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.
than 5%; c) within this limit +H 2 0 content would be less than 3%; d) C0 2 content would be less than 2%. Ninety percent of the selected analyses showed less than 1 % C0 2 content. The 68 analyses, selected according to the above criteria, are suitable for statistical studies to obtain an overview on the volcanic rock types. However, due to these self-imposed restrictions, some known rock types are missing from the evaluation: essexite (TAKÁTS 1933) and augitite (Szilágyi 1981). The Mecsek igneous rocks are mostly characterized by high Fe 2 0 3 /FeO ratio. In 92% of the selected analyses it is higher than 0.5 % due to subsequent oxidation. The degree of oxidation strongly influences the CIPW norms (Cox et al. 1979) : oxidation increases saturation, but decreases normative pyroxene and olivine content. That is why Fe 2 0 3 /FeO values are standardized for oxidated samples. Several methods are applied, usually the Fe 2 0 3 /FeO = 0.2 correction is preferred (HUGHES & HUSSEY 1979). This correction was carried out for samples with less than 54% Si0 2 content. The next step was correction for volatiles, recalculating the data for volatile-free composition. These corrections influence CIPW norms and M-value ( = 100. Mg 2+ /Mg 2+ +Fe 2+ ) in the following way: Fe 2 0 3 /FeO standardization: normative quartz and magnetite decreases, while normative diopside, hypersthene and olivine contents appear or increase ; the THORNTON —TUTTLE Differentiation Index decreases, the colour index increases, M value decreases, while normative plagioclase composition ( = 100 an/an -fab) remains constant (in q- or hy-normative rocks). Recalculation of the composition to volatile-free composition does not cause significant changes, like the former correction: normative calcite disappears, while the other components slightly increase. 25r 20BAS PMO 15 SAN BTA 10 TBA AND TRA BSN TCP TAN PIC Fig. 1. Percentage distribution of volcanic rock-types of Mecsek Mts.: PIC = Picrite, BSN = Basanite, BAS = Basalt, BAN = Basaltic andésite, TBA = Trachybasalt, TEP = Tephrite, BTA = Basaltic trachyandesite, TAN = Trachyandesite, AND = Andésite, Tra = Trachyte, PHO = Phonolite, O = samples, which can't be ranged into either of these rock-types.
