Matskási István (szerk.): A Magyar Természettudományi Múzeum évkönyve 99. (Budapest 2007)
Embey-Isztin, A. ; Dobosi, G.: Composition of olivines in the young alkali basalts and their peridotite xenoliths from the Pannonian Basin
The small size of olivine phenocrysts in the rocks seems to corroborate this interpretation. In the case of Szilvás-kő and Podrecany basalts, olivine is not the only liquidus phase, but it co-precipitated with clinopyroxene (DOBOSI 1989). This fact may also explain the apparent disequilibrium feature. In principle, magma mixing and replenishment by a more basic melt can easily account for the disequilibrium, however the absence of inverse zoning patterns in the olivines seems to exclude this possibility. Outstandingly high differences exist between the calculated and measured olivine compositions in the basalts of Szentgyörgy-hegy and Bulhary, where olivines may have re-equilibrated with the differentiated ground mass through a diffusion process. Rock samples, carrying more magnesian olivines than the supposed equilibrium composition, are also plotted in Fig. 10. As far as Bondoró-hegy and Diszel basalts are concerned, the xenocryst origin seems to be unequivocal i.e. the high Fo olivine grains originate from the disintegration of mantle xenoliths. Fig. 7 shows such xenocrysts that have been overgrown by a less magnesian and a more iron-rich rim. The rock of Bondoró-hegy, the youngest basalt in the Balaton region, is a special case. It contains so many xenocrysts in addition to large mantle peridotite xenoliths, that the bulk rock composition has considerably been changed (EMBEY-ISZTIN et al. 1993). Thus, it is mere chance that the composition of xenocrysts equals that of the equilibrium value. Several basalts from the Nógrád district also contain xenocrysts, and peridotite xenoliths of upper mantle origin (Barna, Eresztvény, Magyarbánya, Fülekkovács). The nephelinite of Stradner Kogel in Styria is one of the most fractionated young basalts of the Carpathian Basin (EMBEY-ISZTIN et al. 1993, EMBEY-ISZTIN Sc DOBOSI 1995). The mantle origin of olivine is likely, even in this extreme case, because the rock also contains small mantle-derived dunite xenoliths (DIETRICH & POULTIDIS 1985). In some other basalts, e.g. Hegyesd (Balaton region), Somló (Little Hungarian Plain), Somos-kő and Ajnács-kő (Nógrád) the more magnesian olivine could have precipitated from a more primitive melt than what is suggested by the actual bulk rock composition. These phenocrysts seem to reflect an earlier, less differentiated state of the magmatic evolution that probably occurred at greater depth, and therefore the olivine phenocrysts are out of equilibrium with their present hosts. Finally, our statistical analysis have shown that a fairly high number of measured olivine phenocrysts fall into the category F085-88 (Fig. 8). Olivines with F087 an d greater may have been in equilibrium with primitive mantle melts even if - as discussed above - the actual host basalts are moderately fractionated.
