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

Abstract

The Proterozoic Lindås Nappe, part of the Caledonides of western Norway, was affected by penetrative Sveconorwegian granulite-facies metamorphism, followed by a fluid-driven eclogite- and amphibolite-facies Caledonian overprint, spatially restricted along fractures and shear zones. In mafic granulites and amphibolites, a luminescent anhedral zircon overgrowth, which gives an average age of 924 ± 58 Ma (Th/U = 0·52; secondary ion mass spectrometry data), 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 μm rounded to flat zircon crystals is observed at the outer margin of ilmenite grains. Baddeleyite and srilankite (Ti2ZrO6) blebs are reported around ilmenite included in feldspar or pyroxene. Baddeleyite is interpreted as an exsolution product from magmatic ilmenite, whereas srilankite, the zircon corona around ilmenite and the luminescent zircon overgrowth were formed as reaction products during granulite-facies metamorphism. Textures suggest that magmatic ilmenite was a main source of Zr to form metamorphic zircon. In massive amphibolites, relic ilmenite grains are surrounded by a corona of titanite and a discontinuous corona of micro-zircons. Amphibolite-facies overprint is not associated with any significant growth or dissolution of zircon. An unsheared eclogite displays a zircon population with a euhedral oscillatory zoned overgrowth giving an age of 455 ± 29 Ma (Th/U ≤ 0·13). A corona of micro-zircon grains is observed at some distance around rutile, and locally these zircons show a prismatic overgrowth. A specific low-Th zircon growth event is related to eclogite-facies forming reactions, involving breakdown of a two-pyroxene + garnet + plagioclase + ilmenite assemblage to form a garnet + omphacite + rutile assemblage in the presence of a fluid. The low Th content of this zircon probably stems from the coeval precipitation of clinozoisite. This oscillatory zoned zircon records fluid infiltration and coeval eclogitization in the crust.