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)

ANNALES HISTORICO-NATURALES MUSEI NATIONALIS HUNGARICI Tomus 77. Budapest, 1985 p. 17-24. Plagioclase twin law statistics of gabbroic inclusions originating from the alkali basaltic rocks of the Balaton Highland (Hungary) by G. NOSKE-FAZEKAS, Budapest Abstract

—Universal stage measurements have been carried out on the gabbroic inclusions of Pliocene basaltic rocks originating from two Hungarian localities. It has been established that both the ratio of Gorai's A- to C-twins and the anorthite content of the plagioclases are different in the gabbroic inclusions of the two localities. This fact, coupled withtextural observations confirm that the gabbroic inclusions of Szigliget experienced a slightly smaller rate of metamorphism than those of Bondoróhegy. With 4 photoplates. The occurrence of gabbroic inclusions among a lot of ultrabasic to basic xenoliths of lower crust to upper mantle origin extruded by the alkaline basalt volcanism in the Balaton Highland area has so far been known only at two localities : on the Bondoróhegy near Kapolcs village and on the hills of Szigliget. In an earlier paper I presentsed the results of a similar study of inclusions from Szigliget, comparing the characteristics and genesis of plagioclase feldspars from the two areas involved (NOSKE-FAZEKAS 1984). Macroscopic characteristics The gabbroic inclusions of Szigliget do not differ very much in size from the lherzolite inclusions occurring here in great numbers (5-7 cm), but they are smaller than their counterparts from the Bondoróhegy which attain even 12 cm in diameter. They constitute a maximum of 10% compared to the number of lherzolite inclusions. Usually uncoated when found on the Bondoróhegy, the ovoid

—shaped gabbroic inclusions in the Szigliget area are often encountered with a lava-coating on. In addition, a few tenths of mm of more or less contiguous carbonate cover the lava coat or the gabbro surface directly (when the gabbro is uncoated). The colour of the gabbro on a fresh surface of fracture is dark grey or a lighter shade of grey, the individual mineral components (feldspars, clinopyroxenes) being quite distinctly differentiated. In some samples vesicles of 1 to 2 mm size are also observable with segregation of carbonate on their surface. Although mineral banding may be observed in one or two gabbros, the gabbroic inclusions here appear to be more homogeneous than it is the case with the inclusions from the Bondoróhegy. Results of microscopic observations The texture of the rocks studied is holocrystalline in all cases, consisting of grains averaging 1 mm in diameter and represented by acidic and intermediate plagioclase, clinopyroxenes and opaque minerals. Secondary minerals are also involved in the constitution of the rock and cavity-filling carbonate, though subordinate in quantity, is also observable (Table 1). 2 Természettudományi Múzeum Évkönyve 1985