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

coated anatase and rutile have been formed. Magnetite and ilmenite have been altered into limonite and leucoxene, respectively. Present in a small amount, the trachyte lava fragments are heavily stained by iron hydroxide. The volcanogenic rock material of the occurence at Öskü is enriched as fineand medium grained detritus in the yellow, pink, hard calcareous parent rock. The parent rock, containing the tuff material, is not homogeneous either, for dolomitic limestone and calcareous marl and chert-rich calcarenite can be readily distinguished even over hardly a few cm distance. The enrichment of calcite-shelled fossil fragments in thin layers can also be observed. The major part of volcanogenic material is constituted by fragments of sanidine, quartz and biotite crystals, the rest by trachyte lava fragments. In dolomitic limestone, sanidine has an average grain size of 500 p.; quartz, of 400 u,; biotite, of 250 u,; lava fragments, of 300 p.. On the other hand, in biogenic limestones the average size of the crystals is reduced to 60-100 p., that of lava fragments to 120150 (X. Magnetite, limonite, and leucoxene are of accessory amount. The magmatogenic clastic material is poorly sorted, the grains are of partly sharp, angular (quartz, lava fragments), partly poorly rounded (sanidine, biotite), a fact indicating some small-scale, local redeposition, to the wearing effect of which the individual grains responded according to their hardness dependent on composition and degree of alteration. On the basis of its mineralogical composition and chemical analyses, the tuff under consideration is a lithoclastic potash-trachytic crystal tuff (Table 2, sample Ös-1). The material of the tuffaceous sequence — looking of best preservation —of the outcrop at Sóly is of low hardness; it can be crumbled by hand. To the naked eye, only pale-green patches, dirty-white calcareous grains and a netlace of pale-brown streaks can be recognized (sample Vm-33). In thin section the rock can be identified with tuffaceous limestone. Its pyroclastic material is miscellaneous: the amount of crystal fragments is somewhat higher than that of trachytic lava detritus. Because of the advanced state of alteration and large-scale carbonatization, however, the original ratios and amounts of the above cannot be reconstructed for sure. The group of mineral fragments is constituted, in form of isolated rags or in the nuclei of calcite-pseudomorphs, by sanidine as well as by biotite, by few quartz grains, pyrite, goethite and limonite. Most of the lava fragments have been preserved as recrystallized, aggregate structured quartz and iron-hydroxide-stained chalcedonized substances. The deeper level of the Lower Ladinian tuffaceous strata explored by drilling at Pécsely still represents potash-trachytic pyroclastic rock (samples T-514, -513, -512), while the uppermost layer, with its rhyodacitic detritus, marks a transition to calcalkab'c rhyolitic tuffs. As one proceeds from the deeper levels towards the surface, the amount of pyroclastics will gradually decrease, so that calcareous siltstones, sandy marls, and sandy-clayey marls can be distinguished. 15 to 20% of the clastic material are constituted by sanidine, comparatively fresh, unaltered, Carlsbad-twinned, with an average grain diameter of 30 to 40 p.. Slightly rounded, quartz is of mosaic-like structure, its minor part being of magmatic origin, averaging 30 to 40 p. in grain diameter. In addition, worth of mention is the amount of biotite, more or less altered, partly showing a magmatic resorption. As shown by micromineralogical analyses, perovskite is present in a strikingly high quantity; the enrichment of pyrite should also be mentioned. Tuffaceous marls contain plenty of mollusca shell fragments, where, beside the shells, some internal molds constituted by pyrite, chalcedony and, in low amount, by collophane, may also occur. Cementing clay