Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 72. (Budapest 1980)
Örkényi-Bondor, L.: Andesite agglomerate from Zebegény village, Börzsöny Mountains (Hungary)
contain similar porphyric constituents. This fact is even more surprising, because in several samples there are varieties oi free silica (tridimite, cristobalite, scarcely even quartz). The alvmina content is uniformly high in every sample, ranging frcm 18.41 to 2C.84%. This corresponds well to the A1.;0 3 content of the amphibole andésite at Nagybörzsöny. The high alvmina values result by no means from clayey decomposition products, because those samples which contain clay minerals do not have higher alumina values. Compared with the analyses of Japanese andésites, among those the highest alumina content is 16.64%. The iron content is widely varying in the Zebegény rocks. Ferrous, ferric and even the total iron content have rather large ranges. However, the iron content is in general lcv\er, than in the data found in the literature for andésites (including also the Nagybörzsöny reck analyses). The degree of oxidation is higher than in the case of andésite lava f ows or subvolcanic bodies. This may be due to the pyroclastic origin (during explosion the erupted matter was submitted to the action of atmospheric oxygène). In such less compact rocks even chemical weathering is more intensive than in subvolcanic bodies, lava sheets or volcanic chimneys. The MgO content also varies widely, from 1.24 to 3.78%, as a function of the percentage of dark silicates, mostly pyroxenes. (With the exception of sample 5/71). In the literature one encounters higher values of MgO. CaO content ranges 6.24-8.78%, in good accordance with the Japanese and Nagybörzsöny andésites. The percentages of Na 2 0 are between 2.27 and 2.94 The Nagybörzsöny data vary in a wider range. The K.,0 contents are rather high, from 1.02 up to 2.17 %. The Nagybörzsöny andésites have similar potassium oxide contents. The Japanese data are, in general, much lower. As for binary correlations, the following can be stated: Ca and Mg show well marked co-variation, which, however, is not proportional. The same can be said of K and Na. The alkalis vary in opposite sense to calcium. (Except for samples 12/75 and 14/75.) Na+K is in negative correlation with total iron. These relationships are most likely characteristics of the original magma, not due to secondary alterations. For illustration, the triangle diagram after H. KUNO was adopted. (MgO, total iron expressed as FeO, Na+K). With the exception of two alkali-rich, Mg-poor samples, the data are located within a small field. Compared with the data published by ISSHIKI on tholeitiic magma, the Zebegény rocks are much poorer in iron. The chemical analyses corroborate the concept that the amphiboles occurring in the Fig. 4. Total iron — MgO — Na 2 0+ K 2 0 diagram of andésite bombs from Zebegény
