Petrology and geochemistry of granulite xenoliths beneath the Nograd-Gomor Volcanic Field, Carpathian-Pannonian Region (N-Hungary/S-Slovakia)

摘要

Abundant upper mantle and rare lower crustal xenoliths have been found in the Plio-Pleistocene alkali basalts of the Nograd-Gomor Volcanic Field, situated in the northern Pannonian Basin, on the border between northern Hungary and southern Slovakia. A few lower crustal granulite xenoliths have been found in a small basaltic pyroclastic cone at Baglyasko. The mafic granulite xenoliths are plagioclase-bearing hornblende clinopyroxenites, plagioclase-bearing clinopyroxene hornblendites, and plagioclase-bearing clinopyroxenites. They contain unusual symplectites, composed of spinel, feldspar and clinopyroxene. These symplectites are interpreted as the product of garnet breakdown. Following the breakdown reaction, the symplectite underwent in situ partial melting. Mineral constituents of these granulite xenoliths have chemical compositions similar to those of other granulite xenoliths worldwide. However, a distinctive positive Pb and Ce anomaly in mineral constituents of these gran-ulites is characteristic. Granulite xenoliths from the Nograd-Gomor Volcanic Field must have experienced granulite facies metamorphism at pressures that correspond to the 'original' thickness of the crust (>l.lGPa; > approx 30km), whereas the breakdown reaction of garnet and subsequent melting and recrystallization of clinopyroxenes in the symplectites happened at shallower depths close to the present-day MOHO (0.6-0.7 GPa; approx 16-19km).