Sapphirine in SW Sweden: a record of Sveconorwegian (–Grenvillian) late‐orogenic tectonic exhumation

Abstract

In the Sveconorwegian granulite region of SW Sweden, sapphirine occurs in reaction coronas in Mg‐ and Al‐rich kyanite eclogites which form parts of mafic complexes. Aluminous to peraluminous sapphirine forms symplectitic intergrowths with plagioclase±corundum±spinel after kyanite. Kyanite and omphacite were the main reactants in the formation of sapphirine. The sapphirine formed during decompression from the eclogite facies (P >15 kbar) through the high‐ to medium‐pressure granulite and upper amphibolite facies at c. 750 °C. Preserved growth zoning in garnet, frozen‐in reaction textures, and chemical disequilibrium suggest a rapid tectonic exhumation. Ductile deformation in the surrounding gneisses and parts of the mafic complex is characterized by foliation development, WNW–ESE stretching and dynamic recrystallization under granulite to upper amphibolite facies conditions, simultaneous with the sapphirine formation. This decompression, high‐grade re‐equilibration and associated deformation took place during the exhumation of the Sveconorwegian eclogites, bracketed between 969±14 and 956±7 Ma. Probable tectonic causes are late‐orogenic gravitational collapse and/or plate divergence following the Sveconorwegian–Grenvillian continent–continent collision. There are no indications of metastability of aluminous and peraluminous sapphirine in the decompressed kyanite eclogites; sapphirine is stable in amphibole‐poor and amphibolitized varieties, including rocks that have undergone dynamic recrystallization. Close similarities between rocks from different parts of the world with respect to reaction textures suggests that sapphirine+plagioclase‐forming reactions are a universal feature in high‐temperature decompressed kyanite eclogites.