Matskási István (szerk.): A Magyar Természettudományi Múzeum évkönyve 92. (Budapest 2000)

Embey-Isztin, A. ; Scharbert, H. G.: Glasses in peridotite xenoliths from the western Pannonian Basin

tie xenoliths could take place at high pressure conditions. The latter two authors convincingly demonstrated that the melting process was preceded and initiated by metasomatic enrichment in incompatible trace elements. PETROGRAPHY OF GLASS-BEARING MANTLE XENOLITHS Texture, mineralogy and geochemistry of the peridotite xenoliths from the western Pannonian Basin (WPB) have been described elsewhere (EMBEY-ISZTIN 1976, 1978, 1984, EMBEY-ISZTIN et al. 1989, KURAT et al 1991; DOWNES et al. 1992; SZABÓ et al. 1995), therefore only a brief characterisation will follow here. The xenoliths have been found in Pliocene alkali basalt flows as well as in basaltic tuffs. Localities include Szentbékkálla (Szt), Bondoróhegy (Bo), Szigliget (Szg) and Gerce (G). All xenoliths contain spinel as an aluminous phase, garnet and plagioclase are typically absent. Textures are highly variable, however protogranular and equigranular types prevail, porphyroclastic textures are restricted to one locality. In addition to these texture types that are very comon in peridotite xenoliths of world-wide localities, WPB peridotite xenoliths show a relatively high frequency of poikilitic texture types that are generally rare or absent from most spinel peridotite xenolith suites of alkali basalts. Xenoliths belonging to this group are either coarse-grained or fine-grained, the latter showing a typical mosaic equigranular texture. Chemically the peridotites vary between fertile and highly depleted compositions. Some of the xenoliths contain clinopyroxenes exhibiting LREE-enriched patterns, whereas in other xenoliths the clinopyroxene is depleted in LREE. A few peridotite xenoliths originally studied by EMBEY-ISZTIN et al. (1989) contain small amounts of glass that have not been described earlier. The glass seems to be unrelated to the host alkali basalts or basanites. They range from small amounts of glass associated with fine-grained clinopyroxenes that formed by disintegration of former larger primary clinopyroxene grains (samples Szt1006, Szt1033, Szt1034, Szt1069, Szt1116; Fig. 1 ) to distinct patches of brown, lilac brown glass (G-1055; Fig. 2) and widely developed interstitial veins of light-coloured glass containing tiny needles of quench plagioclase (Szt1034; Fig. 3). In xenoliths Szt-1

006, Szt1033 (Fig. 4) and Szt-1 116 the small amounts of light brown glass (50-75 um) is found within blebs that are composed mainly of euhedral clinopyroxene crystals (40-320 pm) a few olivine, and abundant spinel microlites (10-40 pm) as well as vugs. In Szt1006 amorphous carbonates were also observed. Spinel grains are more densely concentrated at the border of blebs just as previously described by MAALLOE & PRINTZLAU (1979). Blebs may con-