Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 69. (Budapest 1977)

Embey-Isztin, A.: The Szigliget amphibolitelherzolite compound xenolith as an evidence for diapiric uprise in the mantle below Hungary

Fig. 3. The Szigliget composite xenolith. — Fig. 4. Exolution lamellae of clinopyroxene in orthopyroxene, Magyarbánya, North Hungary. 16-11 kbar). The last alkali-rich strongly undersaturated liquids would have been crystallized as amphibolites in the lower level of the crust about 8 kbar. It is evident that the vein systems of Lherz cannot serve as a precise model for the conditions of igneous bodies at great depth below the Pannonian Basin. However, here we have also evidences of an active diapiric rise of the upper mantle (see in STEGENA et al. 1975) in the Pliocene. The Moho discontinuity is in an elevated position at a depth of 22-26 km only (MITTTCH 1967). The heat flow is markedly greater than the average value. Therefore I conclude that the Szigliget amphibolite vein — like at Lherz in the Pyrenees — is a last product of the differentiation of liquids, the origin of which is due to partial melting of hydrous rising material from the upper mantle, and probably it crystallized at a pressure of 8-10 kbar. — In the Szigliget amphibolite, a partial remelting also took place, probably due to subsequent rapid ascending in the basanitic lava. As the centre of the secondary amphibole is chemically similar to the primary one, it may be concluded that the remelting originated at a considerable depth. However, the Szigliget composite xenolith is not the only petrological evidence for upwelling mantle below Hungary. The examination of inclusions (mostly 4-phase lherzolites) has revealed some facts indicating the complex cooling history of the inclusions (EMBEY-ISZTIN in prep.). First of all, the exolution lamellae of clinopyroxene in enstatite (Fig. 4) can be mentioned. The reverse case is uncommon. Furthermore, it seems probable that not only the clinopyroxene but the spinel can also be an exolution product in the lherzolites. According to these textúrái relationships the following reaction can be assumed: Ca, Al-rich pyroxene —- enstatite -f-clinopyroxene -f- spinel. It is evident that on the left of the equation the t must have been higher than on the right. According to BEST (1975) the Ca, Al-rich pyroxene is stable at 1400 °C, NICOLAS et al. (1972) assume also 1400 °C and 18 kbar. The lherzolites have re-equilibrated at 850-1050 °C and at a pressure range of 10-18 kbar (EMBEY-ISZTIN in prep.). The mechanism that transported these rocks from an environment of higher p, t to another of lower p, t must have been a convection in the mantle. In this regard, another interesting fact may be mentioned. Namely the position of the Hungarian lherzolite «xenoliths in the p-t diagram is very instructive (Fig. 2). One part of the lherzolites is on or near to the oceanic geotherm along with lherzolites of Auvergne (Massif Central, France). Another part of the Hungarian samples shows a clear departure from the oceanic geotherm (EMBEY-ISZTIN in