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

that the spinel inclusions are not evenly disseminated throughout the entire rock that would be expected in the case of such an intensive recrystallization process that is implied in the secondary cycle textures. Neither are spinels even grained : they vary between 1 mm and a few hundredth of millimetre in their longest dimension. These features are clearly more understandable if igneous crystallization is assumed. Plate II, Fig. 2 shows for example three quasi euhedral spinel grains situated side by side. Their resemblence to spinels of igneous cumulates is striking. Silicate grains have mostly gently curving but rarely straight grain boundaries. Generally they show no or little evidence of stress, but in some nodules an incipient stronger tectonization is evident. NIELSON PIKE & SCHWARZMAN (1977) also interpreted some coarse-grained peridotite nodules as possible candidates for a magmatic origin. In Auvergne, Massif Central, France 15% of the 70 peridotite xenolith occurrences were estimated to contain nodules with poikilitic and coarse-grained tabular textures of possible magmatic origin by COISY & NICOLAS (1978). It seems thus probable that the coarse-grained spinel inclusion-bearing xenoliths are magmatic in origin. Independent chemical data are however needed that may help to solve the problem (EMBEY-ISZTIN et al. in prep.). Nógrád basaltic area In the Nógrád basaltic area only very small peridotite nodules can be found in compact basanites. From a few of them thin sections were prepared that show that porphyroclastic and equigranular texture types may be frequent. A porphyroclastic nodule from Magyarbánya (Mb-1001) carries large orthopyroxene porphyroclasts with beautifully developed clinopyroxene exsolution lamellae. Textural properties of the black pyroxene (Al, Ti-aiigite) rich suite xenoliths A general characteristic of these rocks is that they only very occasionally show strain effects in their texture. The pyroxenites are coarse grained, the gabbroic nodules are fine-grained rocks. Clinopyroxenite Szg-3005 has a typical poikilitic texture, with very large (several cm) clinopyroxene crystals poikilitically enclosing rare rounded olivine and more abundant subherdral and euhedral spinel crystals that are unevenly distributed within the clinopyroxene forming spinel-rich clusters. Most of the clinopyroxenites are however almost monomineralic containing very small amounts of accessories. Inclusions of gabbro at Szigliget and Bondoró-hegy are about three times more abundant than clinopyroxenites. They are composed of varying proportions of clinopyroxene and plagioclase with small amounts of opaque grains. In some cases additional accessories may also be associated such as amphibole, apatite, sphene and phlogopite. All the gabbroic xenoliths show more or less pronounced pyrometamorphic evidences such as beginning of melting in the exterior parts of the crystals. Basaltic liquids often penetrated these rocks along narrow cracks replacing large parts of the nodules by quench products (elongated laths of plagioclase, tiny clinopyroxene and opaque crystals as well as glass). The original texture of the gabbroic nodules is more or less even grained (average grain size is ~ 1 mm) At closer examination, however, it became evident that even within the area of a thin section there are parts composed of larger cl inopyroxene and pi agioclase crystals that have rounded and curvilinear grain boundaries while in other parts of the same rock smaller grains of plagioclase and clinopyroxene tend to have rather straight boundaries forming 120° triple point junctions (Plate III, Figs. 1-2). The first part is rich in clinopyroxene, while the second in plagioclase. Effects of stress are totally absent in both parts. Macroscopic examinations of whole nodules clearly indicate the presence of alternation of pyroxene and plagioclase rich bands resembling igneous layering. Finally, some words should be mentioned about the rare but genetically very important composite xenoliths. One of them, an amphibolite/lherzolite composite xenolith (Fig. 4) was treated in detail (EMBEY-ISZTIN 1976). But there are several others similar to those described from world wide localities (e. g. IRVING 1980). The inclusion Szt-1069 e. g. has a narrow Al-augite+spinel vein with irregular apophyses traversing normal Cr-diopside bearing lherzolite (Fig. 4). The composite xenolith Szg-3007 is a remarkable one. It contains numerous rounded and angular fragments of Cr-diopside, Cr-spinel lherzolite set in a "matrix" composed of black, Al. Ti-augite, subordinate opaque and amphi-