Matskási István (szerk.): A Magyar Természettudományi Múzeum évkönyve 80. (Budapest 1988)

Harangi, Sz.: Major element chemistry of Lower Cretaceous igneous rock of Mecsek Mountains (Southern Hungary), I.

Average chemical composition of each rock types are shown in Table 1. CIPW norms were calculated from the average composition. Comparing these average values with published ones (LE MAÎTRE 1976; DE LA ROCHE et al. 1980), we can observe high Ti0

2 and total iron, and relatively low A1

2

0

3 contents. Most of the Mecsek volcanics are of basaltic-andesitic composition, and a more detailed classification is proposed here. MIDDLEMOST (1975) suggested the joint application of Na

2

0 —Si0

2 and K

2

0—Si0

2 diagrams to distinguish between alkaline and subalkaline characters. If the point of the sample lies in the alkaline field in both diagrams, the rock is alkaline, if it lies in the subalkaline field in both ones, then the sample is subalkaline. If one of the plots indicates subalkaline, while the other one indicates alkaline composition, we have a transitional basalt. About half of the Mecsek samples belong to the transitional group, while a quarter of them is alkaline, and another quarter is subalkaline. Tephrites, basanites, some basalt samples, trachybasalts (hawaiite) and basaltic trachyandesites belong to the alkaline group. To distinguish between potassic or sodic character, the samples were plotted on the K

2

0 —Na

2

0 diagram (MIDDLEMOST 1975) (Fig. 5). More than half of the samples are of potassic composition, but the potassium enrichment may be connected to submarine alteration, the K being concentrated in clay minerals. WILKINSON (1986) has based the classification of basaltic and andesitic rocks (AN > 30) on normative plagioclase composition, Si0

2 saturation, A1

2

0

3 content and K

2

0/Na

2

0 ratio. More than half of the Mecsek volcanics belong to the andésites (norm. plag. AN = 100 an/an+ab < 50), while most of the basaltic rocks display tholeiitic character (Fig. 2). Within the andesitic rocks the tholeiitic character is prevalent. However, we should note that the tholeiitic character does not imply genetic classification; we can only say that petrochemically the investigated rocks are of tholeiitic (subalkaline) character. Comparing the average compositions of each groups (Table 2) with the averages provided by W'ILKINSON (1986), we can observe that Ti0

2 content is higher, while A1

2

0

3 content is somewhat lower in the Mecsek samples. According to the K

2

0/Na

2

0 ratio these belong to the middle to low K/Na type. K

2 6 4 2 0 1 fx 1 1 1 I 1 High K-series N. / K-series + \ / + V. — _ / + + * N. * SS Na-series \ 1 t 1 I 1 3 2 4 e Na

2

0 Fig. 5. K

2

0 vs. Na

2

0 diagram (after MIDDLEMOST 1975) for basaltic-andesitic rocks of Mecsek Mts. Annls hist.-nat. Mus. natn. hung., 80, 1988