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
minerals are represented by sericite and illite. The rock occurring in the top of the borehole (1.0-2.0 m) is macroscopically medium grey and hard (sample T-511). Its volcanogenic clastic material consists primarily of oligoclase, of subordinate andesine, sanidine and biotite as well as of volcanic glass with a refraction index higher than that of balsam. The plagioclase crystals are albite- and, incidentally, periclinetwinned, zoned, reflecting partly magmatic resorption, partly crystallization interrupted at sudden cooling. Their size varies between 120 and 800 p., most of them being of 250 to 300 p. size. The sanidine crystals are stubbier than plagioclase, they are markedly altered, being replaced by calcite, chlorite, and quartz. Of the epigenic minerals filling fossil shells within calcareous marls, glauconite and collophane should be quoted. The Lower Ladinian tuffaceous formations of exploratory trench 1 driven near Paloznak village are constituted for the most part by sandy marls, pure trachyte tuff being confined to a single (20 cm) layer. Macroscopically, the rocks of the sequence are of greenish-yellow, dirty white and pale-yellow colour, friable, and alternating. Sample T-605, characterizable by concentration peaks of glauconite and illite, is predominantly ivy green, sample T-604 being constituted by a grey substance. As shown by its mineralogy, the tuff horizon represents potash-trachytic tuff éjecta, the ratio of crystal detritus to rock fragments being variable or, to be more precise, an enrichment of lithoclastic material about the middle of the sequence can be observed. Of the feldspars, alone sanidine is present, mostly in a very advanced stage of alteration (illite, calcite), its average grain size being 200 to 250 p. Because of the advanced alteration state of sanidine, the biotite and, locally also, the quartz show apparent concentration peaks. In the course of alteration, biotite is converted into chlorite and glauconite, ilmenite is transformed for the most part into limonite and leucoxene; the trachytic lava fragments also represent an advanced stage of alteration, as fresh, unaltered matrix and feldspar microlite grains are unfrequent. The tuffaceous sequence exposed on Forrás Hill at Felsőörs is —both macroscopically and on the basis of examination under the microscope—very similar to the tuffaceous formations of Paloznak, i.e. the Felsőörs sequence can also be referred to the occurrences of potash-trachytic pyroclastics. Differences in composition between the closely spaced samples are due to changes in the ratio of the clay minerals deriving from the decomposition of tuffs and to the varying amount of CaC0 3 . As a contrast with the usually more or less vivid light green tuffs there are in the sequence several thin layers of snow-white, pulverulent lime mud of extremely low clay content. In a general characterization like this, it should also be mentioned that among the clay minerals of the Felsőörs occurrence, the wide-spread illite is occasionally accompanied by montmorillonite (sample T-712) and mixed-layered ilhte-montmorillonite; finally, some kaolinite is also present (T-699) as proved by X-ray analyses. In other words, a more advanced stage of clay mineralization appears here. Within the sequence the composition of mineral fragments is rather unvaried and can be characterized in a laconic way as follows: the bulk is constituted by clay-mineralized, calcitized sanidine, the average grain diamètre being 250 to 350 p. Biotite is strongly altered, limonite-stained (in patches) or totally bleached. Quartz occurs sporadically, being totally absent in the middle horizon. The opaque minerals are heavily oxidized, magnetite and ilmenite are scarce; mainly limonite and pyrite,
