Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 73. (Budapest 1981)
Embey-Isztin, A. ; Noske-Fazekas, G.: On the chemistry of the large phenocrysts in the tuff of Godóvár (Börzsöny Mts., Hungary)
ANNALES HISTORICO-NATURALES MUSEI NATIONALES HUNGARICÏ Tomus 73. Budapest, 1981 p. 9-3 E On the chemistry of the large phenocrysts in the tuff of Godóvár (Börzsöny Mts., Hungary) by A. EMBEY-ISZTIN & G. NOSKE-FAZEKAS, Budapest Abstract — Large phenocrysts of clinopyroxene, amphibole in the Godóvár tuff show normal chemical zoning with mg-values decreasing from the center towards the border zones. Clinopyroxenes exhibit a primitive Cr-diopsidic (Mg 48 Fe 7 Ca 45 ) core which with a sudden change in the composition is surrounded by an augitic rim (Mg 44 Fe 14 Ca 42 ). No such abrupt change can be observed in amphibole crystals which are typical pargasites and only the outermost zones may pass into the magnesio-hastingstie field. It has been argued that the Cr-diopside may have separated from a basaltic liquid at depths of about 20 km, while the other pyroxene zones with the pargasite and plagioclase crystals accumulated in a storage chamber at a depth of about 12-15 km. With 4 figures and 6 photoplates. Introduction — The Börzsöny Mts. is a member of the intra-Carpathian volcanic chain. It is a typical strato-volcano, at least in the northern part of the mountains which is composed of andésite lava flows and tuffs (PANTO 1970). The age of the volcanic activity is Miocene (~ 15 m.y.). Besides andesitic rocks, more acid dacites are also participating in the building of the volcanic region but basic rocks are typically absent. Petrography Among the volcanic rocks, the tuff of the Mount Godóvár in the central part of the Mountains distinguishes itself by containing numerous large phenocrysts of amphibole up to 2 cm long, smaller pyroxene up to 0.8 cm long and even smaller plagioclase crystals up to 1-4 mm long. Because of the considerable frequency of the crystals, the rock of Godóvár may well be designated as a crystal tuff (Plate 1 : 1,2). The crystal tuff was recorded firstly by LIFFA & VIGH (1937) but it has never been investigated petrographically. Macroscopically the amphibole is black, whereas in the microscope it is brown with a strong pleochroism (a = yellow, ß = brown, y = dark brown), y/c = 8-18° (at the border smaller). Smaller crystals can be completely euhedral but the larger ones are often subhedral with traces of resorption. In some cases amphiboles are surrounded by an opacitic rim (Plate II, III: 1, 2). In addition, crystals show optical zoning generally with a darker coloured rim overlapping a lighter coloured core. In some cases however, the zoning manifests in a complex manner: darker and lighter coloured bands alternate with each other or more frequently there is only one lighter coloured band sandwiched in dark brown material (Plate IV: 1). The monoclinic pyroxene also exhibits differences in colour between rim and core which is more easy to see macroscopically than in thin section. The core is pale green surrounded by a darker green rim. In thin section, the core is almost colourless and the rim is pale green, y/c = 30-38°, at the border zones a little smaller. Like amphiboles, individual pyroxene crystals show different zoning patterns and this phenomenon is even more sharply accentuated in the latter case. There are e.g. completely or almost completely homogeneous crystals with only a thin darker coloured rim, then in a few cases the small core and the large rim is clearly separated by perfectly formed crystal faces suggesting perhaps that the growing of the crystal was not continuous (Plate IV: 2). In other cases, in addition to the colour zoning, another fine concentrically developed oscillatory zoning is also present which is due to changes in the extinction (Plate V: 1). The last phenomenon may be attributed perhaps to non-equilibrium rapid crystallization. Otherwise, pyroxenes are less corroded than are amphiboles, perfect euhedral crystals are frequent. While in one case, the whole crystal seems to be optically homogeneous, in the other the extinction is undulatory or it reveals. Annls hist.-nat. Mus. natn. hung., 73, 1981
