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

mainly clinopyroxene, olivine, plagioclase, spinel and magnetite (e.g. FREY & GREEN 1

974). Similar glasses have, however, also been described from peridotite xenoliths the origin of which was argued to be cognate with the host lava (ELLIS 1976, IRVING 1980). Most of the glass-bearing peridotites are spinel Iherzolites or wherlites hosted in alkali basalt, basanite, trachyte but more often emplaced in tuffs, diatrems or maars (JONES et ai. 1983a). Breakdown of amphibole (WHITE 1966, FORBES & STARMER 1974, FRANCIS 1976, GIROD et al. 1981), mica (FREY & GREEN 1974) or even garnet (HUNTER and TAYLOR 1981 ) has been considered as a major factor in the genesis of some glasses. However, consideration of simple mass balance requirements suggests that additional factors, e.g. metasomatic enrichment or readjustment of alkalis, must play an important role in the process. The formation of glasses is comonly ascribed to decompression melting during rapid ascent of the xenoliths. Nonetheless, alternative proposals have also been put forward such as melting within the mantle adjacent to a magma body (MAALLOE & PRINTZLAU 1979), different versions of cognate origin (ELLIS 1976, IRVING 1980) or accumulative origin (IRVING 1974). We consider that, except for the xenolith Szt-1

106, the breakdown of amphibole (or another potash-rich phase such as mica) did not play a role in the genesis of glasses in WPB mantle xenoliths. Firstly, even if pargasitic amphibole does occur Table 4. Electrone microprobe analyses of secondary minerals in blebs Szg-3007 Szt006 Szt-1033 ol* ol cpx* cpx Pg ol* ol cpx* cpx cpx cpx ol* ol Si0

2 40.33 39.84 52.19 50.16 52.74 40.35 40.90 52.94 48.72 50.09 47.61 40.38 40.79 TÍO, 0.01 0.00 0.49 1.29 0.19 0.00 0.00 0.31 1.02 0.59 1.04 0.00 0.02 A1A 0.03 0.05 6.83 4.35 29.05 0.00 0.04 2.37 11.54 5.73 8.60 0.01 0.06 Cr

;

0, 0.01 0.00 0.72 0.66 nd 0.00 0.11 2.54 1.75 2.60 2.98 0.00 0.09 FeO 11.25 14.93 2.88 5.19 0.66 10.15 7.91 3.64 2.97 2.69 2.82 9.90 9.22 MnO 0.14 0.24 0.10 0.13 nd 0.16 0.11 0.12 0.05 0.08 0.08 0.17 0.13 NiO 0.41 0.26 nd nd nd 0.35 0.32 nd nd nd nd 0.42 0.35 MgO 48.03 45.06 15.57 15.1 1 0.11 49.09 50.35 18.67 12.18 15.89 13.96 48.86 48.94 CaO 0.09 0.16 19.77 21.93 12.18 0.08 0.21 19.16 19.46 21.42 21.92 0.04 0.18 Na

2

0 nd nd 1.59 0.52 4.07 nd nd 0.75 1.56 0.66 0.78 nd nd K

:

0 nd nd 0.00 0.00 0.43 nd nd 0.00 0.52 0.02 0.02 nd nd Total 100.29 100.54 100.14 99.34 99.43 100.17 99.95 100.48 99.77 99.77 99.81 99.78 99.78