Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 76. (Budapest 1984)

Noske-Fazekas, G.: Universal stage measurements of plagioclase feldspars originating from gabbroic inclusions of the Bondoró-hegy alkali basalts (Hungary)

primary crystallization of the gabbros no penetration twin was formed, which are observable in a low number only on plagioclases of secondary crystallization origin (Plate III: 2). The anorthite content of plagioclases belonging to the ./zrí* generation is rather uniform, varying from 50-55%, though minor deviations from this figure do occur (Table 2). Albite twin law typically prédominantes, acline twins being present in a significant number (Fig. 1). In addition to the above, I was able to identify only in two cases a complex twin law (albiteala twins) on plagioclases of primary crystallization. Secondarily crystallized plagioclases are characterized by an average An content of 70 to 75%. Their most frequent twin law is similarly the albite one (72%). At the same time the quantity of acline twins is strikingly reduced (5%). An additional characteristic feature is the marked increase of the amount of parallel (Carlsbad) and complex (albite-Carlsbad) twins (~20%) (Fig. 1). Albite-Carlsbad twinned plagioclase crystals are certainly greater in number than measured, but because of the low size of the crystals and because of the presence of very thin lamellae many supposedly complex twins could not be measured. Strikingly enough, the points of measurement of the primary plagioclases of gabbros on the BURRT-PARKER-WENK diagrams (1967) fall on the high heat level migration curves. Upon heating, the low heat level optics in these inclusions seems to have changed into a high heat level variant. Unfortunately enough, because of the high anorthite content of secondarily crystallized lathed plagioclases, to distinguish between high and deep heat level migrations is impossible, as in this range of values the curves will practically coincide. Fig. 1. Frequency of the different twin laws in the plagioclases of the gabbro inclusions of the Bondoróhegy basanite. Key: 1 = Albite, 2 = Acline, 3 = Carlsbad, 4 = Albite-Carlsbad, 5 = Albite-Ala. — Fig, 2. Plotting of twin statistics in the A-C-U triangle of GORAI (1951). Key: 1 = Feld of metamorphic plagioclases, 2 = Feld of magmatic plagioclases, 3 = Primary plagioclases from gabbroic inclusions, 4 — Secondary plagioclases of inclusion Bo-3009, 5 = Plagioclases of Tardos-bánya, Bükk Mountains, 6 = Plagioclases of Tóbérc-bánya, Bükk Mountains, Hungary.