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
Basaltic andésites As it is indicated by the name, these rocks are by no means true basalts though they approach the basaltic composition. They represent the most basic members of the andesite-dacite-rhyolite association of the famous Intercarpathian Volcanic Chain. In Hungary the Pilis, Börzsöny, Cserhát, Mátra and the Zemplén Mountains (big. 1) are built of this rock association. At the western part of the chain rocks are about 15 m. y. old, to the East, in the Zemplén Mts. they are only 12 m. y. old and in Transylvania they are even younger. There are many records of basalts accompanying andésites in the case of island arc-type volcanism, and this kind of basalt can be distinguished chemically from basalts of other tectonic settings. In the following, using the discriminant diagrams of Pearce, I intend to test whether or not the basaltic andésites in question fall in one of the fields of volcanic arc basalts. Choice of analyses PEARCE (1976) has shown that a small number of analyses may be sufficient if we need to classify lavas of unknown origin by the aid of his discriminant function diagrams. Where it was possible I strictly respected his recommendations as to the quality and compositional range of the analyses to be chosen: that is, analyses of samples described as altered or with FeO/Fe 2 O 3-<=0,5 were rejected, the total of all major element oxides including H 2 0 was between 99 and 101 per cent everywhere, and the range of CaO+ MgO was between 12 and 20 per cent. From the young alkali basalts described under section A, as well as from the diabases (Section C) a higher number of analyses was published from which 30 and 22 have been chosen, respectively, all of course satisfying the conditions mentioned above. Analyses of the young alkali basalts were chosen so as to represent all the basalt eruptions of the country. Though it is obvious that the discriminant diagrams can be used exclusively for lava rocks, I plotted there discriminant functions of some gabbros associated with diabases (Section C), too. Since there are transitional rocks between diabases and gabbros of the Bükk Mountains, the eventual compositional shifting caused by increasing gabbroic character may thus be deciphered. Unfortunately, there is only a smaller number of analyses published from the Lower Cretaceous alkali basalts of the Mecsek Mountains. Thus, I had to plot rocks with greater and lesser amount of MgO+ CaO than the normal basaltic range, too. In this way, compositional shifting caused by the differentiation could be evaluated. As these rocks are partly zeolitized and chloritized, oxidation rate and H 2 0 is higher than in fresh basalts. Finally, the basaltic andésites are much lower in MgO than are average basalts and the sum of MgO+ CaO is around the lower limit (12 per cent), in some cases only approaching this value. The list of analyses and the sources of data are summarized in Table 1. Basalts of different tectonic settings Before proceeding to the discussion of the results, it may be appropriate to review the basalts of different tectonic settings that can be distinguished from each other in PEARCE'S diagrams. There are six magma types that can be defined on the basis of modern plate tectonic processes according to PEARCE & CANN (1973): 1. Ocean-floor basalts (OFB) are erupted at diverging plate margins, either within large oceans (mid-oceanic ridges) or within the small ocean basins behind island arcs. — 2. Island arc tholeiites (LKT) are erupted on oceanic crust at converging plate margins. They are typically erupted close to the deep-ocean trenches. — 3. Calk-alkali basalt (CAB) are erupted on continental crust at converging plate margins, and on oceanic crust well behind the deep-ocean trenches. — 4. Shoshonites (SHO) are erupted at converging plate margins, either a long way behind the deep-ocean trenches in mature arcs, or in post-orogenic situations after subduction has ceased. — 5. Ocean-island basalts (OIB) are erupted as ocean islands within ocean basins, largely within plates but occasionally as islands on ridge crests. — 6. Continental basalts (CON) are erupted through continental crust, largely at continental rifts in a within-plate setting. In the discriminant diagrams ocean-island and continental basalts form only one field (WPB) that is the field of within-plate basalts, since they cannot be separated on the basis of their major element chemistry. This fact may prove that the nature of the crust, oceanic or continental does not modify these basaltic rocks by chemical interaction of the magma and wall-rocks.
