Metamorphic evolution of kyanite-bearing garnet pyroxenites from the Variscan orogenic root: Gföhl Unit, Moldanubian Zone, Austria

Abstract

Kyanite-bearing garnet pyroxenites occurring in centimetre to several hundred metres sized lenses of (ultra-)mafic lithologies embedded in felsic granulites of the Gföhl Unit in the south-eastern part of the Moldanubian Zone recorded a polyphase metamorphic evolution. An early high-pressure–high-temperature (HP–HT) stage at temperatures of 900 °C to 1000 °C and pressures in excess of 2 GPa is represented by the assemblage garnet–kyanite–omphacitic clinopyroxene–rutile (± quartz). Subsequent re-crystallisation under medium-pressure–high-temperature (MP–HT) conditions of about 1.0–1.3 GPa and 900–1000 °C is documented by the replacement of omphacitic clinopyroxene by symplectic intergrowth of diopsidic clinopyroxene and plagioclase, formation of sapphirine-bearing symplectites after kyanite, and a pronounced secondary compositional zoning of pre-existing garnet. Systematic compositional variations in the garnet cores over mm- to cm-sized mineralogically and microstructurally distinct domains indicate that equilibration volumes were small during the early HP–HT stage. In contrast, the compositions of the minerals that formed, or re-crystallised, during the MP–HT overprint are uniform throughout the samples, indicating substantially larger equilibration volumes during that stage. The metamorphic evolution is discussed in the light of deep subduction and relamination of felsic metaigneous crust, which subsequently experienced buoyancy-driven exhumation to mid-crustal levels and incorporated slivers of mafic and ultramafic material from the lithospheric mantle and lower crust en route.