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.

ANNALES HISTORICO-NATURALES MUSEI NATIONALIS HUNGARICI Tomus 80. Budapest, 1988 p. 11-24. Major dement chemistry of Lower Cretaceous igneous rocks of the Mecsek Mountains (Southern Hungary), I. by Sz. HARANGI, Budapest Sz. HARANGI: Major element chemistry of Lower Cretaceous igneous rocks of the Mecsek Mountains (Southern Hungary), I. — Annls hist.-nat. Mus. natn. hung. 1988 80: 11-24. Abstract — Available chemical analyses of Lower Cretaceous volcanic rocks of the Mecsek Mts. (S Hungary) were statistically evaluated. Sixty-eight analyses of the total of 359 were selected according to the following criteria: +H

2

0 < 3%, C0

2 < 2%. The Fe

2

0

3

/FeO ratio were standardized to 0.2. Based on the selected data and using 3 methods (Cox et al. 1979, DE LA ROCHE et al. 1980, TAS diagram) the Mecsek volcanics were classified into 4 groups : basaltic rocks (picrite, basanite, basalt), transitional rocks (basaltic andésite), alkaline transitional rocks (trachybasalt, basaltic trachyandesite, trachyandesite) and intermediate-acidic rocks (andésite, phonolite, trachyte). Among the basaltic-andesitic rocks the tholeiitic character was found to predominate. Average chemical composition of each rock-types is given. Using univariate statistical methods, petrochemical character of these rocks was determined. The Mecsek volcanics bear features of continental rift magmatism, however, the ratio of tholeiitic rocks is somewhat high. Introduction — Investigations on the Lower Cretaceous igneous rocks of the Mecsek Mts. in southern Hungary have been carried out for more than one hundred years. The first detailed petrographical and petrochemical investigation was made by MAURITZ (1913). Characters of the submarine volcanism were described by BILIK (1983), who distinguished effusive rock types (hyaloclastite. pillow lava, massive lava, lava breccia, pillow breccia, etc.). He stressed the alkaline character and continental nature of the volcanism. Discrimination analyses (EMBEY-ISZTIN 1981) corroborated the intraplate affinity and relationships with continental rift magmatism. Studies on pyroxenes of the alkaline basalt — trachybasalt — trachyandesite — phonolite series (Dobosi 1986, 1987) also indicate intraplate magmatism and alkaline magma, differentiated by low-pressure fractional crystallisation. — The subvolcanic phonolite is a product of the Lower Cretaceous volcanic activity (VICZIÁN 1971), being the acidic, alkali-rich differentiate of the alkaline basaltic magma. The first data on the major element composition were published by MAURITZ (1913). Further geochemical analyses were carried out to help the detailed survey of the Mecsek Mts. in the 1960s, and part of them were published in the series "Explanations to the 1 : 10000 geological maps of the Mecsek Mts.". The chemical analyses were made in the Komló Laboratory of the National Geological Prospecting and Drilling Company (OFKFV) by É. Pánczél, G. Csordás, M. Vigh, G. Csajághy. and in the Chemical Laboratory of the Hungarian Geological Institute by L. Jankovich and L. Soha. The data are deposited in the Documentation Department of the Hungarian Geological Institute. Analytical data — The results of 359 chemical analyses were collected and examined; only a minor part of them have been published (MAURITZ 1913, NAGY et al. 1978, BILIK et al. 1978, VICZIÁN 1971). The unpublished data are not accompanied by petrographical descriptions and the names of the rocks, raising several problems during evaluation. Therefore the conclusions of the present paper are exclusively based on the chemical data; classification of the rocks was made according to the normative composition. Classification of the Mecsek igneous rocks is hindered by the almost total absence of fresh rocks ; most of them suffered alteration, such as submarine alteration, influence of host rocks on dykes and weathering. The often high CaC0

3 content may be due to the assimilation of carbonate rocks by the rising magma. The first step of the analyses was to delete volatile-rich samples. Generally accepted criteria of fresh rocks are: +H

2

0 «= 2.0% and C0

2 < 1.0%. However, only 18 samples fulfilled this requirement. As this small number of data is not enough to characterize the types of igneous rocks in the Mecsek, we modified the criteria. The following conditions were applied: a) the sum of the components would be between 98 and 102%; b) total volatile content ( +H

2

0, - H

2

0, COj would be less