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

Relatively many of the Bondoróhegy inclusions (25%) have fine-grained true equigranular tabular texture (average grain size—

0.6 mm). An excellent foliation and lineation may also be observed (e.g. Bo-1061, 1115). A few of the inclusions (7%) are coarse grained (>2 mm) with spinel) inclusions in the silicate minerals (e. g. Bo-1026, 1028, 1117). Their characteristics are the same as in the Szentbékálla and Szigliget nodules of similar nature. Their textures are interpreted therefore either as secondary protogranular or more probably igneous. Inclusions Bo-1021 and Bo-1055 have unique textures among the Bondoró-hegy xenoliths. Bo-1021 has protogranular characteristics but it is highly strained, Bo-1055 is similar but it is much more recrystallized with a marked foliation and lineation. The elongated olivine grains (1.5 4 mm) have partly straight boundaires and resemble those of the granuloblastic tabular type xenoliths described above. Szentbékálla Large nodules of peridotite (up to 25 cm) are very frequent at this locality. The majority of the Szentbékálla xenoliths (67%) have a medium-grained transitional texture that lies between the protogranular and porphyroclastic types of MERCIER & NICOLAS (1975) forming a slightly increasing continuous recrystallization series. A few nodules (e.g. Szt-1002, 1012, 1025; Fig. 3) may be classed in the protogranular texture type in the strict sense of the word, while typical porphyroclastic texture seems to be absent just like at Szigliget. The protogranular nodules with an average grain size of 2-2.5 mm have curvilinear grain boundaries and the equidimensional olivines and enstatites show only slight development of kink bands. Exsolution of clinopyroxene in orthopyroxene and the reversed case could not be observed here. Spinel is generally interstitial but it can fairly often be found in the vicinity of ortho- and clinopyroxenes and rarely as vermiform inclusions in pyroxenes. With increasing tectonization the average grain size diminishes: in most of the nodules of this series it is between 1.2-1.5 mm. Straight grain boundaries due to polygonization also appear such as a weaker fabric. These are clearly porphyroclastic features but grains remains rather equidimensional and it is difficult to find large crystals that would be surrounded solely by small newly formed minerals (e.g. Szt-1015, 1031, 1060; Fig. 3). A few of these nodules show stronger strain effects. 13% of the Szentbékálla peridotite xenoliths are fine-grained (exceptionally medium-grained), largely equidimensional and polygonal. Grain boundaries are straight, 120° triple point junctions predominate. Small spherical spinel grains often occupy these triple point junctions, however they are present as tiny inclusions in the silicate minerals as well, that form mosaic or tabular aggregates. Mineral banding (lineation) is well developed, foliation is not so marked as in the case of the equigranular tabular texture type of Szigliget and Bondoró-hegy. However, these nodules are very similar to other fine-grained xenoliths of Szigliget containing also small mostly spherical, rarely subhedral spinels as inclusions in the silicate minerals including the fact that a large part of the clinopyroxene originally present in the peridotite was removed and replaced by a probable process of partial fusion. The most probable interpretation is that the texture is an equigranular one and spinels suffered inclusion in the silicates by intensive recrystallization in response of the strain field (minerals are now totally unstrained). Certain resemblance, however, just like in the case of the Szigliget nodules of similar texture facies to the coarse-grained (igneous?) xenoliths should be emphasized. An unusually high number (19.5%) of the Szentbékálla nodules have coarse-grained (2-4 mm) texture with subhedral, euhedral but also spherical spinel inclusions in the silicates (e.g. Szt-1006, 1010, 1018, 1026, 1034; Fig. 3). In these inclusions clinopyroxene was also very frequently replaced by ill-identifiable secondary minerals of very small grain size just as in the finer-grained texture type treated above. It is very characteristic of this kind of nodules 3 Természettudományi Múzeum Évkönvve 1984