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

enstatites and Szg-1012 also of amphibole. It is true that this texture resembles closely the equigranular mosaic subtype of MERCIER & NICOLAS (1975) and spinel was considered to form small spherical inclusions in all of the silicate minerals at the end of the primary tectonic cycle by the same authors, there are however features that they have in common with some coarser-grained Szigliget and Szentbékálla nodules that can perhaps be interpreted to have an igneous origin. Such features are relatively frequent occurrence of subhedral silicate grains and especially spinels (sometimes euhedral in the case of the last one), general low grade of tectonization and a strong tendency of disagregation of clinopyroxene grains probably by partial fusion which cannot be observed in nodules of other texture types. The interpretation of the coarser-grained variety of this texture type is also problematic (e. g. Szg-1033, 1059, 1070, 1083). The grain size is important, 4-6 mm or even larger olivines and enstatites are frequent. Since spinel forms inclusions in silicate minerals, these rocks may theoretically be interpreted as having annealed secondary protogranular texture similar to that described by MERCIER & NICOLAS (1975). In this case they should have experienced a highly complex tectonical evolution, therefore, it seems more probable that at least a part of these rocks may have had a more simple tectonic history, that is a magmatic crystallization followed by eventual textural changes due to stress effects of varying degrees. For example, inclusion Szg-1070 containing euhedral spinels is highly stressed with undulatory extinction and incipient polygonization at the borders of the large silicate grains. Gerce The textural properties of the Gerce peridotite nodules are more uniform than those of inclusions originating from other Hungarian localities. This is also the only place where other types of nodules (e.g. pyroxenites, gabbros) are completely lacking. Spinel peridotite inclusions can be found here in a great number within a layered tuffring situated east from the village Gerce (Fig. 1), but only in one of the layers, the rest being completely barren. Most of the xenoliths measure 5-10 cm and have the form of an ellipsoid. The overhelming majority (81%) of these peridotites have porphyroclastic texture (e.g. G-1002, Fig. 2). They carry 4-8 mm large enstatite or olivine porphyroclasts showing strain effects and sometimes clinopyroxene exsolution lamellae in orthopyroxene set in a matrix of small unstrained grains of the same minerals. The interstitial spinel is generally holly-leaf shaped. In many of these inclusions, however, porphyroclasts are ruptured and/or polygonized (transitional feature toward the equigranular type). In nodules G-1021, 1026 and 1028 the smaller grains have frequently straight or smoothly curving grain boundaries and they define a weak foliation, so they are really close to the equigranular texture type. A very few nodules may perhaps belong to the secondary cycle texture type of MERCIER & NICOLAS (1975) e. g. G-1009, 1023; but G-1005 (Plate I, Fig. 2) seems to be a unique case. It has a rather equant (1-2 mm) granular texture with a marked tendency to form rational crystal faces. The abundant rounded subhedral, rarely euhedral spinels are either interstitial or form inclusions in the silicate minerals that show no signs of strain effects. It is not easy to imagine indeed that this texture would be the result of a complicated tectonic evolution, in contrast, a primary igneous origin seems much more probable. It is interesting to note in this respect that G-1005 is very poor in Crdiopside just like strongly depleted xenoliths, yet containing a relatively appreciable amount of spinel. This anomaly may also be interpreted in favour of an igneous origin. Electron probe determination of mineral chemistry (EMBEY-ISZTIN et al., in prep.) may help to elucidate the problem. Bondoró-hegy Peridotite inclusions can be found here both in the lower lava flow and the upper scoriacious lava where they often form cores of volcanic bombs. They are generally larger than the Szigliget or Gérce nodules (up to 20 cm). The texture types are rather variable. 25 % of the xenoliths have a more or less typical porphyroclastic texture (e.g. Bo-1064, Fig. 3).