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
Fig. 8. Alkali-silica diagram after MACDONALD & KATSURA (1964). Dots: diabases, Crosses: gabbros; X: soda gabbros. For other symbols and explications see: Fig. 4 cause of the low concentrations in MgO. In this respect they are very similar to the amphibole and biotite andésites and even to the dacites with which they are intimately associated. However a primary and probably an undifferentiated nature cannot be categorically excluded for these magmas since experimental work by NICHOLLS (1974), KUSHIRO (1974) and others indicate that hydrous peridotite can yield andesitic melts at water pressures of 10-15 kbar. At the same time andésites and dacites can be considered as partial melts derived from eclogite or amphibolite (subducted oceanic crust; GREEN & RINGWOOD 1968; GREEN 1972). The chemical composition of the basaltic andésites from different volcanic mountains are rather homogenous and CIPW-norms calculated on the basis of mean values reveal a hy-qu characterized quartz tholeiitic composition (Table 3). Tectonic implications It has been suggested, that present-day geophysical peculiarities (high heatflow-values Moho discontinuity and LVZ in elevated position, gravitatioal anomalies etc.) of the Pannonian Basin may be explained if we suppose the presence of a thermal mantle diapir below this territory. According to STEGENA et al. (1975) and GÉCZY et al. (1975) the mantle diapir is a consequence of the late Oligocène —early Miocene subduction of the oceanic basement of the Carpathian flish trough. In the late Miocene and especially during Pliocene times the Pannonian Basin was formed by subsidence. At the same time the earlier compressional tetonism gave way to an essentially extensional tectonic regime. Thus, the youngest tectonic evolutionary stage of the Pannonian Basin is similar to that of continental rift zones, such as Auvergne and the Rhein Graben in Europe or the Baikal district in Siberia. The presence of primitive peridotite nodule-bearing alkali basalts with marked WPB-affinities are ubiquitous in the rift zones, the generation of which must be connected with the mantle diapirism. Some features of the lherzolite xenoliths and especially an amphibolite /lherzolite composite nodule seem to support this suggestion (EMBEY-ISZTIN 1976a, 1977, 1978). The basaltic (pyroxene) andésites along with other andésites dacites and rhyolites could have been formed during the Miocene due to the subduction mentioned above. The SHOaffinities of basaltic andésites may be in accordance with this suggestion. Since the oceanic or suboceanic area subducted could not have been too large in the case of the Carpathian arc, judging from geological conditions, magmatic evolution may have reached a mature stage characterized by SHO-affinity very soon or else magmas of earlier stages with LKT and CAB-affinities were completely lacking.
