Evolution of the middle crust beneath the western Pannonian Basin: a xenolith study

摘要

Felsic to mafic granulite xenoliths from late Neogene basalt pyroclastics in four localities of the western Pannonian Basin (Beistein, Kapfenstein, Szigliget and Kaptalantoti (Sabar-hegy) were studied to find out their meta-morphic and fluid history. The characteristic mineral assemblage of the granulites consists of Pl + Opx + Qtz ± Cpx ± Bt ± Grt ± Kfs. Based on abundant magmatic relic microstructural domains occurring in these rocks, the potential precursors might have been predominantly felsic igneous or high to ultrahigh temperature rocks. Ternary feldspar thermometry provides a rough estimate of temperatures of about 920-1070℃. The first fluid invasion event, which is linked with this early high to ultrahigh temperature stage is characterised by primary pure CO_2 inclusions in apatite and zircon. The densest primary CO_2 inclusions indicate 0.52-0.64 GPa pressure at the estimated temperature range of crystallization. According to mineral equilibria and geothermobarometry; the high to ultrahigh temperature rock cooled and crystallized to granulite of predominantly felsic composition at about 750-870℃ and 0.50-0.75 GPa in the middle crust, between 20 and 29 km depths. The second fluid invasion event is recorded by primary CO_2-rich fluid inclusions hosted in the granulitic mineral assemblage (plagioclase, quartz and orthopyroxene). In addition to CO_2, Raman spectroscopy revealed the presence of minor N_2, H_2S, CO and H_2O in these inclusions. Partial melting of biotite-bearing assemblages could be connected to the next fluid invasion shown by secondary CO_2-rich fluids recorded along with healed fractures in plagioclase, clinopyroxene and orthopyroxene. This event could have happened at depths similar to the previous ones. The final step in the granulite evolution was the sampling in the middle crust and transportation to the surface in form of xenoliths by mafic melt. This event generated temperature increase and pressure decrease and thus, limited melting of the xenoliths. The youngest fluid inclusion generation, observed mostly in healed fractures of felsic minerals, could be associated with this event.