Granulites and charnockites of the Gruf Complex: Evidence for Permian ultra-high temperature metamorphism in the Central Alps

Abstract

We present a detailed field and petrological study of charnockites and ultra-high temperature (UHT) granulites from the Gruf Complex, eastern Central Alps. Charnockites occur as up to 0.5km wide and 8km long, internally boudinaged, opx-bearing sheet-like bodies within the regionally dominant migmatitic biotite-orthogneisses. Granulites occur as garnet–orthopyroxene–biotite–alkali feldspar-bearing schlieren (±sapphirine, sillimanite, cordierite, corundum, spinel, plagioclase, and quartz) within charnockites and as residual enclaves both in the charnockites and the migmatitic orthogneisses. Thermobarometric calculations, P–T pseudosections and orthopyroxene Al content, show that both charnockites and granulites equilibrated at metamorphic peak conditions of T=920–940°C and P=8.5–9.5kbar. Peak assemblages were subsequently overprinted by intergrowth, symplectite and corona textures involving orthopyroxene, sapphirine, cordierite and spinel at T=720–740°C and P=7–7.5kbar. We suggest that granulites and charnockites are lower crustal relicts preserved in the migmatitic orthogneisses. Garnet diffusion modelling shows that metamorphic garnet–opx±sapphirine±sillimanite peak assemblages and post-peak reaction textures always involving cordierite developed during two separate metamorphic cycles. Peak assemblages reflect UHT metamorphism related to post-Varican Permian extension, but post-peak coronae and symplectites formed during the mid-Tertiary, upper amphibolite facies, Alpine regional metamorphism. Fluid-absent partial melting of pelitic and psammitic sediments during the Permian UHT event lead to the formation of charnockitic magmas and granulitic residues. Intense melt loss and thorough dehydration of the granulites (although retaining biotite) favoured the partial preservation of peak mineral assemblages during Alpine metamorphism.