Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 77. (Budapest 1985)
Noske-Fazekas, G.: Plagioclase twin law statistics of gabbroic inclusions originating from the alkali basaltic rocks of the Balaton Highland (Hungary)
during re-melting and crystallization, the material from Szigliget did not contain even one sample suitable for measuring secondary feldspar crysts. As evident from Table 2, the twinned feldspars of samples measured with the universal stage largely vary in quantity, being between 11 and 71%. In the light of earlier universal stage measurements of samples from the Bondoróhegy and the textural similarity of gabbroic samples from the two localities, Gorai's A-twins were with good reason expected to abound, being represented, as evidenced by the measurements, in amounts of 73 to 100% in the individual samples. It is surprising, however, that whereas the primary plagioclases of the gabbros in the inclusions from the Bondoróhegy locality (except for the 2% C-twin content of a single sample) were all twins of type A, in the material examined now the amount of the C-twins shows a considerable increase at the expense of the A-twins: a total of 18% in terms of the albite-Ala, Carlsbad, Manebach and Baveno twin laws. Curiously enough, the crystals obeying the A-type albite twin law are represented in almost equal quantity in the two areas (Bondoróhegy 66%, Szigliget 63%). At the same time, in the Szigliget samples the aclinic twins of type A (19%) showed a considerable decrease, in contrast with the 32% amount in the samples from Bondoróhegy. Comparing the anorthite contents of the plagioclases from the two localities, it is obvious from the results that the feldspars of the inclusions from Szigliget are more acidic than their counterparts from Bondoróhegy: the anorthite content of plagioclase feldspars from Szigliget is, on the average for the analyzed samples, by about 5% lower (Table 2). As was also observed when analyzing the gabbros from Bondoróhegy locality, that the points of measurement of these plagioclases on the BURRI —PARKER — WENK diagrammes (1967) coincidence with the high-temperature curves. In all probability, as a response to a subsequent high-temperature impact, the originally low-temperature optics in these samples would change into a high-temperature variant. Table 2. Frequency of different plagioclase twin laws and their average A-contents. Szg-3001 Szg-3003 Szg-3004 Szg-3006 Szg-3008 Twinned crystals % 11 40 71 20 25 Untwinned crystals % 89 60 29 80 75 A-twins % 80 79 80 100 73 C-twins % 20 21 20 — 27 Average An-content % 46 53 46 44 42 In light of the above considerations, I conclude that though the plagioclases of gabbroic inclusions extruded from great depths (probably from the lower crust) by the basalt volcanism at Szigliget and Bondoróhegy, respectively, are similar in textural and optical characteristics, they carry differences that are formulable in numerical terms. As shown even by the results of a twin-statistical comparison, the metamorphic effect to which the samples from Bondoróhegy were exposed during their preceding history was greater than it was in the case with the gabbro samples from the Szigliget locality.
