Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 72. (Budapest 1980)

Embey-Isztin, A.: Major element patterns in Hungarian basaltic rocks: an approach to determine their tectonic settings

ANNALES HISTORICO-NATURALES MUSEI NATIONALIS HUNGARICI Tomus 72. Budapest 1980.. Major Element Patterns in Hungarian Basaltic Rocks: an Approach to Determine their Tectonic Settings by A. EMBEY-ISZTIN, Budapest Abstract — Discriminant diagrams have been employed to establish the most likely tectonic settings of four different basaltic rock-associations in Hungary. The young Upper Pliocene — Pleistocene basalts (Balatonfelvidék, Cserhát) have marked WPB-afflnities the eruptions of which are connected with recent mantle-diapirism and continental rifting. The Lower Cretaceous basic rocks (Mecsek Mts.) plot also as WPB-baslts and therefore they are interpreted as witnesses of a Lower Cretaceous rifting process. The Mesozoic diabases of the Bükk Mts. show OFB-affinities and it has been argued that they were emplaced tectonically along the most important tectonic line of the Pannonian Basin. Finally, the basaltic andésites show shoshonitic affinités (SHO) and they could have been generated by a late Oligocène early Miocene subduction process occurred in the Carpathian region. With 8 figures. Introduction — Ever since the plate tectonic model of tectogenesis has gained a general acceptation among earth scientists, and our understanding as to the magma genesis has become as deep as never before, petrologists have been seeking for an eventual correlation between the element patterns of lavas and the tectonic environment in which they were erupted. In this respect several successful methods, especially in the case of basic volcanic rocks have been proposed, e. g. that of PEAR CE & CANN (1973) which uses minor element patterns, as well as PEARCE'S (1976) method which on the contrary makes use of major element patterns to establish the most likely tectonic settings of basic volcanic rocks. A similar approach to the second one has been proposed by NISBET & PEARCE (1977), the greatest difference being that they used major element pattern of clinopyroxenes instead of whole rocks. In the present paper I employed PEARCE'S method, that is, I calculated discriminant functions on the basis of major element data to represent visually the separations of four well-defined basaltic rock associations of Hungary. In the following, geology and petrology of the rock associations will be discussed briefly. Geology and petrology of the basic rock associations Young alkali basalts The first basaltic formation to be considered is that of the young basalts erupted partly north of the Lake Balaton (Balatonfelvidék) and partly in the Cserhát Hills (Fig. 1). However, basaltic eruptions of the same volcanic activity can be identified well beyond the frontiers of the country, so the Cserhát basalt area is continuing to the North in Slovakia and there is another basalt area near Graz in Austria and several others in Transylvania. The volcanic areas are mainly composed of lava flows, scoriacious lavas and tuffs. At several localities they contain nodules of peridotite, megacrysts, and xenoliths of cumulate origin. Lavas show typical alkaline affinities, they are fresh, although at some localities zeolithization is widespread. Differentiation processes could not have operated to any large extent, since the chemistry of individual lava flows is rather uniform (see MAURITZ et al. 1948). The age of the eruptions is Upper Pliocene—Pleistocene, and it seems that the Cserhát basalts are younger («2 m.y.) than those of the Balaton area ((«3-5 m.y., BALOGH KADOSA pers. comm). Since from Upper Pliocene time only a few million years have passed we can expect that tectonic conditions at that time could not have been very different from that prevailing at present. The essential characAnn. Hist.-nat. Mus. Nat. Hung., 72, 1980 2*