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

Embey-Isztin, A.: Major element patterns in Hungarian basaltic rocks: an approach to determine their tectonic settings

composition that may be designated by the name of olvine tholeiite according to Ringwood's nomenclature. In the diagram of Macdonald and Katsura (Fig. 8) diabases plot at either side of the Hawaii alkalic-tholeiitic division line and in the field of mid-Atlantic Ridge basalts. The differentiation trend of the Bükk rock association is a peculiar one. Since the concentrations of potassium are very low, the differentiation proceeds along the join Na-Ca of the triangle Na-K-Ca (Fig. 6). A smooth descent line cannot be ascertained in Fig. 5, though it shows some tholeiitic affinity. Considering the OFB-affinity of the Bükk diabases it seems appropriate to examine whether spilitization could have played a role in the chemistry of these rocks. Low potash and high soda concentrations are characteristic of spilites, however there is only a slight defficiency in Ca and none in Al, 'though the latter is considered to be very mobile during spilitization (CANN 1969; MELSON & VAN ANDEL 1966), finally there is no excess Si. Consequently, diabases of the Bükk Mts. could not have •been modified by spilitization to any greater extent. Summing up, we can say that the mesozoic diabases of the Bükk Mts. are olivine tholeiitic in composition and they show OFB-affinities. The abyssal tholeiitic nature of these rocks was earlier emphasised by ONUOHA (1977) and NOSKE-FAZEKAS & EMBEY-ISZTIN (1978). The primary magma must have been modified by olivine and clinopyroxene fractionation and modifications due to spilitization could not be ascertained. Basaltic andésites The sum of MgO+CaO in the analyses plotted in Figs. 2 and 7 is only 11-13 per cent, so they are close to the lower limit of basaltic composition to be considered in the discriminant diagrams. Both in Figs. 2 and 7 they largely plot as shoshonites. An interesting feature of these rocks is that while their F

x and F

3-values remain essentially constant, F

2 is changing considerably. Alteration of the rocks could produce this effect (PEARCE 1976) but only analyses of fresh rocks were used. On the other hand, we can explain the effect from the antipathetic variation of K

3

0 and MgO, oxides that have the largest scores on the second discrimi,nant function (+0.70 and —0.74 respectively). The 100 MgO/Mg+Fe

2- (Mg-values) of the basaltic pyroxene andésites are low, be ;Fig. 7. Discriminant diagram (PEARCB 1976). Crosses: diabases; Dots: gabbros; Open circles: sodagabbros; Triangles: basaltic andésites