Evidence for xenolith-host basalt interaction from chemical patterns in Fe-Ti-oxides from mafic granulite xenoliths of the Bakony-Balaton Volcanic field (W-Hungary)

We investigate the effects of xenolith-host basalt interaction on lower crustal mafic granulite xenoliths from the Central Pannonian Basin. The xenoliths are devoid of any signs of melting, nevertheless various phenomena are identified, which indicate that the original mineralogy and chemistry of the xenoliths was modified during interaction with the host basalt. The rock-forming silicates are only slightly affected by alteration, but the Fe-Ti-oxides are overprinted to a significant extent. Complex chemical zoning patterns are detected using high-resolution element mapping in ilmenites and in lamellar titanomagnetite-ilmenite intergrowths. The chemical alteration of the Fe-Ti-oxides was diffusion-controlled and, hence, time and temperature dependent. On the basis of diffusion profiles in titanomagnetite we estimate the duration of xenolith-host basalt interaction to be at least 9-20 h. This is comparable to the time necessary for the ascent of the host basalt to the surface. It is too short to reflect alteration during granulite facies metamorphism in the deep crust. The chemical alteration of the titanomagnetite thus reflects the total duration of the xenolith-host basalt interaction.