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.
{basanites, alkali basalts and ol-tholeiites) belong to the alkaline, and half of them to the tholeiitic field. Another method indicates alkaline character, if it contains normative nepheline. It indicates subalkaline character for most of the Mecsek volcanics, except phonolites, but part of them belong to the transitional (ol- and/or hy-normative) group. Tectonic position may be determined by applying the discrimination diagram of PEARCE (1976), which is mainly useful for high Ti0 2 and high K 2 0 rocks. Applying this method EMBEYTSZTIN (1981) has shown the intraplate affinity of the Mecsek volcanics. The analytical data examined by us, which fulfill the requirements of Pearce, fall into the same field (WPB) without exception. The K 2 0 —Ti0 2 —P 2 0 5 triangular diagram (PEARCE et al. 1975) can be used to distinguish between continental and oceanic character. Of course, the exclusive application of this method seldom yields acceptable results, because both K 2 0 and Ti0 2 can be enriched in oceanic and continental volcanics. The Mecsek samples (bearing less than 20% total alkalis on the AFM diagram) display a scattered distribution, part of them in the continental, part in the oceanic field. The latter contains the samples with extremely high Ti0 2 content (Fig. 7). To make more detailed distinctions on the tectonic character, the MgO — FeO tot — A1 2 0 3 diagram (PEARCE et al. 1977) was applied with good results on the basaltic andésites with 51-56% Si0 2 . Almost all the samples fall into the continental field (Fig. 8). Discrimination diagrams and chemical composition enable us to draw the conclusion that the Mecsek volcanics bear features of a continental rift magmatism, compared with the products of recent rifts (BARBERI et al. 1982), but the ratio of tholeiitic rocks is somewhat higher. This study is based on major element analyses, with inherent uncertainties. Yet it can call attention to some problems which may be solved by other methods (trace element and REE analyses). Besides studying the classification of the Mecsek rocks based on chemical analytical data, we should like to call attention to the unsolved problems in geochemistry in this area, hoping that further detailed studies will follow. References BARBERI, F., SANTACROCE, R., VARET, J. (1982): Chemical aspects of rift magmatism — In: PÁLMASON, G. (ed.): Continental and oceanic rifts. — Geodynamic Ser. 8: 223-258 BILIK, I. (1966) : Problems of the nomenclature of the Lower Cretaceous volcanic rocks in the Mecsek Mts. (Hung, with English abstract) — Ann. Rep. Hung. Geol. Inst, for 1964: 59-74 BILIK, I. (1983): Lower Cretaceous submarine (rift) volcanism in South Transdanubia (South Hungary) — Proc. 17 th Assembly of the ESC, Budapest 1980: 569-576 BILIK, I., HÁMOR, G., HETÉNYI, R. & NAGY, I. (1978): Explanation to the 1 : 10 000 geological maps of Mecsek Mts., Kisbattyán (Hung.). — Publ. Hung. Geol. Inst. (Budapest). Cox, K. G., BELL, J. D. & PANKHURST R. J. (1979): The interpretation of igneous rocks. — London, Georg Allen and Unwin. DE LA ROCHE, H., LETERRIER, P., GRANDCLAUDE, P. & MARCHAL, M. (1980): A classification of volcanic and plutonic rocks using the R1-R2 diagram and major element analyses. Its relationships with current nomenclature — Chem. Geol. 29: 183-210. DOBOSI, G. (1986): Clinopyroxene compositions of some mesozoic igneous rocks of Hungary: the possibility of identification of their magma type and tectonic settings — Ofioliti 11: 19-34. DOBOSI G. (1987): Chemistry of clinopyroxenes from the Lower Cretaceous alkaline volcanic rocks of the Mecsek Mountains, South Hungary. —N. Jb. Miner. Abh. 156: 281-301. EMBEY-ISZTIN, A. (1981): Statistical analyses of major element patterns in basaltic rocks of Hungary. — Acta geol. hung. 24: 351-368. HUGHES, C. J. & HUSSEY, E. M. (1979): Standardized procedure for presenting corrected Fe 2 O a /FeO ratios in analyses of fine grained mafic rocks — N. Jb. Miner. Mh. 12: 570-572.
