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 young absolute age of the basaltic volcanism itself justifies the idea that correlation should exist between the tectonic picture evaluated from the investigation of the textures of xenoliths and the present day geophysical pecularities of the Pannonian Basin. Statistical investigations of textures based on a large number of green pyroxene-rich peridotite inclusions are very rare with a major exception of the Auvergne (Massif Central, France) volcanic province. Here, COISY & NICOLAS (1978) as well as BROWN et al. (1980) investigated 1650 collected xenoliths from which 850 had been sectioned for deformation analysis by P. Coisy. The 315 sectioned Cr-diopside, Cr-spinel peridotite xenoliths constituting the basis of this work may seem modest, but it should not be forgotten that they originate from only four volcanic vents (70! in the case of Auvergne). The statistical study of the different texture types in Hungarian nodules confirms the fact underlined by MERCIER & NICOLAS (1975), and COISY & NICOLAS (1978) according to which a single volcanic vent usually produces xenoliths of one dominant texture type, accompained by a smaller number of inclusions in continuous textural relation with it. Thus, at Szentbékálla the dominant texture type is a transitional one between the ideal protogranular and porphyroclastic textures and in the xenoliths of Gérce porphyroclastic texture type predominates. However, in the area of Szigliget in spite of the predominence of equigranular tabular texture, a large number (31%) of coarser-grained transitional (protogranular-porphyroclastic) nodules does occur. Moreover, as typical porphyroclastic texture seems not to be represented among the Szigliget nodules, the finer- and coarser grained xenoliths should be regarded as isolated from each other in the sense of a tectonic evolutionary cycle. This may mean that besides the more or less regular changes of the texture, that is the tectonic state in the mantle depending on the horizontal location of the volcanic vent in respect to the mantle diapir demonstrated by COISY & NICOLAS (1978) important vertical inhomogeneities may also be revealed if a large number of xenoliths is considered to be originating from a single volcanic vent. Geophysical especially seismic (POSGAY 1975) and heat flow measurements (ÁDÁM 1976) have revealed the presence of a mantle diapir with an abnormal uppermost mantle below the Pannonian Basin. However, as it can be seen in Fig. 5 two rather than a single diapir may be located beneath Transdanubia. The negative correlation between the surface of Moho and the topography is evident : below depressions the Moho is in an elevated position, while under the mountainous region of Bakony the crust has a maximum of thickness. It is interesting to note that volcanic vents tend to be located rather more over inflection points of the Moho than over maximums. As it was proposed by WILSHIRE & PIKE (1975), EMBEY-ISZTIN (1976) diapiric movement of mantle material may provoke partial melting and the formation of composite rocks such as the Szigliget amphibolite/lherzolite composite xenolith may be the result of invasion of certain mantle regions by more or less modified partial melts. It should be emphasized, however, that the liquids trapped in the upper mantle have nothing in common with the alkali basalt that have brought the inclusions to the surface. The compound inclusion Szg-3007 containing spinel peridotite fragments of very different texture types in a "matrix" of clinopyroxenite should be considered as an evidence for important transportation processes of solid particles by basaltic liquids for considerable distances within the mantle. There is an interesting correlation between the diopside+spinel depletion and the grade of recrystallization in the case of the dominant texture type xenoliths of Szentbékálla. Namely, xenoliths close to the porphyroclastic stage tend to be more depleted in these minerals than those having more pronounced protogranular features. This observation was chacked by bulk chemical analyses of 14 xenoliths. It was found that the more recrystallized rocks are indeed more depleted in basaltic constituents with a mean mg-number of 91.3 in contrast of 90.0 in the case of the less tectonized group. The equigranular inclusions of Szigliget are