Phase relations during peak metamorphism and decompression of the UHP kyanite eclogites, Pohorje Mountains (Eastern Alps, Slovenia)

Abstract

Phase relations among the mineral assemblages of UHP kyanite eclogite were investigated in the Pohorje Mountains of the Eastern Alps. Ultrahigh-pressure metamorphism resulted from intracontinental subduction during the Cretaceous (ca. 92Ma). Kyanite-bearing eclogites are associated with meta-ultramafic rocks including UHP garnet peridotites and are embedded in metapelitic gneisses and micaschists. The kyanite eclogites contain a peak metamorphic assemblage of garnet, omphacite, kyanite and phengite. Pyrope-rich garnet is unzoned and almost free of inclusions. The non-stoichiometric supersilicic omphacite contains up to 5mol% of Ca-Eskola molecule. Breakdown of omphacite during decompression resulted in exsolution of oriented rods of silica. Phengite contains up to 3.5 Si a.p.f.u. Polycrystalline quartz inclusions in peak-pressure minerals – garnet, omphacite and kyanite – are surrounded by radial fractures diagnostic of the former presence of coesite. Peak-pressure minerals are replaced by symplectites of diopside+plagioclase+amphibole after omphacite, plagioclase+biotite after phengite and sapphirine+corundum+spinel+anorthite after kyanite. Sapphirine has composition close to (Mg, Fe)12.4 Al38.9 Si4.5 O80 in average, which is amongst the most aluminous yet reported. Peak metamorphic conditions were constrained from calculated phase equilibria in the NKCFMASH system with the fixed bulk-rock composition, and conventional geothermobarometry. This approach led to consistent results, the calculated peak P–T conditions of 3.0–3.7GPa and 710–940°C, in the stability field of coesite and the same range as metamorphic conditions recorded by the associated garnet peridotites. This implies that eclogites and their host rocks were subducted to depths of about 100km. The relatively high temperature at peak pressure, compared to UHP rocks of Tertiary age in the Western Alps where mostly oceanic crust was subducted, probably resulted from radiogenic heat production by subducting continental crust, in the intra-continental setting of the Cretaceous subduction zone in the Eastern Alps.