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.
(Cox et al. 1979; LE BAS et al. 1986). The IUGS Subcommission suggested the application of the TAS (total-alkali-silica) diagram for the classification of volcanic rocks. SABINE and co-workers (1985) examined the influence of alteration on the TAS-diagram classification, and good correlation was observed between the petrographical and chemical nomenclature, except for strongly altered and rare rocks. DE LA ROCHE and co-workers (1980) suggested a chemical variation diagram containing the main cations. Their method provides more information than the Si0 2 — alkali diagrams considering 3 components only. Classification of the Mecsek volcanics based on modal composition raises problems, therefore we need chemical classification, too. Besides the TAS diagram, plots of Cox et al. (1979) and DE LA ROCHE et al. (1980) were applied for the classification of the Mecsek rocks. As the latter one contains more rock names, some rock types were amalgamated for the purposes of the comparison (basalt = alkaline basalt, basalt and tholeiite; trachybasalt = hawaiite and mugearite; andésite — andésite and latiandesite; phonolite = phonolite and trachypohonolite). Plotting the corrected compositions on the three diagrams,, we found that there is a 90% agreement between the TAS and Cox's classification, and 80% between the TAS and DE LA ROCHE'S classification. Summing up the results, the Mecsek volcanic rocks were ranged into 11 groups (Fig. 1), while the relations of the 5 samples with any of the groups is unclear, therefore these ones were excluded from the calculation of average compositions. In spite of the large number of rock types, 4 groups are easily separated: basaltic rocks (picrite, basanite, basalt); transitional rocks (basaltic andésite); alkaline transitional rocks (trachybasalt, basaltic trachyandesite); intermediate-acidic rocks (andésite, trachyandesite, phonolite, trachyte) (Figs 3 and 4). Na>0+ rCO Fig. 4. Na 2 0 +K 2 0 vs. Si0 2 diagram for volcanic rocks of Mecsek Mts. (discrimination curve after MACDONALD & KATSURA 1974). A = Alkalic field, Th = Tholeiitic field (symbols are explained in Fig. 3).
