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.

the petrochemistry of the Mecsek igneous rocks, however, we know that there are several kinds of rocks in the Mecsek: augite diabase, alkaline diabase, albite diabase, spilite, augitite and several rock types: basalt, trachyte, essexite, phonolite. DOBOSI (1987) published brief petrographical descriptions of alkaline basalts, trachybasalts, trachyandesites and phonolites. A detailed study of the phonolite was carried out by VICZIÁN (1971). A new system for the classification and nomenclature was put forward by the IUGS Subcommission on the Systematics of Igneous Rocks (STRECKEISEN 1979). Among others the term "diabase", applied for altered basalts was rejected. The classification was based on the modal mineralogical composition, plotted on a QAPF diagram. We know that modal composition cannot be well determined for some volcanic rocks (due to the presence of microcrystalline or vitreous groundmass), therefore classification must be based on chemical composition. STRECKEISEN & LE MAÎTRE (1979) suggested the application of the BarthNiggli norms for volcanic rocks classification, providing a diagram constructed from the fields of the QAPF diagram based on a large number of analytical data. The latter method is not widely applied; classification methods are rather based on Si0

2 vs. Na.

2

0+K

2

0 Si0

2 Fig. 3. Si0

2 vs. AN diagram for volcanic rocks of Mecsek Mts. : 1 = basanite, 2 = basalt, 3 = tephrite, 4 = trachybasalt, 5 = basaltic trachyandesite, 6 = basaltic andésite, 7 = andésite, 8 = trachyandesite, 9 = trachyte, 10 = phonolite; I = basaltic rocks, II = transitional basaltic-andesitic rocks, III = alkaline transitional basaltic-andesitic rocks, IV = intermediate-acidic rocks