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.

Petrochemically the igneous rocks of the Mecsek Mts. form an alkaline (alkaline basalt — trachybasalt — basaltic trachyandesite — phonolite) series and a subalkaline (tholeiitic —) transitional series (tholeiite — basaltic andésite — ?trachyte). If we have many samples and even more chemical analyses, statistical treatment of data cannot be avoided. In this paper only univariate statistical methods are applied. Histograms were constructed for all oxides and some characteristic ratios, expressing the characteristic frequency distribution of the oxides; besides, the maximum values are usually characteristic for the rock types (Fig. 6). Si0

2 (x/average value) = 53.71, SD (standard deviation = 5.35): there are three maxima in the Si0

2 distribution of the Mecsek volcanics (43-50, 50-57 and 57-70 Si0

2

%), corresponding to a basic a transitional basaltic-andesitic and an intermediate-acidic group, in accordance with the results of classification. TiO, (x = 2.88, SD = 1.45): the examined compositions show extremely high Ti0

2 contents, indicating intraplate volcanism. Phonolite contains average amount of Ti0

2 only. Defining a small class width (0.25) for the histogram, phonolites (0-0.5%), trachytes (1-1.5%), andésites (2-3%) and basalts (> 3%) can be easily separated. A minor peak between 4.5 and 6.0% shows the location of Ti-rich basalts from the Jánosipuszta area. A1

2

0

3 (x = 14.76, SD = 3.3): AI

2

0

3 contents are somewhat lower than published average values, except for the phonolites. Especially low values have been observed in andésites and trachytes. Applying 0.5% class width in the histogram, a low A1

2

0

3 (11-14%) group was observed, containing iy-tholeiites and alkaline basalts among the basaltic rocks. FeO

tot (x = 10.18, SD = 3.76): total Fe contents are high in basaltic and andesitic groups; a significant positive excursion was observed in transitional intermediate rocks (trachybasalts). Phonolites and trachytes show lower FeO

tot content. MgO (x = 4.23, SD = 2.96): Mg-poor phonolites (0-1.5%), and alkaline basic rocks, containing pyroxene and olivine, with higher MgO content (basanite, alkaline basalt, ol-tholeiite) (MgO =-7%) are separated. Fig. 7. K

2

0- Ti0

2- P

2

0

5 diagram (after PEARCE et al. 1975) for basaltic-andesitic rocks of Mecsek Mts. Oc = Oceanic field, Cont = Continental field. Chemistry of the igneous rocks in the Mecsek Mts. K

2

0