Abstract
A deformed composite peridotite-pyroxenite xenolith in Pliocene alkali basalts from the Pannonian Basin (Szentbekkalla, Bakony—Balaton Highland Vol- canic Field) has been studied in detail. A narrow shear zone of intense deformation marked by porphyroclast elongation and recrystallization runs along the perido- tite-pyroxenite contact. The xenolith contains a large volume of euhedral olivine neoblasts and tablet grains of olivine away from the ''shear zone'' interpreted as products of annealing and recrystallization in the pres- ence of grain boundary fluid. Estimates of the time re- quired for growth of recrystallized olivine grains suggest that the annealing took place in situ in the mantle and not during transport of the xenolith in the basalt mag- ma. The grain boundary fluid present during recrystal- lization was a vapor rich silicate-melt different from the host basaltic melt that entrained the xenolith. This study demonstrates that high-stress deformation zones and associated fluid-assisted recrystallization, which are common features in kimberlite mantle xenoliths, also occur in some mantle xenoliths from alkali basalts. The suggested high-stress deformation zones in the mantle beneath the Pannonian Basin may be produced by paleoseismic events in the lithosphere associated with faulting related to the ascent of basalt magma.
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