Reduced carbonic fluids in mafic granulite xenoliths from the Bakony–Balaton Highland Volcanic Field, W-Hungary

Abstract

Listen

Translate article

Amphibole-bearing and amphibole-free garnet granulite xenoliths (clinopyroxene, plagioclase, garnet ± orthopyroxene, ± amphibole) from two localities of the Bakony–Balaton Highland Volcanic Field (Sabar-hegy and Szentbékkálla) were investigated by means of textural analysis, mineral chemistry, mineral equilibria, geothermobarometry and fluid inclusion studies. We have determined the P– T path of these lower crustal xenoliths, which begins at peak pressure between 0.9 and 1.4 GPa. The peak pressure was followed by temperature increase of about 200–300 °C and 0.3–0.7 GPa pressure decrease which is marked by breakdown reactions of garnet and pargasitic amphibole. Garnet broke down first to orthopyroxene–anorthite–spinel (type 1) then to olivine–plagioclase ± spinel (type 2) symplectite. Amphibole broke down to form clinopyroxene–plagioclase–Fe–Ti oxide-melt ± orthopyroxene ± olivine mineral assemblage. These changes in P– T conditions were caused by crustal thinning. The present day crustal thickness (max. 32 km) indicates that garnet granulites are no longer stable in the lower crust of the Pannonian Basin. The calculated crustal thinning factor is between 1.6 and 1.8 in accordance with the values calculated from tectonic methods. A unique CO 2–CO–C fluid system—which has not been described from the lower crust so far—occurs in inclusions of the rock-forming minerals, mainly in plagioclase and clinopyroxene. A primary and two secondary fluid inclusion (FI) generations were identified on the basis of textural analyses and distribution of homogenization temperatures. Primary FIs are connected to the formation of garnet granulite. The older secondary generation was trapped after the main temperature increase and some pressure decrease during crustal thinning. The younger secondary generation trapped within the depth limits of the present lower crust at pressures between 0.6–0.7 GPa. The overall composition of the fluid system has not changed during crustal thinning. The calculated oxygen fugacity values are slightly above and under QFM (between 10–8.5 and 10–10.1 at 1200 °C).