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

crystals. These are albite, Carlsbad and pericline twins or monocrystals. Chloritized, resorbed biotite can be encountered in small quantity. Coarse-crystalline calcite and chert have replaced a part of the crystals. Pumice and volcanic glass, about 800 to 1000 [i in diameter, are heavily altered into illite. Progressing carbonatization and silicification will replace these, too. Limonitization of minor magnetite grains is also frequent. Viewed macroscopically, the Lower Ladinian tuffaceous layers exposed in road-cuts on Öcs-Hill at Balatonakaii show advanced state of alteration (sample Ba-312, -314). The rock of better preservation is a slightly granular calcareousargillaceous tuff of pale-green to pale-brown colour and of medium hardness; the more altered variety is a pale-green to pale-brown tuffaceous marl with dirty-white mottles. Among the volcanoclastic constituents glassy trachyte lava, vesicular volcanic glass fragments and pumice are abundant. Illitization, montmorillonitization and limonitic staining are rather advanced. Grain size varies between 250 and 400 u.. Of crystal fragments it is only sanidine that can be identified for sure, its grain size being very variable (120-500 u,). On a few specimens Carlsbad-twinning is still recognizable. Sporadically, the presence of some largely altered plagioclase is also probable. Quartz is represented by a few grains only, and even these are partly secondary. It belongs to the characterization of the rock, that rhombohedral, limonite-rimmed calcite is also present in large quantity. On the opaque minerals, only limonite is present in considerable amount, magnetite, pyrite, and leucoxene occurring sporadically in the rock. On the basis of the above, the rock can be identified with a younger member of the Lower Ladinian tuff formation, representing a transition between trachyte and rhyolite. During the investigations carried out in the Balaton Highland, a Triassic tuffaceous limestone xenolith was found in Pliocene basalts near Szentbékkála (sample Sztb-27). Most of the volcanoclastic material is lava fragment of trachytic composition; recrystallized and partly altered volcanic glass fragments are abundant; the ratio of crystal detritus is subordinate. The average grain size of detritus varies between 250 and 350 u,, the grains are sharp-edged, angular. The glassy matrix of trachyte lava has been montmorillonitized ; the microlites of acid plagioclase are rather fresh, unaltered, magnetite has been converted into limonite. Most of the scarce crystal detritus are represented by comparatively fresh, unaltered, haloic biotite of 180 to 350 p size and of pale-brown to brown pleochroism. Quartz is limpid, concave in outline, scarce, averaging 250 p in diameter. Beside limonite,. some magnetite and sporadical zircon can be encountered. On the basis of the shape and size of the calcite crystals of the country rock some conclusions can be drawn as to post-feldspar pseudomorphs. For lack of additional information, the xenolith can be termed just: vitro-lithoclastic tuff, any more precise identification being avoided. In the authors' opinion, the significance of the xenolith consists in the fact that it records the presence of Middle Triassic tuffaceous formations in a locality situated far away from the investigated area, in tectonically deeper-seated strata..