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

Fig. 4. Alkali-silica diagram for the Mecsek basaltic and trachyandesitic recks (circles). Alkalitholeiitic division line after MACDONALD & KATSURA (1Ç64), encircled area is the field of analyses from mid-Atlantic Ridge at 45° N after AUMENTO & LONCAREVIC (1969), AUMENTO (1968). Triangle is average spilite from POLDERVAART (1955); Square: average oceanic tholeiite after ENGEL et al. (1965) and K

2

0 (+0.70). Therefore the high magnesium and low potash cumulative rocks have typically low F

2-values and the high potash and low magnesium trachyandesitic rocks high F

2-values. The calculated mean of the analyses (Table 2) can provide only a rough estimate as to the composition of the parental magma. The standard deviations are very high since the compositional range of the rock analyses is also large. The "mean" composition is that of an alkali basalt which is very low in Si0

2 and high in Ti0

2

. The calculated norms (Table 3) represent a typical ol-ne characterized basanite. The alkaline nature of the Mecsek basalts and trachyandesites is also obvious if we plot them in the diagram of Macdonald and KATSURA (1964) It is remarkable that not only fall all the analyses to the left-hand side of the Hawaii alkalic-tholeiitic division line (I ig 4), but none of them fall in the encircled area (field of analyses from mid-Atlantic Ridge at 45° N, AUMENTO 1968) In the triangle Alk-í e-Mg (F ig. 5) the diflerentiation trend of the Mecsek alkali basalt-trachyandesitephonolite series is very similar to the Hawaii alkalic trend of diflerentiation, however in the triangle Na-K-Ca (1 ig. 6) diflerentiation trend seems to be disturbed. Especially intermediate and highly differentiated members were modified by some process of soda-enrichment. This process was probFig. 5. Alk-Fe-Mg diagram. Circles: alkaline basaltic-*

trachyandesitic-»phonolitic reck series of the Mecsek Mts. Cresses: diabases and acid différenciâtes ol the Btkk Mts S-S: difierentation trend in the Skaergaard intrusion aittr WAGER & MITCHELL (1951), D-D: diabase-* granophyre series, Dillsturg, Pennsylvania alter FJOTZ (1953), A-A: Hawaiian alkaline basait-» trachyte series after NOCKOLDS & ALLEN (1954)