Ilmenite as a Source for Zirconium during High-grade Metamorphism? Textural Evidence from the Caledonides of Western Norway and Implications for Zircon Geochronology

Abstract

distance around rutile, and locally these zircons show a prismatic The Proterozoic Lindas Nappe, part of the Caledonides of western overgrowth. A specific low-Th zircon growth event is related to Norway, was affected by penetrative Sveconorwegian granulite-facies eclogite-facies forming reactions, involving breakdown of a twometamorphism, followed by a fluid-driven eclogiteand amphibolitepyroxene + garnet + plagioclase + ilmenite assemblage to form facies Caledonian overprint, spatially restricted along fractures and a garnet + omphacite + rutile assemblage in the presence of a shear zones. In mafic granulites and amphibolites, a luminescent fluid. The low Th content of this zircon probably stems from the anhedral zircon overgrowth, which gives an average age of 924 ± coeval precipitation of clinozoisite. This oscillatory zoned zircon 58 Ma (Th/U = 0·52; secondary ion mass spectrometry data), records fluid infiltration and coeval eclogitization in the crust. surrounds a magmatic zoned core with an age of 952 ± 32 Ma (Th/U = 1·27). In the granulites, a continuous rim of zircon or a discontinuous corona of >10 m rounded to flat zircon crystals is observed at the outer margin of ilmenite grains. Baddeleyite and