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

Embey-Isztin, A.: Texture types and their relative frequencies in ultramafic and mafic xenoliths from Hungarian alkali basaltic rocks

The fraction of spinel peridotite xenoliths with probable igneous texture is small except for Szentbékálla where it is important (19.5%). In comparison 15% of the 70 localities of Auvergne produced xenoliths with textures that may be interpreted magmatic and the mean fraction of these texture types amounts to 13% (COISY & NICOLAS 1978). Only a minority of these nodules in Hungary show strain effects, most of them seem to have escaped any kind of tectonization. The great majority of the black, Al, Ti-rich augite pyroxenites are not tectonized either and the most probable origin for them is a magmatic cumulus one within the upper mantle. The textural properties of the gabbroic inclusions are equivocal since the coarser-grained clinopyroxene-rich parts of these rocks having curvilinear grain boundaries suggest rather magmatic, while the finer-grained, plagioclase-rich parts where straight line boundaries with 120° triple point junctions prevail are rather indicative of equilibrium crystallization. The large fraction of untwinned plagioclase crystals and the absence of Gorai's C-twins are also characteristic of metamorphic conditions (NOSKE-FAZEKAS in this volume). Perhaps the most probable interpretation is an igneous origin followed by a partial metamorphic re-equilibration. This metamorphic overprint may have occurred as early as the final stages of the consolidation of the basaltic liquid perhaps due to increasing pressure on the magma chamber situated somewhere in the lower parts of the crust. References ÁDÁM, A. (Ed. in chief) (1976): Geoelectric and geothermal studies (East-Central Europe, Soviet Asia). — KAPG Geophysical Monograph, Akadémia Kiadó, Budapest. AVE LALLEMANT, H. G. & CARTER, N. L. (1970): Syntectonic recrystallization ot olivine and modes of flow in the upper mantle. — Geol. Soc. Am. Bull., 81: 2203-2220. BALOGH, K., JÁMBOR, Á., PARTÉNYI, Z., RAVASZ-BARANYAI, L. & SOLTI, G. (1982): A dunántúli bazaltok K/Ar radiometrikus kora. (K/Ar radiometric age of the Transdanubian basalts). — M. Áll. Földt. Int. Évi. Jel. 1980 p. 243-260. BEST, M. G. (1974): Contrasting types of chromium-spinel peridotite xenoliths in basanitic lavas, Western Grand Canyon, Arizona. —• Earth Planet. Sei. Lett., 23: 229-237. BOIVIN, P. A. (1982): Interactions entre magmas basaltiques et manteau supérieur: Arguments apportes par les enclaves basiques des basaltes alcalins. Examples du Deves (Massif-Central Français) et du volcanism de Carthagene (Espagne). — Ph. D. Thesis, University of ClermontFeirandll.,p. 1-344. BOULLIER, A. M. & NICOLAS, A. (1973): Texture and fabric of peridotite nodules from kimberlites at Mothae, Thaba Putsoa and Kimberley. — NIXON, P. H. (ed.) : Lesotho Kimberlites : Lesotho National Development Corp. p. 57-66. BOULLIER, A. M. (1975) : Structure des peridotites en enclaves dans les kimberlites d'Afrique du Sud. — Ph.D. Thesis, University of Nantes,p. 1-112. BROWN, G., M., PINSET, R. H. & COISY, P. (1980): The petrology of spinel-peridotite xenoliths from the Massif Central, France. —Am. J. Sei., 280-A: 471-498. CARTER, J. L. (1970): Mineralogy and chemistry of the Earth's upper mantle based on the partial fusion—partial crystallization model. — Geol. Soc. Am. Bull., 81: 2021-34. COISY, P. & NICOLAS, A. (1978): Regional structure and geodynamics of the upper mantle beneath the Massif Central. —Nature, 274: 429-432. COLLÉE, A. L. G. (1963): A fabric study of lherzolites with special references to ultrabasic nodular inclusions in the lavas of Auvergne, France. — Leid. geol. Meded., 26: 1-102. DEN TEX, E. (1969): Origin of ultramafic rocks, their tectonic setting and history: A contribution to the discussion of the paper "The origin of ultramafic and ultrabasic rocks" by P. J. Wyllie. — Tectonophysics, 7: 457-488. DICKEY, J. S. (1968): Eclogitic and other inclusions in the mineral breccia member of the Deborah volcanic formation at Kakanui, New Zealand. —Am. Min., S3: 1304-1319. EMBEY-ISZTIN, A. (1976): Amphibolite/lherzolite composite xenolith from Szigliget, North of the Lake Balaton, Hungary. — Earth Planet. Sei. Lett., 31: 297-304. 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: 27-44. Annls hist.-nat. Mus. natn. hung.. 76. 1984