Szekessy Vilmos (szerk.): A Magyar Természettudományi Múzeum évkönyve 62. (Budapest 1970)

Szabó, I. ; Ravasz, Cs.: Investigation of the Middle Triassic volcanics of the Transdanubian Central Mountains, Hungary

Conclusions With a view to the thickness and extension of the tuffaceous formations, to the distance between the occurrences and, finally, to the degree of alteration of analysed samples and their mixing with calcareous sediments, the authors—abstaining from any generalization —draw the following conclusions. In the territory of the Transdanubian Central Mountains, explosive volcanism began in the Middle Triassic Late Anisian time and can be traced to have lasted till the end of the Early Ladinian. No lava product of this volcanic cycle is so far known to occur in the territory under consideration. Petrographically, the overwhelming majority of the pyroclastic material can be ranked among the trachytic tuffs (Upper Anisian and Lower Ladinian), representing their extremely K-rich varieties. Pumiceous rhyolitic tuffs of stratigraphically higher (Lower Ladinian) position,, marking the completion of the cycle, occur in comparatively smaller amount. In other words, the final products of Mediterranean volcanism of alkali (K) type grade into calcalkalic volcanism. This igneous activity belongs to the initial phase of Alpine orogeny which in' the Central Mountains Mesozoic is represented mainly by basic, in smaller measure by acide magmatites (Bükk Mountains). That the magmatites, in the Transdanubian, Alpine, sedimentary basin of the Central Mountains are of strikingly alkalic (K) nature, very different from the above, seems to be accounted for by the rock type of the crystalline basement. The Hungarian—Mesozoic sedimentary basins were formed in the NE-SW—trending grabens of earlier crystalline basements and the volcanic rocks under consideration belong to the type of alkalic volcanism of the continents. Pétrographie evidence concerning the crystalline basement of the Transdanubian Central Mountains is still scant. Nevertheless, it is known that the Mesozoic mass of the sector being considered of the Transdanubian Central Mountains can be traced even in outcrops on the NE, being in contact, along the SWtrending line of Lake Balaton, with the granitic basement. Out of the numerous hypotheses concerning the origin of continental alkalic magmatites, it is in the first place the theories developed by TURNERVERHOOGEN and MARINELLI-MITTEMPERGHER that have been taken into consideration. As for the origin the well-known Italian alkalic magmatites, MARINELLI and MITTEMPERGHER advocate their anatectic origin; more precisely, they consider them to be a filtration differentiate of the anatectic (sialitic) magma. TURNER and VERHOOGEN suggest that the K-rich magmas may have been produced by an assimilation reaction due to the interaction of a normal basaltic or nephelin-basaltic magma with the granitoid rocks of the continental crust. The last-mentioned theory seems to apply in the studied territory with reservation of wiew-point namely,that the reacting rock masses have resulted contamination rather than assimilation. Irrespective of the theories concerning the origin of magmatites, it is a matter of fact that the relationship between the granitoid basement and the alkalic nature of the present detailed magmatites is the only potential explanation for the contrast with the ophiolitic nature of the initial magmatism of the surrounding Alpine sedimentary basins. Easily mobilizable elements characteristic of granitoid rocks,K, Ba, U, Th, are intensively enriched in this hybrid magma. Apatite and zircon are radioactive, as evidenced by the pleochroic halo of apatite and zircon inclusions in biotite as well as by the frequent occurrence of the metamict variety (Table 1). The pyroclastic nature of magma products is the result of an interaction of the still water-rich sediments of the geosyncline with the magma. The grain size and