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

type, others are closer to the prophyroclastic group, so the entire suite of the relatively coarser-grained xenoliths clearly demonstrate a gradually and slightly increasing recrystallization series attesting that the main texture types transfom in each other in a given order that was originally proposed by MERCIER & NICOLAS (1975). The inclusion Szg-1065 is one of those that stay relatively close to the protogranular texture type (Fig. 2). Indeed, at first sight the texture may seem rather protogranular with anhedral and relatively equidimensional grains and curvilinear grain boundaries. However the average grain size is < 2 mm (in most of the cases it is between 1-1.5 mm) and all the classifications consider peridotites as protogranular if the average grain size is > 2 mm. On the contrary Szg-1018 is one of the very rare inclusions having truly protogranular texture (Fig. 2). The average grain size is 3 mm in this case and a part of the spinel grains form inclusions in pyroxenes especially in enstatites resembling the vermiform spinel inclusions that were interpreted as products of exsolution from an alumina-rich orthopyroxene originally formed at a much higher temperature and at a considerably greater depth than the site of incorporation of the xenolith by the basaltic magma (MERCIER & NICOLAS 1975). This implies that even the protogranular texture cannot be regarded as a primordial one because it is also metamorphic in origin. Whether the original texture was a magmatic one and whether it was formed by the intensive melting process in the upper mantle at the very beginning of the Earth' history remains a territory of speculations. Inclusion Szg-1041 is one of those that show stronger prophyroclastic feature (Fig. 2). The average grain size is diminished ( ~ 1.3 mm) due to more intensive polygonization of olivine and enstatite. Some relict olivine and orthopyroxene crystals measuring 3-4 mm surrounded by smaller newly formed grains of the same minerals have to be regarded as real porphyroclasts. However, such porphyroclasts are rare and the relatively even grained nature of these xenoliths distinguishes them from the typical porphyroclastic texture type. It is characteristic of the entire suite of transitional texture type nodules that apart from some exceptions such as inclusion Szg-1025 they were only moderately effected by stress judging from the relatively poor development of kink bands in olivines. Exsolution of clinopyroxene from orthopyroxene is only occasionally observed. Slight foliation and mineral banding may be present in the textures of tectonically more evolved members. The majority of the Szigliget peridotite nodules are fine grained and belong to the equigranular tabular subtype of MERCIER & NICOLAS (1975). The type locality of this texture is in Dreiser Weiher, W. Germany, therefore it was once called "Dreiser Weiher structure". In Harte's terminology it is named "granuloblastic tabular", while Nielson Pike & Schwarzman call it "foliated". Not all of these nodules are typically equigranular, many contains larger relict olivines or enstatites. About every third xenolith of this texture type contains a few grains of brown amphibole, while amphibole could not be observed in the coarser grained nodules, neither is it present in periodite nodules of any other Hungarian locality (except for one small xenolith from the basanite of Magyarbánya, Nógrád District). An amphibolite/lherzolite composite xenolith was also found at Szigliget (EMBEY-ISZTIN 1976). Inclusion Szg-1007 containing amphibole is a good example for the equigranular tabular texture type (Plate I, Fig. 1). It is largely equidimensional (average grain size ~0.7 mm), with straight or smoothly curving grain boundaries that converge often into 120° triple point junctions sometimes occupied by small spherical grains of spinel. The fabric is strong with a remarkably parallel foliation and lineation. Some very rare (4%) Szigliget inclusions (e. g. Szg-1020, Szg-1037) have highly stressed tabular texture which is coarser grained than the equigranular tabular type. This variety may correspond to the granuloblastic tabular group of HARTE (1977). 8 per cent of the Szigliget peridotite nodules have rather problematic textures. A common feature in all of them is that they have spherical and subhedral spinel inclusions in the silicate minerals. A part of them (e. g. Szg-1012, 1013, 1064) are composed of fine-grained mosaic-like olivines and