Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 73. (Budapest 1981)
Embey-Isztin, A. ; Noske-Fazekas, G.: On the chemistry of the large phenocrysts in the tuff of Godóvár (Börzsöny Mts., Hungary)
since Na tend to decrase in the outer zones. From Figs. 2a-b it is evident that while the compositional changes seem to be gradual between the outermost rim and intermediate zones, there is a sudden jump at the border of the innermost primitive light coloured core. As it was mentioned earlier, discontinuity between the core and outer zones can be frequently observed in thin section (Plate IV: 2). In contrast, compositional shifting is less important and seems to be rather continuous across whole amphibole crystals. One of the traverses (continuous line in Fig. 4a-b) does show large scale and rather irregular variations but this originate from a thick opacitic rim. A bulk chemical analysis of phenocrysts separated from the tuff and representative microprobe analyses of individual zones are presented in Table 2. The wet chemical analysis, where Fe 2 + :Fe 3+ has also been determinated and most of the microprobe analyses fulfil the definition of pargasite in the nomeclature of amphiboles compiled by LEAKE (1978) in having Si near to 6.0, Ti <<c 0.5, 100 Mg/Mg+27Fe (mg-values) generally higher than 70 and at least in the analysis No. 28 Fe' i+ <s: A1 V1 . In the outermost zones however, total Fe increases considerably and só does very probably Fe 2+ consequently in these zones FV i+ may be equal or greater than Al vi and since with increasing proportions of Fe 3 + , 100 Mg/Mg+Fe 2+ also increases, the composition of these zones may fulfil the definition of magnesio-hastingsite. As a matter of fact, the only important chemical variation is that of the increasing substitution of Fe for Mg proceedig from the light coloured core to the dark brown rim. Besides this, Ti tends to increase slightly in the outer zones and K in the core. The other elements do not vary much, especially the curve of Ca is remarkably smooth. In contrast, all elements change considerably and in a rather irregular manner in the opacitic rim where witrous material may also be present. A traverse of one plagioclase crystal revealed normal zoning, that is more calcic and less sodic in the core and less calcic and more sodic at the border zones. Table 3 presents the two extreme analyses the most basic one (Ab 12 .i An 875 ® R OA Das i c bytownite) and the most acid one (Ab 36l An 629 Or 10 labradorite).* Universal stage measurements yielded an average An-value of 73.1 per cent. Conculding remarks The high concentration of clinopyroxene, amphibole and plagioclase crystals as well as the presence of xenolithic aggregates of these same minerals indicate that they ^.re early cumulate phases crystallizing from the parent magma. The nature of this parent magma must have been basaltic judging from the chemical features of the phenocrysts, especially from their high 100 Mg/Mg -f JCFe (mg-values) in most zones. This is surprising since the most basic lavas present on the surface are clearly of andesitic nature. Indeed, rock forming minerals of the Börzsöny andésites seem to be very different chemically. Partial microprobe analyses (PANTO 1970) of a few pyroxene and amphibole phenocrysts from different andésites of the Börzsöny Mts. compared to the present results show that the former ones must have segregated from liquids rich in Fe (from two pyroxene traverses mg-values of 70-76 and 60-71 can be obtained and amphiboles are extremely rich in Fe with mg-values of only 30-66 and 40-64 respectively). In addition, these amphiboles are less calcic than those of the present work. The last remark is valid also for the plagioclases: hundreds of universal stage measurements (NOSKE-FAZEKAS 1972, 1974, 1977, 1979, 1980) show that on average plagioclases from * Individual measurements in the rim and core of different crystals yielded a composition range between Ab 37 .5 An fíu « On.; and Ab 9 .4An 00 .i;Oro.4 (see Table 3).
