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)

Examining statistically the regularities of twinning, it can be concluded that these gabbro inclusions are characterized by Gorai's A-twins (albite +acline), only in the molten and recrystallized parts do C-twins (Carlsbad, albite-Ala) occur in significant quantity. Even if because of very thin lamellar twinning only a part of the secondary plagioclases could be measured, the results of measurements of the fifty secondary plagioclases of sample Bo-3009 largely deviate from the case of the primarily crystallized feldspars of the gabbros. In Gorai's A-C-U triangle (A-twins, C-twins and untwinned plagioclase) the primary plagioclases lie clearly in the metamorphic field, whereas the secondary feldspars of sample Bo-3009 have got already nearer to the field of eruptive, i.e. volcanic rocks (Fig. 2). An interesting comparison can be made with the gabbro from the Bükk Mountains which we analyzed in an earlier case (NOSKE-FAZEKAS & EMBEY-ISZTIN 1978). According to universal stage measurements, the plagioclase of the gabbro exposure of Tardosbánya proved there to include A-twins sharing 46%, while the pegmatitic gabbro of Tóbércbánya •contained considerably fewer (33%) A-twins. Consequently, the bulk of the twin crystals were identified as C-twins, and so, in terms of twin statistics, the gabbro in the Bükk Mountains is a real subvolcanic rock, whereas the gabbro xenoliths being discussed here contain plagioclases that are much more metamorphic in character. Summarizing the above discussion, let us conclude that the twin law frequency distribution pattern of the gabbro xenoliths brought up by the alkali basalt from great depth (probably from the lower crust) is of metamorphic character. The same character is suggested by those parts of the texture of these rocks, in which an equilibrium crystallization took place (120" triple point junctions). Other parts of the texture, however, are suggestive of a magmatic crystallization. The contradiction may probably be eliminated, if it is assumed that these rocks crystallized from a melt, but the very high hydrostatic pressure characteristic of the lower crust resulted in an added metamorphic pattern produced quasi "syntectonically". A crystallization under high pressure seems to be suggested also by the fact that the primary plagioclases are of intermediate, the secondary ones, however, of basic character, as the pressure is known (SMITH 1958) to enhance the incorporation of the albite molecule in the plagioclases. The twinning law frequency and higher anorthite content of the secondary plagioclases is indicative of a typical quench crystallization which appears to have resulted from the incorporation of the gabbro fragment in the basalt magma (its heating) and from the sudden drop of pressure during eruption. References BURRI, C. PARKER, L. & WENK, E. (1967): Die optische Orientierung der Plagioklase. — Basel: 334. pp. EMBEY-ISZTIN, A. (1976a): Amphibole/lherzolite composite xenolith from Szigliget, north of the Lake Balaton, Hungary. — Earth Planet. Sei. Lett., 31: 297-304. EMBEY-ISZTIN, A. (1976b): Felsőköpeny eredetű lherzolitzárványok a magyarországi alkáli olivinbazaltos bazanitos vulkanizmus kőzeteiben. (Lherzolite nodules of upper mantle origin in the alkali olivine basaltic, basanitic rocks of Hungary.) — Földt. Közi., 106: 42-51. EMBEY-ISZTIN, A. (1978): On the petrology of spinel lherzolite nodules in basaltic rocks from Hungary and Auvergne, France. — Annls hist.-nat. Mus. natn. hung., 70: 28-44. GORAI, M. (1951): Petrological studies on plagioclase twins. — Amer, Mineral., 36: 884-901. NOSKEFAZEKAS, G. & EMBEY-ISZTIN, A. (1978): New data to the minerology of the basic intrusion around the village Szarvaskő, Hungary. — Annls hist.-nat. Mus. natn. hung., 70: 13-25. SMITH, J. V. (1958): Effect of temperature, structural state and composition on the albite. pericline and acline-A twins of plagioclase feldspars. — Amer, mineral., 43: 546-551. SMITH. J. V. (1974): Feldspar Minerals. — New York: 690 pp. Author's address: DR. GABRIELLA NOSKE-FAZI KAS Mineralogical Department Hungarian Natural History Museum Budapest VIII. pf. 330 H-1370