can also be observed. The lilac-red, so-called oxy-hornblende andésite in which the mafic silicates (mainly hornblende and less pyroxene) are almost totally altered by oxidation, is assigned to a separate group in the relevant literature. The porphyric feldspars are by more than 10% poorer in quantity compared to the case of the oher andésites. At the same time, the weathered mafic silicates already referred to are present in much higher percentage (26%). The rock is considered to be the product of an eruption subsequent to the main lava outpourings (CSESZKÓ 1958, KUBOVICS & PANTO 1970). In the parts of the Börzsöny thus far investigated by the author andésites of high carbonate content in only one place was found: the Rókahegy to the northeast of Szokolya village where it is made up of light grey, biotitic hornblende andésite. Characterized by a carbonate content of nearly 10%, this rock contains hosts of round xenolithes in which CaC0 3 attains or even exceeds 50%. Brought to the surface by the andésite lava, these products gave interesting results also when examined optically in the course of investigations of feldspars from the study area (NOSKE-FAZEKAS 1977). In the western Börzsöny area, dacites were extruded in the final phase of the volcanism (CSESZKÓ 1958), while the biotite dacites of the eastern area are believed to be early products (KUBOVICS & PANTO 1970). The two major dacite exposures studied by the present writer are different both volcanologically and in terms of mineralogical composition: in the case of the Csákhegy at Szob the hornblende dacite forming the mount's top represents the final phase of the volcanism that produced the hornblende andésite mass underneath, while the Várhegy at Nógrád is an independent biotite dacite •cumulo-dome. Produced as a continuation of the andesiteeruption, the Csákhegy dacite is both mineralogically and in chemical composition more akin to the hornblende andésite forming the bulk of the mountain (CSESZKÓ 1958, NOSKE-TAZEKAS 1974). The difference between the two facies is to some extent manifested in the frequency of the plagioclase twin laws as well. Regarding the average of the two areas, however, the dacites fit well in a twin-statistical series correlated with the chemistry of the rock. Coming closer to the central part of the Börzsöny, in the vicinity of Nagyirtás puszta, we could observe hornblende-bearing pyroxene andésites and a compact tuff-like rock to occur folded and squeered into each other. While examining the rock under the microscope, we found in the pyroxene andésite inclusions of a few mm size. The examination of these with the universal stage gave astonishing results. Composed mainly of plagioclase feldspars and, in smaller percentage, of hornblende and devoid of any matrix, the inclusions showed more than half of the measured plagioclase twin crystals to obey the acline twin law. In addition, the albite twins are also considerable in number, so that the quantity of Gorai's C-twins is as high as 84 %. As it can be established on the basis of the An content, the feldspars of the inclusions are much more basic than the enclosing rock, and along with other mineral optical observations, this is another indication of a cognate origin (NOSKE-FAZEKAS 1979). Examination of feldspars with the universal stage The main mineralogical components of the lava rocks of the Börzsöny are plagioclase feldspars. Therefore, conclusions as to the genetic circumstances of the rocks can be drawn primarily from the characteristics of this predominant mineral assemblage. Namely, an
