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

The glass-bearing xenolith Szt-1

106 is the only one to contain amphibole in the form of texturally well equilibrated grains, proving that the formation of amphibole - presumably by metasomatic replacement of clinopyroxene - preceded the recrystallisation process. In this rock, glass is mainly associated with a thick reaction rim surrounding amphibole grains (Fig. 6). Xenolith Szt1069 is a composite one, containing a thin vein of Type II clinopyroxenite crossing normal Type I protogranular peridotite. Glass seems to be confined to the border zone of Type II and Type I material. Small glass patches (2-133 pm) occupy interstitial space around Type II pyroxenes. In glassy areas, a peculiar partly empty spherical structure can sometimes be seen probably due to contraction caused by loss of a fluid component. Sample Szg-3007 is a rare type of composite xenolith that has numerous fragments of small coarse-grained and fine-grained Type I peridotite cemented by amphibole-bearing Type II pyroxenite (EMBEY-ISZTIN 1984). Here again, glass patches are associated with Type II material. The light-coloured and brownish glass around Type II clinopyroxenes and amphiboles may contain quench crystals of plagioclase. The glass may also show spherical structures associated with vugs (Fig. 7). The sample G-1055 is a porphyroclastic peridotite containing irregular shaped small patches of lilac brown glass (200-400 pm) at orthopyroxene, clinopyroxene and spinel contacts (Fig. 2). The glass is rather poor in quench crystals, only a few microliths of spinel are embedded in it. All around the glassy areas clinopyroxene shows signs of disintegration and spherical vugs (250-500 pm). In addition to mantle-derived peridotites, some of the WPB volcanic vents delivered lower crustal granulite xenoliths that have also been described earlier (EMBEY-ISZTIN & SCHARBERT 1988, EMBEY-ISZTIN et al. 1990, KEMPTON et al. 1997). These xenoliths are mafic granulites with a mineralogy of clinopyroxene ± orthopyroxene + plagioclase + spinel ± garnet ± magnetite ± ilmenite. In general, granulite xenoliths are much more susceptible to melting than peridotites and indeed, many of the WPB granulites show melting phenomena, especially those found at the locality of Bondoróhegy (Bo). However, we have analysed the composition of glass only in one of these xenoliths (Bo-3007). In this sample, disintegration of primary plagioclase and clinopyroxene grains is widely developed. Patches of dark-coloured glass contain quench crystals, especially abundant euhedral laths of plagioclase, but a few clinopyroxene and olivine crystals are also present (Fig. 8).