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)

a mosaic structure. This astonishing variety in zoning and fine structure suggests highly complex and variable physicochemical conditions during the growing of the crystals. Among the plagioclases, besides broken grains completely formed euhedral crystals are highly frequent. In general, they are often strongly zoned optically, suggesting non-equilibrium and relatively rapid crystallization. The matrix is composed of dust glass, microlites and opaque grains, the latter is frequent also as larger crystals set in the ground mass. Occasionally lithic fragments also contribute to the composition of the rock. Regarding textural evidence, the order of crystallization of the different mineral constituents cannot be easily deciphered. For example, inclusions of amphiboles in clinopyroxene seem to be much more frequent than the reversed case (it is true however, that it can rather be attributed to a later amphibolitization from morphological features) and in a very restricted cases if amphibole and monoclinic pyroxene are juxtaposed the latter seems to be older judging from the morphology of their borders. More often however, the contact between the two minerals is irregular and no oriented intergrowth can be deciphered, suggesting simultaneous growth of the grains at least during a major period of their development (Plate VI: 2). In addition, there is evidence for the ulterior growing of the dark green rim of the clinopyroxenes and the dark brown rim of amphiboles because in the case of juxtaposed amphiboles and pyroxenes these border zones have never been developed between the two minerals. There can be no doubt however, as to the ulterior growing of amphibole in a few cases, when corroded clinopyroxene is overgrown by amphibole (Plate V: 2). Some pyroxene crystals contain abundant opaque inclusions (Plate VI: 1), while amphiboles are relatively inclusion free but rarely they contain irregular opacitised parts with associated plagioclase probably due to a prior melting of small size and subsequent crystallization at shallower levels. The position of the plagioclase in the crystallization order is also difficult to establish. Rarely amphiboles may include plagioclase but preferentially in the border zones. In a cognate xenolith of small size, small subhedral opaque oxide bearing pyroxenes are surrounded by great subhedral amphiboles and plagioclases and the whole is surrounded again by other plagioclase laths. From this, the following order of precipitation seems to be the most probable: oxides, clinopyroxene, amphibole, plagioclase. However overlapping between the crystallization intervals of pyroxene and amphibole as well as of amphibole and plagioclase is very likely. Fig. 1. Composition of the clinopyroxenes in the triangle of Poldervaart and Hess. A compositional gap between the diopsidic core and the outer zone auigitc pyroxenes is evident. For comparison, pyroxenes from andesitic lavas of the Börzsöny Mts. (partial microprobe analyses of PANTO, 1970) have also been plotted. 1 = outer zone augitic pyroxene; 2 = diopsidic core; 3 = augites from andesitic lavas